Bioinspired Surfaces with Superwettability: New ... - ACS Publications

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Bioinspired Surfaces with Superwettability: New Insight on Theory, Design, and Applications Shutao Wang,†,‡ Kesong Liu,§ Xi Yao,∥ and Lei Jiang*,†,‡,§ †

Laboratory of Bio-inspired Smart Interface Science, Technical Institute of Physics and Chemistry, and ‡Beijing National Laboratory for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China § Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry and Environment, BeiHang University, Beijing 100191, People’s Republic of China ∥ Department of Biomedical Sciences, City University of Hong Kong, Hong Kong P6903, People’s Republic of China 4.1.4. Spin-Coating Methods 4.1.5. Spraying Methods 4.1.6. Electrohydrodynamics/Electrospinning 4.1.7. Ion-Assisted Deposition Method 4.2. Chemical Methods to Superhydrophobic Surfaces 4.2.1. Sol−Gel Methods 4.2.2. Solvothermal Methods 4.2.3. Electrochemical Methods 4.2.4. Layer-by-Layer Methods 4.2.5. Self-Assembly Methods 4.2.6. Bottom-Up Fabrication of Micro-/Nanostructure 4.2.7. One-Step Methods 4. 3. Combination of Physical and Chemical Methods 4.3.1. Vapor Deposition Methods 4.3.2. Etching Methods 4.4. Functional Surfaces with Special Wettability 4.4.1. Stable Superhydrophobic Surface 4.4.2. Superhydrophobic Surfaces with Unique Optical Properties 4.4.3. Superhydrophobic Surface with Conductive Properties 4.4.4. Superhydrophobic Surfaces with Magnetism Properties 5. Superhydrophilic Surfaces 6. Responsive Surfaces with Switchable Wettability 6.1. Single Stimuli-Responsive Surfaces 6.1.1. Thermoresponsive Surfaces 6.1.2. Photoresponsive Surfaces 6.1.3. pH-Responsive Surfaces 6.1.4. Solvent-Responsive Surfaces 6.1.5. Electricity-Responsive Surfaces 6.1.6. Stress-Responsive Surfaces 6.1.7. Gas-Responsive Surfaces 6.1.8. Enthalpy-Driven Surfaces 6.1.9. Ion-Responsive Surfaces 6.2. Dual- and Multiresponsive Switchable Surfaces 7. Adhesion-Controlled Liquid/Solid Surfaces 7.1. Theoretical Understanding of Liquid/Solid Adhesion

CONTENTS 1. Introduction 2. Fundamental Understanding of Superwettability 2.1. Definition of Hydrophilicity and Hydrophobicity 2.2. Definition of Superhydrophobicity and Superhydrophilicity 2.3. Different Superhydrophobic States 2.4. Explanation of Wettability on Water/Oil/ Solid Interfaces 2.5. Measurements of Surfaces with Superwettability 2.5.1. Static Contact Angle 2.5.2. Dynamic Contact Angle 3. Natural Examples with Superwettability 3.1. Lotus Leaf 3.2. Rice Leaf 3.3. Butterfly Wing 3.4. Water Strider Leg 3.5. Mosquito Compound Eye 3.6. Poplar Leaf 3.7. Gecko Foot 3.8. Red Rose Petal 3.9. Salvinia Leaf 3.10. Fish Scale 3.11. Nepenthes Leaf 3.12. Stenocara Beetle 4. Fabrication of Superhydrophobic Surfaces 4.1. Physical Methods to Superhydrophobic Surfaces 4.1.1. Plasma Treatment 4.1.2. Phase Separation Methods 4.1.3. Template Methods © 2015 American Chemical Society

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Chemical Reviews 7.2. High Adhesive Surfaces 7.3. Adhesion-Tunable Surfaces 7.3.1. Chemical Composition 7.3.2. Surface Topography 7.4. Stimuli-Responsive Surfaces with Tunable Adhesion 7.4.1. Mechanical-Responsive Adhesion 7.4.2. Thermal-Responsive Adhesion 7.4.3. Magnetic-Responsive Adhesion 7.4.4. Photoelectrical-Responsive Adhesion 7.4.5. Heat, pH, and Ion Multiresponsive Adhesion 8. Superoleophobic Surfaces 8.1. In-Air Superoleophobic Surfaces 8.2. Underwater Superoleophobic Surfaces 8.2.1. Electrochemical Control of Oil/Solid Adhesion on Underwater Superoleophobic Surfaces 8.2.2. Thermal Control of Oil/Solid Adhesion on Underwater Superoleophobic Surfaces 9. Applications 9.1. Anticorrosion 9.2. Antifogging 9.3. Anti-icing 9.4. Drag Reduction 9.5. Medicine 9.6. Oil/Water Separation 9.7. Printing and Reprography 9.8. Self-Cleaning 9.9. Water Harvesting 9.10. Other Applications 10. Conclusion and Perspective Author Information Corresponding Author Notes Biographies Acknowledgments Abbreviations References

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Figure 1. Possible extreme states of surface wettability. Surface wettability on a flat solid surface commonly shows four basic states in air (inside inner circle), including hydrophilic (HL), hydrophobic (HB), oleophilic (OL), and oleophobic (OB). Their combination will generate four possible extreme states in air, that is, superhydrophilic (SHL), superhydrophobic (SHB), superoleophilic (SOL), and superoleophobic (SOB). When air is changed to water or oil, there are four possible extreme states: underwater superoleophobic (WSOB), underwater superoleophilic (WSOL), underoil superhydrophobic (OSHB), and underoil superhydrophilic (OSHL).

1805.1 Wenzel explained the contribution of roughness to hydrophobicity in 19362 and 1949.3 Cassie and Baxter showed that the composite interface of solid and air can affect the superhydrophobicity in 1944.4 Later, Cassie, Bartell, Fowkes, Zisman et al. discussed the hysteresis of CA.5 In the following half a century, most efforts focused on theoretical studies and analytical techniques. In 1997, Barthlott and Neinhuis revealed that the self-cleaning property of lotus leaves was caused by the microscaled papillae and the epicuticular wax, providing a monomicrostructure model. 6 Subsequently, Jiang et al. disclosed that there are micro- and nanoscaled hierarchical structures on the lotus leaf surface, that is, branch-like nanostructures on the top of micropapillae, which result in the superhydrophobicity.7 Their mimicking study by using aligned carbon nanotube array further demonstrated that nanostructures can induce the high CA of superhydrophobic surfaces and multiscale structures can effectively reduce the sliding angle (SA). Thus, the origin of superhydrophobicity of a lotus leaf becomes clear from a monoscaled model of microstructures to a dual-scaled model of hierarchical structures. These studies on the lotus leaf activated the field of wettability from surface chemistry, biomimetic study, fundamental theory, and material science. In the following decade, a mass of surfaces with superwettability have been fabricated, besides superhydrophobic surfaces, and have shown promising potential applications in self-cleaning, antibiofouling, anti-ice, antifogging, oil/water separation, smart membrane, sensor, and microfluidic devices. In this Review, we will summarize the recent progress of bioinspired surfaces with superwettability. Our content includes 10 parts: introduction, fundamental understanding of super-

1. INTRODUCTION The wettability of solid surfaces is a renewed old topic that has impacted most fields of science and technology for a long time, from cave painting in the ancient to microfluidic devices in the modern. In the past decades, bioinspired surfaces with superhydrophobicity that is one of the extreme states of surface wettability have been intensively explored and accelerated by discoveries of superwetting phenomena in nature. Accordingly, several possible extreme states of superwettability were disclosed (Figure 1), including superhydrophilic (SHL), superhydrophobic (SHB), superoleophilic (SOL), and superoleophobic (SOB). When air is changed to water or oil, several possible extreme states appear: underwater superoleophobic (WSOB), underwater superoleophilic (WSOL), underoil superhydrophobic (OSHB), and underoil superhydrophilic (OSHL). Those terms, going far beyond new terminologies, have dramatically accelerated the development of new surface technologies and deepen the understanding of fundamental knowledge of wettability. Historically, as the father of contact angle (CA) and wettability, Thomas Young first described CA in 1804 and 8231

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wettability, natural examples with superwettability, fabrication of superhydrophobic surfaces, superhydrophilic surfaces, responsive surfaces with switchable wettability, adhesioncontrolled liquid/solid surfaces, superoleophobic surfaces, applications of bioinspired surfaces with superwettability, and conclusion and perspective.

2. FUNDAMENTAL UNDERSTANDING OF SUPERWETTABILITY Wetting at solid and liquid interfaces, one of the most common phenomena, is governed by surface chemistry and surface roughness. In general, CAs are applied to evaluate the static wettability of a solid surface, while SAs or advancing/receding angles are considered to characterize the dynamic wettability of a solid surface. With the field of surface wettability dramatically developing, some unique wetting phenomena and interfacial molecular structures of water droplets bring new challenges, even contradictions, to the traditional theory of surface wettability, such as Young’s equation, Wenzel model, and Cassie and Baxter model. Therefore, in this section, we try to make some fundamental concept clear and give a clue to understanding the unique wetting phenomena on a solid surface.

Figure 2. Berg limit based on hydrophobic force measurement. As CA on hydrophobic surfaces is higher than 65°, the adhesion tension of pure water is less than 30 dyn/cm, exhibiting long-range attractive forces at a distance of tens of nanometers. In contrast, when CA on hydrophilic surfaces is less than 65°, the adhesion tension of pure water is less than 30 dyn/cm, indicating repulsive forces. Reproduced with permission from ref 10. Copyright 1998 Elsevier Science B.V.

the depletion of hydrogen bonding in water close to hydrophobic surfaces could result in drying of hydrophobic surfaces and long-ranged forces at nanometer scale.12 By measuring the intrinsic and apparent CA of a series of polymer surfaces, Jiang et al. also concluded that the limit of hydrophilicity and hydrophobicity could be around 62.7°.13 On the basis of the above studies, 65° seems a new limit of intrinsic CA between hydrophilicity and hydrophobicity for a smooth solid surface. The criterion angle of 65° shows more chemical and physical meaning than the mathematical angle of 90° and will bring a wide and instructive impact in the design of interfacial materials such as biofouling coatings.14 Surely, further extensive studies should be done in this direction from both experimental observation and theoretical interpretations. In comparison to hydrophobic and hydrophilic surfaces, there are not many efforts devoted to the study of oleophobic and oleophilic surfaces probably because of the complexity and diversity of the oil phase. In a pervasive understanding, solid surfaces with oil CAs θ < 90° are considered to be oleophilic, and those with oil CAs θ > 90° are oleophobic. There remain some difficult conundrums, for example, the influence of surface chemical structure of oil droplets, the influence of oil types, surface chemistry, and topography of solid materials.

2.1. Definition of Hydrophilicity and Hydrophobicity

Generally, solid surfaces that have CAs θ < 90° are considered to be hydrophilic, and those that have CAs > 90° are hydrophobic. The CA limit of 90° between hydrophilicity and hydrophobicity originates from Young’s equation:8 γSV = γSL + γLV cos θ or cos θ =

γSV|γSL γLV

where γSV, γSL, and γLV represent solid/gas, solid/liquid, and liquid/gas interface tensions, respectively. θ is the balance CA or the intrinsic CA of the materials and can be written as θe. The CA is determined by the interactions across the three interfaces on the smooth surface. It is an ideal and simple mathematics and physical model. Here, the solid surface should be an ideally smooth surface that is isotropic, undeformed, and homogeneous, and also the water droplet is considered as a mathematic entity whose surface and bulk phase are consistent. However, by considering the actual chemical and structural state of the water droplet, Berg et al. showed a new limit between hydrophilicity and hydrophobicity that may be 65°.9 From the view of physical chemistry, the structure and reactivity of water at the surface and the bulk phase are vastly different and change along with varying native properties of solid surfaces. Volger thought that the water structure should be a manifestation and quantitative definition of hydrophobicity and hydrophilicity.10 A relative less-dense water region with an open hydrogen-bonded network forms against hydrophobic surfaces, while a relatively more-dense water region with a collapsed hydrogen-bonded network forms against hydrophilic surfaces. Recently, Yoon et al. measured the local changes of chemical potential of water by the surface tension apparatus and other assistant technology at the scale of tens of nanometers.11 As Figure 2 shows, between two hydrophobic surfaces with CAs higher than 65°, the adhesion tension of pure water is less than 30 dyn/cm, indicating long-range attractive forces in distance of tens of nanometers. In contrast, between two hydrophilic surfaces with CAs less than 65°, the adhesion tension of pure water is less than 30 dyn/cm, showing repulsive forces. Chandler et al. further demonstrated theoretically that

2.2. Definition of Superhydrophobicity and Superhydrophilicity

The term superhydrophobicity was introduced in 1976 by Reick to describe hydrophobic particle coating made of hydrophobic fumed silicon dioxide, where the shape of water remains almost spherical and the force of adhesion is negligible. The finding of the lotus effect accelerates the wide use of superhydrophobicity, especially in the past decade. According to the general definition, a superhydrophobic surface has a CA θ > 150°. Surface chemistry and surface roughness are two critical factors to obtain the superhydrophobicity. However, the maximized CA on the flat surfaces is around 118° that depends on the modification of hydrophobic fluoride. The role of surface roughness has been further discussed in the following two main models. In 1936, to explain the superwettability on rough surfaces, Wenzel modified Young’s equation by introducing a factor of 8232

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surface roughness, r, which is defined as the ratio of the actual area of a rough surface to the geometric projected area.2,3 Assuming θ* to be the apparent angle on the rough solid surface, it can be evaluated by considering a small displacement dx of the contact line along the parallel direction of the surface, as indicated in Figure 3.15 The total free energy difference dF can be written as

The angle corresponding to the lowest free energy of this system is given by the Cassie−Baxter equation: cos θ* = f1 cos θ1 + f2 cos θ2

In this equation, θ1 and θ2 are the apparent CAs of the liquid on substance 1 and substance 2, respectively. f1 and f 2 are the apparent area fractions of substance 1 and substance 2 (f1 + f 2 = 1). This equation can also be applied on the solid surfaces composed of solid substance and air, for example, the surfaces with porous structures or other rough structures that can preserve air. At this time, f 2 is the area fraction of the trapped air. Because the CA between air and water θ2 is 180°, the equation can be rewritten as

dF = r(γSL − γSV ) dx + γLV cos θ*

cos θ* = f1 cos θ1 − (1 − f1 )

Figure 4 shows the apparent CA versus Young’s angle for the hydrophobic surface assuming the roughness remains constant.

Figure 3. Scheme of contact edge between the droplet and the rough surface. θ*, the apparent angle on the rough solid surface, can be evaluated by considering a small displacement dx of the contact line along the parallel direction of the surface.

As F is a minimum, the system will reach a thermodynamic equilibrium. Thus, the equilibrium condition yields Young’s equation when r is equal to 1 and leads to Wenzel’s equation for r is larger than 1: cos θ* = r cos θe

As r is always larger than 1, surface roughness will enhance surface wettability. That is, (a) when θ < π/2, θ* decreases with the increase of surface roughness and the surface becomes more hydrophilic; and (b) when θ > π/2, θ* increases with the increase of surface roughness and the surface becomes more hydrophobic. By simulating the CA of a water droplet on idealized sinusoidal surfaces, Johnson Jr. and Dettre showed that the possible hysteresis decreases as r decreases with the maximum slope of the solid surface.16 Roughness made it possible to have more than one metastable position of equilibrium, and Wenzel’s equation is thus derived by comparing these adjacent and nonequilibrium states, in contrast to Young’s equation, which is derived from an equilibrium position. They also tested the difference between the advancing and receding CAs on fluorocarbon wax surfaces with different roughness, qualitatively confirming the inference of Wenzel’s equation. For the surfaces with high roughness or high porosity (r ≫ 1), the absolute value of the right side in Wenzel’s equation could be larger than 1. In this case, the Wenzel model is no longer valid. In 1944, Cassie and Baxter derived an equation describing CA hysteresis for composite smooth surfaces with varying degrees of heterogeneity:17 cos θ* =

Figure 4. Cassie and Wenzel regimes of superhydrophobic states: the apparent CA versus Young’s angle for the hydrophobic surfaces assuming the roughness remains constant. Roughness makes droplet possible to have more than one metastable position of equilibrium, and the drop can transfer from one metastable equilibrium to the other, if the energy barrier can be conquered. The resulting Cassie configuration corresponds to the lowest energy state in the open-air regime, while the Wenzel configuration represents the absolute minimum energy in the wetted hydrophobic state. Reproduced with permission from ref 20. Copyright 2003 Nature Publishing Group.

Roughness makes it possible for a droplet to have more than one metastable position of equilibrium, and the drop can transfer from one metastable equilibrium to the other, provided that the energy barrier can be overcome. The resulting Cassie configuration corresponds to the lowest energy state in the open-air regime, also known as the metastable composite state, while the Wenzel configuration represents the absolute minimum energy in the wetted hydrophobic state. Patankar19found that both Cassie and Wenzel configurations are the local energy minimum, and which energy is lower depends on how the drop formed. If θc* > θw*, the drop forming an openair regime will have higher energy than that of the Wenzel regime. On the basis of the principle of energy barriers, he believed that the transition could be induced between Cassie

∑ fi cos θi

where f i is the fractional area of the surface with a CA of θi and ∑f i = 1. Assuming that the solid surface is composed of substance 1 and substance 2, these two kinds of substances are well distributed in the form of extremely tiny part. The total free energy difference dF can be described as18 dF = f1 (γSL − γSV )1 dx + f2 (γSL − γSV )2 dx + γLV dx cos θ* 8233

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and Wenzel configurations under various disturbances from the environment. Lafuma and Quéré approved irreversible transitions from air to Wenzel regimes of superhydrophobic states.20 These transitions as a function of different roughness parameters are further furnished by Hoffmann et al.21 Marmur also discussed these transitions by putting the Wenzel and Cassie−Baxter equations into proper mathematical thermodynamic perspective.22 By using a molecular dynamic simulation, Zeng et al. recently showed that the two states could coexist on a nanopillared surface.23 The physical parameters can strongly affect the transition, including the height of nanopillars, the space between pillars, the intrinsic CA, and the impinging velocity of water nanodroplets. Here, we must point out that the Wenzel and Cassie’s theories are valid only to these homogeneous surfaces. As early as 1945, Pease discussed that the CA measurement was a onedimensional issue to evaluate wettability.24 McCarthy et al. demonstrated, from the view of experiment, that the Wenzel and Cassie’s theories cannot be applied to the chemically and topographically heterogeneous surface.25 For the heterogeneous surfaces, the CA behaviors can be determined by the interactions of the liquid and the solid at the three-phase contact line not by the contact areas. On the other hand, a superhydrophilic surface is a surface on which the water apparent CA is less than 5° or 10°. We believe that a superhydrophilic surface should have a CA of 0°. A clean glass or freshly cleaved mica surfaces are simply naturally hydrophilic, not superhydrophilic ones, although water seems to spread over them with a CA of a few degrees. A CA of 0° cannot be achieved from a flat homogeneous surface, but can be realized on a heterogeneous surface such as TiO2 surface.26 Moreover, Drelich and Chibowski suggested to refer to surfaces as superhydrophilic surfaces only for structured surfaces (rough and/or porous) possessing a roughness factor defined by a Wenzel equation larger than 1 (r > 1), on which water spreads completely.27 Also, it can be deducted from the Wenzel equation. Superhydrophilic surfaces cannot be obtained without manipulation of surface roughness of hydrophilic materials.

Figure 5. Different states of superhydrophobic surfaces: (a) Wenzel’s state, (b) Cassie’s superhydrophobic state, (c) the “lotus” state (a special case of Cassie’s superhydrophobic state), (d) the transitional superhydrophobic state between Wenzel’s and Cassie’s states (including “petal” state with high adhesion), and (e) the “gecko” state of the PS nanotube surface. The gray shaded area represents the sealed air, whereas the other air pockets are continuous with the atmosphere (open state). Reproduced with permission from ref 28. Copyright 2007 Wiley.

contacts most practical samples (Figure 5d). The CA hysteresis can be determined through measuring the difference between the advancing CA and the receding CA or through directly measuring the SA.30 When the water droplet can be hung, even the surface turns upside down, like a water droplet on a rose petal, this special transition state is named the “petal” state.31 In addition, there is a high-adhesive case of the “gecko” state (Figure 5e) that is different from the Cassie’s state. The origin of the “Gecko” state comes from the superhydrophobic surface of the PS nanotube.32 In the Cassie state, the atmosphere is linked to the trapped air pockets (open state). However, in the “gecko” state, there are two kinds of trapped air-sealed air pockets trapped in the PS nanotubes and open air pockets linked to the atmosphere. The trapped air resulted in a high CA, and the negative pressure from the sealed air in the nanotubes produced the adhesive force. To accurately study the CA hysteresis, a high-sensitivity microelectromechanical balance system was applied to quantify the adhesive force between the water droplets and these novel superhydrophobic surfaces, instead of measurements of the advancing/receding CA or SA. A deeper understanding of the different wetting states is paramount for the further theoretical improvement and rational design of novel superhydrophobic surfaces. Furthermore, the exploration of superhydrophobic surfaces with different states should have a far-reaching impact on both fundamental research and practical applications.

2.3. Different Superhydrophobic States

In general, the superhydrophobic surfaces refer to the surfaces with a static CA larger than 150°. When the CA hysteresis is considered, the superhydrophobic surfaces often show five possible states (Figure 5):28 Wenzel’s state, Cassie’s state, the so-called “lotus” state, the transitional state between Wenzel’s and Cassie’s states (including “petal” state), and the “gecko” state. In Wenzel’s state (Figure 5a), there is a wet-contact mode between water droplets and substrates and the water droplet is pinned on the surface, resulting in a high CA hysteresis. In Cassie’s state (Figure 5b), the water droplets adopt a nonwetcontact mode on solid surfaces and can roll off easily due to air trapped between water droplets and substrates. Here, the SA can reflect the CA hysteresis in Cassie’s state but not Wenzel’s state. The “lotus” state should be considered as a unique case of Cassie’s state (Figure 5c), because a lotus leaf exhibits the selfcleaning effect. Micro- and nanoscaled hierarchical structures on lotus leaves are critically important for the self-cleaning effect. However, single micro or nanoscaled structures often occur with CA hysteresis, even in some cases of a surface in the Cassie state.7,29 Additionally, a transitional state between Wenzel’s and Cassie’s states often exists when a water droplet

2.4. Explanation of Wettability on Water/Oil/Solid Interfaces

Although the Young’s equation was initially applied to study a liquid droplet on a solid surface in air, it has also been applied to a liquid droplet on a solid surface in the presence of another liquid. Through Young’s equation, we could get the following equation:33 cos θ3 =

γl1 − g cos θ1 − γl2 − g cos θ2 γl1 − l2

where γl1−g is the liquid 1/gas interface tension, θ1 is the CA of liquid 1 in air, γl2−g is the liquid 2/gas interface tension, θ2 is the 8234

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Figure 6. Typical biological materials with superwettability and corresponding multiscale structures. (a) Lotus leaves demonstrate low adhesive, superhydrophobic, and self-cleaning properties, due to randomly distributed micropapillae covered by branch-like nanostructures. (b) Rice leaf surfaces possess anisotropic superhydrophobicity arising from the arrangement of lotus-like micropapillae in one-dimensional order. (a and b) Reproduced with permission from ref 7. Copyright 2002 Wiley. (c) Butterfly wings exhibit directional adhesion, superhydrophobicity, structural color, self-cleaning, chemical sensing capability, and fluorescence emission functions due to the multiscale structures. Reproduced with permission from ref 45. Copyright 2007 American Chemical Society. (d) Water strider legs have robust and durable superhydrophobicity arising from directional arrangements of needlelike microsetae with helical nanogrooves. Reproduced with permission from ref 46. Copyright 2004 Nature Publishing Group. (e) Mosquito compound eyes demonstrate superhydrophobic, antifogging, and antireflection functions due to HCP microommatidia covered by HNCP nanonipples. Reproduced with permission from ref 47. Copyright 2007 Wiley. (f) Poplar leaves possess superhydrophobic and antireflection properties originating from dense hairs with the hollow fibrous structure. Reproduced with permission from ref 48. Copyright 2011 The Royal Society of Chemistry. (g) Gecko feet present superhydrophobic, reversible adhesive, and self-cleaning functions due to the aligned microsetae splitting into hundreds of nanospatulae. Reproduced with permission from ref 49. Copyright 2012 The Royal Society of Chemistry. (h) Red rose petals exhibit superhydrophobicity with high adhesion and structural color arising from periodic arrays of micropapillae covered by nanofolds. Reproduced with permission from ref 31. Copyright 2008 American Chemical Society. (i) Salvinia leaves demonstrate the superhydrophobic and air-retention properties due to the Salvinia Effect. Reproduced with permission from ref 50. Copyright 2010 Wiley. (j) Fish scales present drag reduction, superoleophilicity in air, and superoleophobicity in water due to oriented micropapillae covered by nanostructures. Reproduced with permission from ref 33. Copyright 2009 Wiley. (k) Clam shell shows low adhesive superoleophobicity underwater arising from the surface multiscale structures and special chemical composition. Reproduced with permission from ref 51. Copyright 2012 Wiley. (l) Peanut leaves exhibit high adhesive superhydrophobicity and fog capture properties originating from the special surface multiscale structures and chemical composition. Reproduced with permission from ref 52. Copyright 2014 Wiley.

water droplets of different volume from 2 to 5 μL were used to monitor the values of CAs. Importantly, the different fitting modes of the static CA also were used to fit the shapes of water droplets on the surfaces, such as ellipse fitting, circle fitting, tangent searching, and Laplace−Young fitting. These differences always result in different observed values of the CAs from around 150° even to 179° with similar shape.34 For a water droplet of 5 μL, the CA is about 156° by using ellipse fitting; however, the CA can be more than 179° under the Laplace− Young fitting mode. Therefore, the fitting mode should be clearly noted with static CAs to reflect the real situation of solid surface wettability. When comparing the superhydrophobicity of different surfaces, the CA must be measured using a water droplet with the same volume. Because the CA is affected by the volume of water droplets and the gravity force, 2 or 3 μL might be a suitable volume when measuring CA of water droplet on solid surfaces. However, it is very difficult to obtain a water

CA of liquid 2 in air, γl1−l2 is the liquid 1/liquid 2 interface tension, and θ3 is the CA of liquid 1 in liquid 2. In an oil/water/solid system where the rough surface is composed of solid and water, the Cassie model can be written as the following expression: cos θ3′ = f cos θ3 = f − 1

where θ3 is the CA of an oil droplet on a smooth surface in water, θ′3 is the CA of an oil droplet on a rough surface in water, and f is the area fraction of solid. In the water/oil/solid system, the wettability of the solid surface is commonly evaluated by the CA given by the above modified Young’s equation. On the other hand, surface roughness still can be considered by the modified Cassie model. 2.5. Measurements of Surfaces with Superwettability

2.5.1. Static Contact Angle. The static CAs measurement is often used to characterize the superwettability. Generally the 8235

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droplet with volume lower than 4 μL, because of the low water adhesion of many superhydrophobic surfaces.35 To reduce the influence of the deformation of the water droplet caused by the gravity force, Zhang et al.34a proposed the new method to measure the CAs by using a much smaller water droplet. First, a 5 μL water droplet is dropped on a superhydrophobic surface with CA around 154°. The volume of the water droplet reached 0.3 μL with a CA of 173° after vaporization for 40 min under ambient conditions.49 2.5.2. Dynamic Contact Angle. The contact angle hysteresis (CAH) can be used to reflect the different superhydrophobic state. For example, a self-cleaning superhydrophobic surface with Cassie state often shows a high static CA and a low CAH. Thus, a series of methods have been developed to measure the CAH, including advancing/receding angle, tilt angle, CA measurements with surfactant solution, and others. Actually, the difference of advancing/receding angle (θAdv/ θRec) is referred to as the CAH. θAdv/θRec can be measured by slowly pumping water into or sucking water from the water droplets on solid surfaces. The critical CA that causes the change of TCL is defined as θAdv or θRec.36 Another important parameter is the tilt angle (TA) or sliding angle (SA), which refers to the critical angle allowing the water droplet to move when tilting the solid substrate. A superhydrophobic surface with a TA of lower than 10° often indicates that this surface own the self-cleaning property. It must be noted that the TA does not equal the difference between θAdv and θRec. The combination of micro- and nanoroughness is important for the construction of superwetting surfaces with high CA and low TA.37 Moreover, a surfactant solution can be used as a probe to measure CA to reflect the CAH. Adding surfactant molecules to water has been demonstrated to decrease the CAs on some superhydrophobic surfaces with high hysteresis by McCarthy36 as well as Ferrari et al.35 However, for superhydrophobic substrates with low CAH, the CAs are similar no matter whether a pure water droplet or a water droplet containing surfactant is used as a probe. Therefore, this approach can be developed to distinguish between a self-cleaning surface and a normal superhydrophobic surface. Kwoun and co-workers38 introduced a MTSM sensor to differentiate superhydrophobic surfaces with similar CAs but different CAHs (probably different superhydrophobic states) using high-frequency shear acoustic waves generated by a piezoelectric quartz resonator thickness-shear mode (TSM) sensor. Under the TSM measurement in the frequency range of 1−100 MHz, the penetration depth is in the order of tens to thousands of nanometers. For a superhydrophobic surface with a low CAH, the water layer can only wet the top of the surface and is hard to penetrate into the interspaces of the rough structure. Furthermore, the harmonic frequency shift of a low CAH surface is much smaller than that of a high CAH surface. By moving the surfaces to touch a water droplet hanging from a needle, the CAH of a nonplanar surface, for example, gold thread that has a larger radius, can be measured.39 If there is almost no CAH, the water droplet just shifts to one side of the gold thread with a slight shape deformation. In contrast, if there is CAH, the water droplet will adhere to the surface and cannot move. The results indicate that the interaction between water droplets and the gold thread is much weaker than that between water droplets and the needle.

In addition, McCarthy et al. reported a method to distinguish the CAH by lowering the superhydrophobic surface onto a droplet and compressing/releasing several times.40 It has been demonstrated that the surface with low CAH cannot adsorb the water droplet after release; otherwise, the superhydrophobic surface with high CAH has a strong affinity toward the water droplet. The bouncing drop method also proved to be versatile in the characterization of the CAH. When a droplet drops onto a solid surface without wetting it, it will bounce with remarkable elasticity.41 Therefore, the bouncing ability of a droplet on superwetting surfaces can also reflect the CAH.

3. NATURAL EXAMPLES WITH SUPERWETTABILITY After billions of years of evolution, nature creates countless mysterious livings that exhibit charming functions.42 To take wettability as an example, people have disclosed many functional natural interfacial materials (Figure 6),7,42,43 such as the superhydrophobic, low adhesive, and self-cleaning lotus leaf (Figure 6a); the anisotropic superhydrophobic rice leaf (Figure 6b); the superhydrophobic, directional adhesive, structural color, and self-cleaning butterfly wing (Figure 6c); the durable and robust superhydrophobic water strider leg (Figure 6d); the superhydrophobic, antifogging, and antireflection mosquito compound eye (Figure 6e); the superhydrophobic and antireflection poplar leaf (Figure 6f); the superhydrophobic, high adhesive, reversible adhesive, and selfcleaning gecko foot (Figure 6g); the superhydrophobic, high adhesive, and structural color red rose petal (Figure 6h); the superhydrophobic and air-retention Salvinia leaf (Figure 6i); the superoleophilicity in air, superoleophobicity in water, and drag reduction fish scale (Figure 6j); the underwater low adhesive superoleophobic clam shell (Figure 6k); the superhydrophobic, high adhesion peanut leaf with fog capture (Figure 6l); the anisotropic, slippery, and amphiphilic Nepenthes leaf; the directional water collection, superior mechanical, and adhesive spider silk; and the water collection and structural color desert beetle. These findings will bring us unique strategies to design functional interfacial materials with superwettability. 3.1. Lotus Leaf

As a symbol of purity in many cultures, the lotus leaf is famous for its low-adhesive superhydrophobicity and self-cleaning property. In 1997, W. Barthlott and C. Neihuis discovered that the microstructures and wax materials on the lotus leaf surface contribute to its self-cleaning properties.6,44 When a water droplet falls on the lotus leaf, the complementary role of micropapillae and hydrophobic wax keeps the droplet with a CA of about 160°. This droplet can easily roll off the surface immediately taking away the adherent dirt particles, the socalled “lotus effect”. Later, Jiang et al. discovered the synergetic effect of micro/nano hierarchical structures on the superhydrophobic lotus leaf.7 Hierarchical structures in the form of branch-like nanostructures on each papilla are observed. These multiscale structures effectively prevent the underside of pallillae from being wetted. The calculation results from the Koch curve give a theoretical relationship between hierarchical structures and CA, which strongly demonstrate the self-cleaning property originates from the cooperative effect of dual-scaled structures and wax layer on lotus leaves. Inspired by the lotus effect, enormous artificial self-cleaning surfaces have been designed and fabricated by creating 8236

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smart fluid-controllable interfaces used in microfluidic devices, and directional self-cleaning coatings.43a

hierarchical-structured surfaces with superhydrophobic properties. The great progress of scientific research has been facilitating practical applications and benefiting modern industry and daily life, such as antisticking windows, waterresistant textiles, and snow-repellent antennae.

3.4. Water Strider Leg

Water strider is famous for its ability of walking freely on the surface of ponds, marshes, and slow streams. How can the water strider effortlessly stand on the water surface, even rapidly move or jump? Previous investigators thought that the water strider’s leg was hydrophobic and its weight was supported by the surface tension generated by the curvature of the free surface. The hydrodynamic propulsion of the water strider was assumed to rely on momentum transfer by surface waves. This assumption results in Denny’s paradox: the infant water strider, whose legs are too slow to generate waves, should be impossible to propel itself along the water surface. In 2003, Bush et al. resolved this paradox,53 and captured the walking process of different insects on the water surface. Their results indicated that capillary waves do not play a critical role in the propulsion of the strider. The strider transferred momentum to the underlying fluid mainly through hemispherical vortices shed by its hairy driving legs. In 2004, Jiang et al. further revealed that the secret of water strider standing effortlessly on the water surface lies in the huge superhydrophobic force of its legs.46 It is remarkable that the leg does not pierce the water surface until a dimple of ca. 4 mm depth is formed. This robust and durable repellency force from just one leg is enough to support 15 times the total body weight of the water strider. Therefore, the water strider can move freely on the water surface even in a violent storm or torrent. Jiang et al. also found that water strider legs are covered with a great number of oriented, needle-shaped microsetae, arranging at an inclined angle of about 20° from the surface. Each microseta consisted of helical nanogrooves. These unique hierarchical structures can effectively trap air to form an air cushion between the leg and water that endows the legs with robust antiwetting ability. Water striders provide a special example with robust and durable superhydrophobicity despite prolonged contact and dramatic pressure disturbances with water, which provides an avenue for the design of novel aquatic devices.

3.2. Rice Leaf

It is well-known that water droplets can roll freely in all directions on the lotus leaf surface, a typical phenomenon of isotropic superwettability. On the contrary, the rice leaf shows a unique anisotropic wettability.7 Although the hierarchically structured papillae on rice leaves is similar to that of lotus leaves, the arrangement is quasi-one-dimensional order parallel to the leaf edge, but randomly in the perpendicular direction to the leaf edge. Sliding experiments of water droplets impart the different wetting behavior between these two dimensions. Along the direction of the leaf edge, the SA is 3°−5°; perpendicular to the direction of the leaf edge, the value of SA is 9°−15°. The directional arrangement of micropapillae on the rice leaf surface provides a different energy barrier of wetting in these two directions, which can cause a sliding anisotropy tendency on the surface. This research offers an important avenue to control anisotropic wetting on an artificial solid surface. 3.3. Butterfly Wing

The wing of butterfly is another example that displays the anisotropic wetting. When a butterfly slightly waves its wings, water droplets roll off the surfaces along the radial outward (RO) direction of the central axis of the body. Recently, Jiang’s group demonstrated that the surface of butterfly (Morpho aega) wings exhibits superhydrophobicity with directional adhesive properties.45 Butterfly wings are covered by a large number of quadrate scales, which overlap each other to form a periodic hierarchy along the RO direction. Each scale is composed of well-oriented nanostripes, which are stacked stepwise by tilted periodic lamellae along the RO direction. Interestingly, the nanotips on the top of stripes tilt slightly upward. These highly directional structures strongly affect the wetting behaviors of water droplets. As a result, a water droplet can easily roll off the wing surface along the RO direction, but the droplet is tightly pinned in the opposite direction. These two inherent states can be adjusted through the control of the wing posture or the direction of airflow across the surface. It suggests that these two distinct contact states could coexist on the orientation-tunable surface and thus form contrasting adhesion. When the wing is tilted downward, the microscale with oriented nanostripes can be spatially separated from each other. In this case, air is efficiently trapped between the wing and the water droplet. Water droplets on the wing not only present a “composite” contact state with the surface but form a discontinuous solid−liquid− gas three-phase contact line (TCL), resulting in the droplet easily rolling off the surface. When the wing is tilted upward, the flexible nanotip and microscales take a close arrangement, which result in the existence of little air between the wing and the water droplet. A quasi-continuous TCL is formed on the wing, demonstrating the pinning state of water droplets on the wing surface. This directional adhesion is crucial for biological materials, which endows the butterfly wings with the ability of directional easy-cleaning in a watery environment. This should contribute to its stability of flight through avoiding the accumulation of dirt particles. The anisotropic wettability on the butterfly wing provides new inspiration for the design of

3.5. Mosquito Compound Eye

Mosquitoes are well-known for their excellent vision even in a dim and watery habitat. Recently, Yang and Jiang et al. revealed the secret of antifogging property of the mosquito compound eyes.47 The single mosquito compound eye is composed of hexagonally close-packed (hcp) ommatidia, on which hexagonally nonclose-packed (hcp) nipples are elaborately decorated. It suggests that this unique multiscale structure renders the compound eyes with excellent superhydrophobicity. When exposing a mosquito to moisture, it was found that the tiny fog drops could not locate on the compound eye surface. At the same time, the surrounding hairs of mosquito were easily nucleated on a large number of drops. Therefore, the special superhydrophobicity prevents fog drops from condensing on the eye surface, and mosquitoes can keep clear vision even in a humid environment by dry and clear eyes. This finding will provide a novel prospect for the design and development of dry-type antifogging optical devices. 3.6. Poplar Leaf

Normally, the poplar tree looks green. However, sometimes in the summer, the poplar tree shows a white cast. Recently, Song et al. reported that the underside of the poplar leaf possesses 8237

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both superhydrophobicity and high reflection.48 The lower surface of poplar leaves is covered by ribbon-like hairs with hollow fibrous structures. In comparison, the upper surface of poplar leaves is almost glabrous. The reflectance of the lower surface was higher than 55% for the wavelength range of 420− 900 nm. The white color for the poplar leaf underside can be attributed to the broad high reflectance in the whole visible wavelength range. For the upper surface, the reflectance is low, which is below 10% in the visible light range. Therefore, the hair layer played an important role in the high reflectance of the lower surface. The ribbon-like hair consisting of hollow fibers endowed the poplar leaf with an energy efficient “cool roof” to protect it from being burned by strong sunlight in the summer. Inspired by poplar leaf hairs, a highly reflective white coating with superhydrophobicity has been fabricated using the coaxial electrospinning technique, which could be used as an effective “cool roof” in modern buildings.

tinier nanorods with periodic pattern on the nanofolders.57 The cooperation of chemical color arising from pigments and structural color originating from the nanostructures endows flowers with bright color and special functions for human and animals’ visual system. The peanut is a typical plant living in the arid and semiarid regions. Recently, Liu et al. found the peanut leaf exhibits superhydrophobicity and high adhesion toward water, due to the generation of discontinuous and quasicontinuous at the nanoscale and microscale, respectively.52 Furthermore, the superwettability and surface multiscale structures endow the peanut leaf with efficient fog capture. Inspired by the peanut leaf, multifunctional copper surfaces with structural similarity to the natural peanut leaf were fabricated, showing high adhesive superhydrophobicity. 3.9. Salvinia Leaf

Salvinia, a kind of floating plants, possesses the inherent nature of trapping air film underwater for a long time (days to months), to prevent itself from being wetted and submersed.50 Barthlott et al. revealed its air-retention property. The S. molesta surface is densely covered by complex hairs. The terminal ends are connected, forming an eggbeater-shaped structure. Moreover, excluding these terminal cells, the whole leaf is covered with nano wax crystals. This hairy surface shows superhydrophobicity with a bead shape droplet sitting on the surface. Yet surprisingly, the terminal cells are wetted, and the shape of the meniscus of the droplet also indicates the terminal cells are hydrophilic, while otherwise parts are superhydrophobic. Hydrophilic patches prevent the rupture of the contact, while hydrophobic hairs can resist the water approaching the plant’s surface. When lifted from water, the leaf is dry. Therefore, the leaf could trap an air layer for several weeks, which is limited by the lifetime of the leaf. This “Salvinia effect”, that is, paradoxical wetted surface pinning the air−water interface, opens an avenue for the fabrication of artificial surfaces with long-term airretaining properties, possessing potential in fields of drag reducing ship and low friction fluid transport coatings.

3.7. Gecko Foot

The gecko foot is well-known as one of the most effective adhesives in nature, revealed as early as fourth century B.C.54 The gecko foot consisted of well-aligned microhairs called setae (about 5 μm in diameter and 110 μm in length), which are split into hundreds of smaller nanoscale ends called spatulae.55 The van der Waals forces generated by the contact between the gecko spatulae and the solid surface could supply geckos to climb vertical walls or across ceilings. Besides the well-known reversible adhesive, gecko feet with multiscale structures also exhibit self-cleaning and superhydrophobicity with high adhesive forces to water. In 2002, Autumn et al. were the first to discover that gecko setae are superhydrophobic and the water CA of gecko setae is about 160°.54 Liu et al. further investigated the surface wettability of gecko feet and measured the liquid−solid adhesive forces between water droplets and superhydrophobic gecko setae.49 The adhesive force of gecko feet toward water droplets was in the range from 10 to 60 μN. The wide range of adhesive forces could be attributed to the complex contact condition between water droplets and setae and the conformational changes in the surface proteins of gecko setae when exposed to water. Gecko feet provide an inspiration for the scientists and engineers to construct multifunctional materials with on-demand adhesive and self-cleaning properties.

3.10. Fish Scale

Fish scales exhibit a self-cleaning property underwater, in addition to the drag-reducing function.33 Jiang et al. revealed that fish scales (Crucian Carp, Carassius carassius) have superoleophilicity in air and superoleophobicity underwater.33 Fish scales are usually covered by a thin layer of mucus, which results in their hydrophilic nature. The fan-shaped fish scales with diameters of 4−5 mm are densely arranged. There are oriented micropapillae on each fish scale. Each micropapillea has a length of 100−300 μm and a width of 30−40 μm. Nanoscale roughness can be observed on the surface of micropapillae. These multiscale structures could trap water and form a composite interface on fish scales to resist oil, which might play an important role in the oil wetting reversion. This fascinating phenomenon should have potential for many applications in an oil/water/solid system, such as marine antifouling, prevention of oil spills, microfluidic technology, and bioadhesion.43b,58 Recently, the clam shell was disclosed to possess underwater low adhesive superoleophobicity.51 This unique underwater superwettability can be attributed to the cooperation of surface multiscale structures and high energy chemical composition of CaCO3. Inspired by the clam shell, high-energy CuO coatings with underwater superoleophobicity were prepared using the oxygen-adsorption corrosion approach in aqueous ammonia. Furthermore, the oil adhesion underwater can be adjusted from

3.8. Red Rose Petal

Unlike the self-cleaning lotus leaves, red rose petals usually keep the spherical water droplet sticking on their surfaces. Small water droplets prefer to pin on the surface maintaining the fresh look, while the bigger ones such as raindrops roll off. To illustrate the origin of this high adhesion, Feng et al. investigated the microstructures of the red rose petal.31 The surface of a red rose petal consisted of an array of micropapillae, and at the top of each micropapillae there are many nanoscale creases. This superhydrophobic surface with high solid−liquid hysteresis could be illustrated by a Cassie impregnating wetting state, that is, water penetrating into the micropapillae, but air gaps retaining in the nanoscale folds. Bhushan et al. further selected two kinds of superhydrophobic rose petals to study the mechanism for adhesion characteristics.56 The understanding of the red rose petal would aid in fabricating artificial films with high adhesive superhydrophobicity. These high adhesive superhydrophobic surfaces provide an effective solution to transport small volumes of liquids in microfluidic devices. In addition to the superhydrophobicity and high water adhesion, red rose petals also exhibited structural color arising from the 8238

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physical methods

8239

template methods

phase separation

laser treatment

plasma treatment

method

fast and low cost, but too complicated to form large-scale surfaces

low cost and ease of production, but difficult to control

simple, fast, and easily tuned, but needed special equipment

advantage and disadvantage

Si(100)/quartz APFB/PI LDPE cellulose HFB C6H6/PP TFE PBD MDS PTFE/PFA

CF4 plasma plasma polymerization plasma plasma pulsed plasma RF plasma plasma deposition plasma fluorination plasma polymerization plasma deposition

PODR1 PDMS copoly(imide siloxane) steel Teflon PTFE steel/Ti-6Al-4 V alloys/ FOTS PDMS

laser laser ablation laser ablation femtosecond laser pulsed UV laser ultrashort pulse laser

i-PP LDPE PVC styrene/HFBM PS PC PDMS

mixed solvents crystallization solvent coating phase separation phase separation phase separation template

ultrafast laser

steel

laser ablation

CF4 plasma

PET PEFT PBD

plasma deposition

materials PTFE PTFE PBD

oxygen plasma oxygen plasma plasma fluorination

specific technology

160° 155°

152°

154.3°

173.08° 155.3°

160°

>170°

152°

140°

166.3°

157° 175°

157°

150°

150° 160° >150°

168° 166.7° >160° 167° 150−165°

θA/θR = 174.8°/ 135.8° 159.8° 170°

θA/θR = 172°/169°

170° 152° 175°

water CA

the microfeature induced surface roughness stable transparent superhydrophobic surfaces with controlled roughness the deposition of well-defined polymeric nanospheres the nanometer and micrometer scale roughness the rough film with a lot of particles the structure of the reagent hydrocarbon combined high fluorination degree and surface texture roughness ribbon structures and PTFE crystalline chemistry and nanoscopic roughness of the films low surface energy of PTFE and textured with different scale structures microgeometrical structure and chemistry of the surface surface microstructuring and fluorosilane that are used to lower the surface energy micro- and nanoshapes like lotus leaves the controllable surfaces’ topography and the low surface energy components laser-induced periodic surface structures on the submicrometer level the cotton-like microstructure and Teflon inherently water repellent property micro/nano roughness with an inherent low-surface energy coating material rough PDMS surface in micro-/nanoscales that imitate a lotus leaf gel-like porous coating, a simple and inexpensive method porous micro-/nanostructures of LDPE crystals diffusion, tension break, and micro-/nanophase separation the surface morphology of the films similar to that of the lotus leaf well replicated the micro-/nanostructures of the lotus leaf PC coatings present various surface morphologies honeycomb-like surface with hierarchical micro-/ nanostructures

the fluorinated species and surface roughness

the modification of surface chemistry and roughness plasma chemical roughening of PTFE substrates a combination of attack by atomic fluorine and surface roughening the shape and continuity of the three-phase (vapor/ liquid/solid) contact line

characteristics

Table 1. Summary of Fabrication Methods of Superhydrophobic Surfaces Including Physical, Chemical, and the Combination of Physical and Chemical Methods ref

94 95

93

92

90 91

89

88

87

86

85

83 84

82

81

78 79 80

73 74 75 76 77

71 72

67

69

66 67 68

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a simple and convenient technology, but limited to small areas

simple, high throughput, and applicable for a variety of substrates and not limited to small areas, but hard to finetune surface morphology

versatile, effective, and applied to a variety of materials, but needed special equipments

spray coating

electrohydrodynamics/ electrospinning

advantage and disadvantage

spin coating

method

Table 1. continued

8240

Teflon AF/PCL PTFEMA

PVA/PCL/PAN/ /PVdFHFP Teflon AF PVDF/FSM

electrospinning electrospinning

electrospinning

electrospinning

electrospinning

POSS/PMMA

electrospinning

ZnO/PMC

electrospray

PS PS/PVC/TiO2 PVA/ZnO

polymeric material

spray deposition

electrohydrodynamic electrospinning electrospinning

TiO2/PS/PFOA CNT/PS

PFDA/boehmite PS PPS/H-SiO2

spin coating one-step casting spray

spray spray casting

CuI

spin coating

metal alkylcarboxylate

CaCO3/SA

spin coating

spray

163°

PMMA-SiO2/slide

spin coating

170.2°

151°

158°

154°

158°

163°

162.1° 178° 165°

168°

178°

166° 160°

160°

>150° 166° 158°

157.3°

152.8°

154° 162° >150°

Pt/Ti/Si Al PS/Al PS/Al

water CA 161° 152.8° >150° 158° 170° 173.8°

AAO template AAO template template-wetting template-wetting

materials paper/PTFE CaCO3 /PSS PDMS paraffin fluorosilane PAN

specific technology template template template AAO template AAO template AAO template

characteristics

nanosized fiber diameter mats, and low surface energies of the Teflon AF nanobeads, with high aspect ratio, were formed on the rough surface

a lotus-leaf like structure lotus seedpod like structure the replicated papillary microstructures micro-/nanoscopic range of roughnesses the solid fraction adapts on an arbitrary rough surface nanostructure of the nanofibers and their lower density both the micro-/nanometer scale the fabricated double-roughened surfaces traditional superhydrophobic and strong adhesion high adhesion, different van der Waals forces, and different negative pressures produced rough two-dimensional hierarchical lotus leaf-like structure the micro-/nanosized structure, together with modification of stearic acid two length-scale roughnesses and the nature of the material itself nanoparticles coated by perfluorodecanoic acid unitary microscale structure the micro-/nano structure and the hydrophic SiO2 nanoparticles the sprayed coating with the flowerlike hierarchical structures perfluotooctanoic acid modified coatin the porous CNT-network structure and the PS grafted on the MWCNTs surfaces a rough structure similar to that of a lotus leaf, which features a high water CA a low surface energy polymer, and compatibilizing nanoparticles porous microspheres increasing surface roughness nanoscale inclusion and surface porosity/roughness the high surface roughness of the fibrous films and the hydrophobic FAS modification re-entrant surface curvature and chemical composition macroroughness (spacing between fibers) and microroughness (striation of individual fibers) superoleophobic property of the PTFEMA webs depends on the fiber diameter and the gap distance between the fibers the fluorocarbon, and pores in nanofibre membranes

ref

124

123

122

121

120

119

116 117 118

115

114

112 113

111

108 109 110

107

106

105

101 102 103 104

96 97 98 99 16 100

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chemical methods

chemical reaction

ion assisted deposition

method

Table 1. continued

convenient and easy to manipulate, but somewhat hazardous and not an environment−friendly method

the roughness can be easily controlled by varying the voltage, but limited in small scale

advantage and disadvantage

PS PDLL/PLL PTFE TiO2/PTS

electrospinning electrospinning ion-plated coatings ion assisted deposition

water CA

8241

162°

163.2° 158°

PE/silica NPs Ag/silicon Ag

PMMA/SA Al/SA ODA-GO BPUR/PTFE

silver mirror reaction silver mirror reaction

aminolysis reaction interface reaction the nucleophilic substitution reaction cross-linking reaction

172°

BMA/EDMA SiO2 PE

in situ polymerization catalytic ethylene polymerization

169°

157°

155° 159.2°

Co3O4 ZnO/CPPs

solid-state reactions ligand generating reaction in situ polymerization

160°

154°

174°

169°

Al/boehmite

152.2°

153°

159° 161° 150°

154.2°

154°

154.7°

161.2°

167° 155°

154.5°

solution-phase synthesis synthesized reaction

Pb

TPU

electrospinning

one-pot reaction

PS

electrospinning

Cu/SA

PVDF/SiO2 NPs

electrospinning

one-step reaction

PS PVDF

electrospinning electrospinning

materials TiO2/silicon oil

specific technology electrospinning

characteristics the combination of low surface free energy and high surface roughness the surface morphology the high surface area of the formed fibers that ranges from nanometer to submicrometer scale nanosized particles irregularly inlayed in the surface of the microsized PVDF mini-islands to form a dual-scale structure the property of composite film can be controlled by altering the mass ratio of bead-on-string fibers/ microsized fibers the increase in roughness of the TPU film as well as the low surface energy of the nanosilica the regular nanostructural protuberant morphology and the microstructural surface roughness beadlike morphologies were the most adequate nanometer-size surface roughness the photo catalytic degradation of the accumulated pollutants roughening a surface and lowering their surface energy dodecanethiolate surface morphology with packed aggregate “rice-like” particles of micrometer scale in thickness the dual scale roughness and the low surface energy of stearic acid coating the combined effect of hydrophobicity of synthesized polyethylene and microstructures the micronanostructured silver clusters were fabricated via the seed-induced silver mirror reaction the combined effects of chemical composition through the presence of air at the interface and high surface roughness rough surface morphology and the hydrophobic carbon chains the complex micro-/nanoscale binary structures along with the low surface energy the long hydrocarbon chain in ODA reduces the surface energy of the GO sheet an irregular rough structure with dispersed grooves and protuberances hierarchical micro- and nanostructures ZnO particles maintain the morphology and superhydrophobicity of the precursor CPPs both surface chemistry as well as the length scale of surface roughness the superhydrophobic film with a microscale and nanoscale hierarchical structure special surface structures containing micrometersized islands, submicrometer particles

ref

149

148

147

145 146

144

143

142

141

140

139

138

137

136

135

132 133 134

131

130

129

128

126 127

125

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solvothermal method

sol−gel methods

method

Table 1. continued

a well-known and widely used method, but poor controllability

can form transparent superhydrophobic surfaces and easily tuned, but poor controllability

advantage and disadvantage

specific technology

154.8°

TEOS/silica/glass PDMS/DTS/SiO2 metal alkoxides AIP/EAA HDTMS/silica silica Al/Zn(NO3)2/C6H12N4 silica/TMCS/HMDZ TiO2/OTS TEOS/MTES PES

sol−gel sol−gel sol−gel sol−gel sol−gel sol−gel sol−gel sol−gel sol−gel sol−gel sol−gel

8242

hydrothermal method

hydrothermal hydrothermal hydrothermal hydrothermal hydrothermal

method method method method method

157° 168° >151° 164°

MTMS MTEOS supraTES HMDS/PPG/PEG/PVP silica/RF silica TEOS/RF C/SiO2

161° 154° 156° 154°

150°

ZnO TiO2 ZnO/TiOPc/FAS SnO2 ZnO/SiO2 K2CO3/Fe3O4 /VTE-silane

154°

150°

166°

162°

160.8° 158.7°

θA/θR = 165°/115° 173° 156° >150° 156.8° >150°

sol−gel sol−gel sol−gel sol−gel sol−gel sol−gel sol gel sol−gel

sol−gel

165° 154°

Al2O3/TiO2/ FAS perfluoroalkyl chain-containing organogelators CTMS/TFCS

165° 150°

152.3°

PS-block-PHFMA Al2O3 PET/Al2O3

150°

HFBA/Cu/TMS

water CA 152.2°

materials PE/MMT

sol−gel sol−gel

ziegler-Natta polymerization surface graft polymerization atom transfer radical polymerization sol−gel sol−gel

characteristics

a wide variation in the pore and particle sizes porous materials roughnes an intrinsically phase-separated structure the surface roughness of the films the surface roughness is about 20 nm silica nanoparticles enhance the surface roughness the hydrophobic groups bonded onto the rough films the superhydrophobic property of 5-layer films and low surface energy controlled surface roughness was obtained layers of SiO2 particles, concentrations of DTS solution, and surface topography surface adjustable roughness and composition a roughness structure on two levels the morphology and surface roughness the porous structure with pore sizes typically ranging from 200 nm to 1.3 μm the special flowerlike and porous architecture, along with the low surface energy a crater-like morphology with each bowl-shaped hole and the TMCS-modified film the thin films with a hierarchical structure the micronano dual size structure and the low energy surface of −CH3 group the hydrophobic groups and the surface roughness structure stimuli-responsive surfaces with nanostructure as-prepared multiscale rough surface microstructures surface roughness, the orientation of the nanorods a micronano composite structure is beneficial to hydrophobicity 3D flower-like micro-/nanoflakes and chemical modification with vinyl tirethoxy-silane

the film made from nanocomposite without any modification combining both low surface energy and rough structure introducing fluorine into the copolymer and the grain-like structure combination of geometric and chemical approaches a small roughness of about 20−50 nm formed on the Al2O3 gel films enhanced by a fine roughness fibrous aggregates that provide nanometer-scale roughness on the surface nanoclusters and nanoparticles

ref

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176 177 178 179 180

175

173 174

172

43m

168 169 170 171

166 167

158 159 160 161 162 163 164 165

157

155 156

153 154

152

151

150

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electrochemical methods

method

Table 1. continued

controlled, less hazardous, and more environmentally safe, but energy consumption

advantage and disadvantage

θA/θR = 151.2/154.7 153° 152° 154.5° 158° 152.0° 153.4° 157° 161° 165° >150° >150° 166.9° >150°

Cu PTh HSCMs/ITO Ag/Ni/Cu PFA/Zn−Fe ZnO Cu

8243

polyETH8/Cu Cu Cu Au Al/POMA ZnO/TiO2 steel/FAS-17

PEDOT

155°

150°

Ag Al/Cu

167°

ZnO/Zn

the aspect ratios of ZnO nanorods and the density of their surface hydroxyls Ag nanoparticles have micronanoscale binary structures a hierarchical structure with nanometer pillars and micrometer clusters low free energy fluorinated chains and a rough morphology

binary microstructures at both micro- and nanoscale

the thin layer of air surrounding the superhydrophobic coating beneath the water a CHS/POAAA reservoir incorporated in the micro roughened alumina surface region wurzite crystal structure and fluorination coating

dendritic and fractal growth on copper base surface

rough surfaces that were like low-order fractals

the formation of fractal surfaces

special micro- and nanohierarchical structure

hierarchical spherical microstructures with a selfassembled monolayer of n-dodecanethiol modifying with n-dodecanethiol and the formation of Ag dendrites special surface compositions and microscopic structures the surface aligned nanostructure

with inborn superhydrophic effect cobalt hierarchical structure the morphology and surface roughness of PTh film

160°

154°

Al

AAO

ITO/n-dodecanethiol

168° 170°

ZnO ZnO/Teflon AF

electrochemical approach electrochemical approach electrochemical approach electrochemical approach electrochemical approach electrochemical approach electrochemical approach electrochemical approach electrochemical deposition electrochemical deposition electrochemical deposition electrochemical deposition electrochemical deposition electrochemical deposition electrochemically deposited electrochemical deposition electrochemical deposition electrochemical deposition electrochemical deposition electrochemical deposition electrochemical deposition

a “rose” like ZnO crystal surface structure a lotus leaves-like ZnO nanoforest surface modification hierarchical flowerlike gold microstructures and chemisorption of n-dodecanethiol the “bird’s nest”-like surface structure is crucial for a superhydrophilic AAO film the multiscale micro-/nanostructures surface

156.3° 154°

characteristics the increase of the surface roughness and reducing of the surface energy ZnO films with well-aligned hierarchical structures surface with rose-like microstructure

ZnO/silica MOR

water CA 151°

hydrothermal method hydrothermal synthesis alkaline hydrothermal alkaline hydrothermal

materials Mg/FAS

specific technology hydrothermal method

ref

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206

205

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203

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201

200

199

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195

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185 186

183 184

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combined the advantages of both electrochemical and LBL methods

can be used to fabricate superhydrophobic surfaces on nonplanar surfaces, but time-consuming

layer-by-layer

advantage and disadvantage

electrochemical deposition combined LBL

method

Table 1. continued specific technology

8244

PVDMA/PEI PAH/PAA/glass/SiO2/ TiO2/SSFS SiO2/silicon

layer-by-layer layer-by-layer layer-by-layer

SiO2/SR-SiO2 FAS/PS/silicon

layer-by-layer layer-by-layer

PVDMA/PEI silica NPs

layer-by-layer layer-by-layer

157°

>150° 152.4°

150° 160°

155° 150°

>150° 172°

160°

Cu−Sn (90−10) alloy PAH/PAA AA/PEI, PAA/PEI-Ag

>150°

Cu/steel/HFTHTMS

172°

PECA

160°

SiO2

165°

166°

PTFE

Au

154°

Cu/HAuCl4

173° 154° 134°

162° 154°

Cu/BA Cu/Ag

Au/ITO Ag Pth/BFEE

porous structures are composed of a large number of nanoparticles aggregate dual-scale hierarchical roughness and heterogeneities

157°

Cu/[Ag(NH3)2]OH

honeycomb-like polyelectrolyte distinguishable hierarchical micro- and nanostructures micro- and nanoscale surface features a hierarchical integration of nanoscale textures and highly fluorinated surfaces dual-size surface roughness the bioinspired combination of low surface energy materials and hierarchical surface structures micro- and nanoscale surface features desired surface roughness on the polyelectrolyte bilayers a LbL-assembled PDDA-silicate/PAA film and CVD of a layer of fluoroalkylsilane

low surface energy materials with high surface roughness the dendritic structure of gold clusters branchlike Ag micro-/nanostructure nearly three-quarters of the total surface area is trapped with air various gold micronano-structures contributed to the very large fraction of air a micrometer-sized pyramid structure consisting of accumulated droplets and nanofibres surface roughness and the surfaces modified by HFTHTMS micro- and nanotextured surfaces

the formation of unprecedented well-defined silver micrometer flower-like structures composed of nanoplates a rough surface with the low surface energy coating the morphology of the Ag layer

156.8°

SA/Cu/Ag

the superoleophobic polymer electrodeposited on micropatterned substrates made of cylindrical arrays the fabrication of biomimetic hierarchical structure with nanometer pillars and micrometer clusters the fractal-like morphology of the silver films

unique textured surface

153°

153°

PEDOP/silicon

characteristics mesogenic segments increases the surface roughness

PATP/Al

167.9°

ZnO

water CA 154°

materials PPR

one-step electroless plating electrochemical reaction layer-by-layer layer-by-layer

electrochemical polymerization galvanic exchange reaction electrostatic

galvanic reactions electroless displacement electroless replacement deposition pulse electron deposition electrophoretic deposition

electrochemical polymerization galvanic exchange reaction galvanic exchange reaction

electrochemical deposition electrochemical deposition electro polymerization

ref

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228 232

230 231

228 229

226 227

225

224

223

222

219 220 221

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to realize the target product with a single or very few reaction steps, but only sutiable to a few materials

solution-immersion Process

vapor deposition

convenient and ease of production, but low durability

advantage and disadvantage

self-assembly method

method

Table 1. continued specific technology

materials

8245

one-step vacuum evaporation

>150°

170°

162°

PTFE/aligned carbon nanotube PFA

Al/FAS-17

solution-immersion

157°

161°

Al/PDES

solution-immersion

161°

DLC

Mg Al/HCl/FA

solution-immersion solution-immersion

151°

150°

Zn/silicon/steel/PFTS

solution-immersion

155° >150°

TMMOS/Si/PMMA

CuO Cu/FOTMS

solution-immersion solution-immersion

162°

161.7° >150°

CuO

solution-immersion

166.7°

157°

162.7°

162° 160.5° 146°

CNTs TMMOS

CuO@Cu2S/Cu

solution-immersion

CuCH3(CH2)12(COO)2 PP−PMMA HPEFs/HPUFs

151°

PBA−PAA

169° 163.7° 176°

>150°

163°

solution-immersion

plasma enhanced CVD microwave plasma CVD microwave plasma-enhanced CVD wave-excited plasma CVD HF CVD

water CA 155°

Au/C16S

Cu/Cu2(OH)3NO3/ PFDTES PM/cotton/paper

solution-immersion

self-assembled monolayer template-assisted selfassembly solution-immersion solution-immersion solution-immersion

Ag/ODT

self-assembly self-assembly self-assembly PFDTS

DPA/FA Cu/DA

polyDADMAC/POTS

self-assembly self-assembly

multilayer deposition

characteristics

ref

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−CF2− groups at the surface lead to this different surface activity

259

258

256 257

255

254

252 253

251

249 250

248

247

246

245

242 243 244

241

240

237 238 239

235 236

234

nanoscale roughness inherent

the surface morphology affinity to needle-like shape

uniform nanoscale roughness protuberances with inherent microscale roughness characteristics micro- and nanoscaled 3D hierarchical structures and modification with C7F15COOH the microflowers transfixed by several nanorods and chemically modified with fluoroalkylsilane a new hierarchical morphology of CuO The special hierarchical structure along with the low surface energy surface structure composed of multilayers of uniformly dispersed particles and the surface modification a honeycomb-like layer and the surface modification the cooperation of rough structure and hydrophobic tail of Al(CH3(CH2)12COO)3 width of the nanoflakes ranges from 20 to 500 nm, and chemical modification with perfluorodecyltriethoxysilane the creation of surface roughness and the presence of the low-surface-energy fluorinated components pillar formed nanostructure in the films control of the surface nanotextures of the deposited films consisting of stacked nanoparticles

roughness of colloidal-crystal films and chemical composition of the latex spheres a special flowerlike structure composed of nanosheets a lot of nanoscale single molecular micelle spheres the very low surface free energies of HPEFs/HPUFs and the woven structure of the cotton microfibers the morphologies and chemical compositions

multilayer deposition, followed by a fluorination treatment nanoscale roughness and low surface free energy a nanoneedle structure copper surface and surface self-assembly with dodecanoic acid a feather-like surface structure microsized pores and arrayed nanofibers or nanorods the role of PFDTS is to lower the surface energy and the essential roughness surfaces micropatterned with aggregates of half-shells

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etching methods

method

Table 1. continued

widely used in surface processing, but needed special equipments

advantage and disadvantage

8246

Al Cu silicon Mg/SA Al/steel SA/steel/Al silicon

chemical etching chemical etching chemical etching chemical etching chemical etching chemical etching

Al/CAM silicon/Cu

chemical etching

fluoroalkylsilane/silicon

femtosecond laser etching electrolytic etching copper catalytic etching

PTFE, silicon surfaces

ODT/Au PDMS

oxygen plasma etching laser etching

photolithography

158.9° 149°

Cu−Zn ATP/Cu silicon glass/FAS

etching etching etching plasma etching

SiOC/PTFE BDD NG/OTS/PFTS

160°

ZnO/Al BMG/FAS

dry etching ion etching

151° >150°

carbon

PFOTS cBN

160.7°

waxes ZnO/glass

thermal evaporation plasma assisted thermal vapor deposition vapor phase transport process noncatalytic CVD etching

ion track etching ion etching

163° 157°

PFA

vacuum evaporation

water CA

>160°

161°

160°

>150° 154°

151°

θA/θR = 176°/156° 150.8°

>160° 160°

161° >150°

170° 162°

161° 155° >173° >135°

>150°

170.9

materials n-hexatriacontane/silicon wafers ⟨100⟩

specific technology physical vapor deposition

characteristics the low surface energy of n-hexatriacontane together with the randomly distributed micro- and nanoscale roughness features −CF2− groups at the surface lead to this different surface activity fabricated structures and surface chemistry nanostructured ZnO without any additional chemical modi cation a lotus-like structrure with micronano hierarchical papillae lotus-leaf-like ZnO micronanostructure films BMG surfaces with binary micronano-scale hierarchical structures coated with FAS film a flower-like hierarchical structure gain rough surfaces by chemical etching uniform formation of double roughness micronetwork of nanopillars and fluoroalkylsilane self-assembled monolayers size of nanostructures the special micro-submicro-nano structures significantly enhance the surface roughness surface morphology that exhibits structure on the micro- and nanoscale micro- and nanosized complex morphologies the hierarchically rough silicon surfaces and chemically modified of Teflon precursor with low surface energy a single-scale roughness of nanometric size surface nanostructuring together with surface fluorination the aspect ratio of the pillars can be controlled substrates with different nanograss densities and chemical functionalization surface roughness, nanometer-size surface asperities, and silanization chemistry a binary structure consisting of microscale crater-like pits and nanoscale reticula the surface roughness is crucial to creating a superhydrophobic surface the formation of a hierarchical structure the flower-like structure and the bonding of the CH3(CH(2))16COOH hierarchical alveolate structures in nano- to microscale synergistic binary structures at micro- and nanometer scales and stearic acid polymer replicas of the surface of leaves were fabricated the fluorinated alkylsilane films

ref

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287

286

284 285

283

282

281

279 280

277 278

275 276

274

272 273

268 269 270 271

266 267

265

263 264

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uncommon methods

bottom up approach

method

Table 1. continued

most for special materials and special requirements

widely applied in the building of larger, more composite structure materials

advantage and disadvantage

continuous film with greatly enhanced roughness high aspect ratio tapered microstructures with nanoporous sidewalls the uneven surface with high roughness and the generated C−F chains a lotus-leaf-like surface structure was clearly observed ribbon-like randomly distributed double-scale structure and the lowest surface energy hydrophobic groups both micrometer and nanometer structure toughness surface chemical modification

159° 174° 170° 154° 153.1° 165° 160° >148°

165° 162° 173°

Cu−Zn

SnO2/titanium substrate PDMS/raspberry-like silica particles/epoxy films BEF fluoro-acrylic resin/colloidal silica silica/FS AKD PTFE LDPE PVA/Cabon NFs Al2O3/HDFTMS/ EPPTMS SU8 SiO2-NWs/ZnS PVC PEEK/PTFE

steel alkylchlorosi lanes/silica AKD

one-step O2 concentration-dependent etching bottom up approach bottom up approach

bottom up technology melting solidification RF magnetron sputtering stretching-controlled micromolding wetting-compatible method electrostatic powder spraying process a backside exposure method thermal evaporation nanocasting method curing

8247

163.2°

PTFE carbon nanofiber

thermal pyrolysis

>150°

155°

OTS/silicon

157° 161°

158.4°

>160° 155°

coating-curing process surface silylation method supercritical CO2solution process aluminum-induced crystallization technique electron irradiation

bottom up approach

155.3° 165°

SDBS/HCl/Al/FDTES CTAB/FDTES/Cu

SDBS/HCl etching HNO3 etching

153°

a rough surface structure with micrometer-sized pores stable in the whole pH range

the average AKD particle size after RESS processing was between 1 and 2 μm introducing OTS SAMs on the silicon micro-/ nanotextured surfaces

a brightness enhancement property by combining roughness dimensions surface functionalization and air entrapped the fractal dimension of the AKD surface distinctly different chemical composition and/or morphology micropapillas much higher than those on the lotus leaf nanostructure on the surface of carbon nanofibers

SnO2 flowers with nanoporous petals dual-size surface roughness that mimics the two-level surface structure of lotus leaves

161°

Cu

154.8°

chemical etching

characteristics the random nanostructure in addition to the regular microstructure binary structure consisting of the irregular microscale plateaus and caves the random nanostructure in addition to the regular microstructure nanomicro mixed structure dense and spherical micropits appeared on copper wafer a flower-like hierarchical structure due to the accelerated alloy etching.

Al

water CA 153°

chemical etching

materials Cu

specific technology chemical etching

ref

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310

309

308

306 307

304 305

303

302

301

300

299

296 297 298

295

293 294

268

291 292

289

290

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spindle-knots, resulting in the coalescence to form larger water droplets. This makes the joints free to perform a new cycle of water condensation and collection. They also found that the special structure of the silk fibers is pivotal for the water collection ability. Aligned nanofibrils on the joints and randomly arranged nanofibrils on spindle-knots produce a different roughness on the silk fibers, resulting in the surface energy gradient that could drive water toward the spindleknots. Furthermore, this effect could be boosted by a geometry gradient, resulting in the deformation of droplets and generation of surface-tension forces, which contributes to the directional movement of water drops toward the spindle-knots. The discovery of directional water movement on spider silk will help in the design of multifunctional fibers that could be used in many different fields. The cactus O. microdasys lives in the arid Chihuahua Desert. Recently, Jiang et al. found the cactus possesses an integrated multifunctional system for efficient fog collection.65 This can be ascribed to the integration of multiscale surface structures with well-distributed clusters of conical spines and trichomes. Each spine has three integrated parts with different roles in the fog collection process. It was found that the Laplace pressure gradient, surface-free energy gradient, and multifunction integration contributed to the effective fog collection system in cactus. This research work should provide more inspiration for the design and construction of fog collection devices with high efficiency.

low to high by changing the corrosion time. This work should be helpful for the design of underwater superoleophobic engineering metals avoiding oil contamination. 3.11. Nepenthes Leaf

The genus Nepenthes is one of most promising carnivorous plants that can capture insects and other small animals as their main nitrogen source. The genus Nepenthes survives in nutrient-poor habitats and has evolved specialized trapping organs, known as pitchers.59 It was found that the welldeveloped peristome possesses multiple functions, exhibiting slippery, anisotropic, and amphiphilic characteristics.59,60 The peristome surface possesses regular microstructures comprising first- and second-order radial ridges constructed by straight rows of epidermal cells. Furthermore, each epidermal cell overlaps the cell adjacent to the pitcher inside. Wax crystals are found on the zone adjoining the peristome to the pitcher inside in N. alata. The special structures and physicochemical properties of peristome surfaces play an important role in preventing the contact between insect feet and the pitcher. Insects that step on the surface at the rim of the pitcher then will slide down into digestive juices at the bottom. Recently, inspired by Nepenthes pitcher, Aizenberg et al. fabricated slippery self-repairing surfaces with pressure-stable omniphobicity.61 3.12. Stenocara Beetle

Stenocara beetle living in the Namib Desert can collect drinking water from fog-laden wind, so that water collected by their backs can trickle down the body to the mouth, which is a mystery.62 In 2001, Parker et al. revealed how the backs of these beetles collect water.63 They noticed that the back of the beetle was composed by hydrophilic smooth “bumps” and superhydrophobic wax-covered valleys. The “bumps” with no covering are distributed in a near-random array, 0.5−1.5 mm apart, each about 0.5 mm in diameter, whereas the superhydrophobic valleys, including their sloping sides, are covered by microscale structures coated by wax consisting of flattened hemispheres with the diameter of 10 μm with a regular hexagonal array. When the early morning fog comes, the water in the fog locates on the bumps peaks, forming fast-growing droplets on the hydrophilic peaks. Water striking the hydrophobic slopes also bounces to the hydrophilic region. The attached water droplet eventually reaches a size that can easily detach and roll down the tilted beetle’s surface. For the desert beetle, the mechanism of collecting water from fog can be ascribed to the combination of hydrophilic areas and hydrophobic sites in the desert beetle’s elytra. These water harvesting surfaces should be utilized in the future to gather drinking water from the rich fog in dry districts and collect water for agriculture. In addition to the Namib Desert beetle, recently, Jiang et al. uncovered the driectional water-collection ability of spider capture silks.64 After exposing dry silk fibers of the cribellate spider Uloborus walckenaerius to humid air, the initial contact with water caused the hydrophilic fibers to restructure. Silk fibers form periodical spindle-knots along the thread axis, separated by elongated joints roughly 4-fold thinner. Nanofibrils distribute differently between spindle-knots and joints, that is, random on spindle-knots and aligned on joints. At the initial stage exposed in moist air, tiny water droplets randomly condense on spindle-knots and joints. As time goes by, water droplets grow rapidly. When they reach the critical size, water droplets attached to joints will move toward the nearest

4. FABRICATION OF SUPERHYDROPHOBIC SURFACES Recently, more attention has been paid to the control of surface wettability by mimicking the structures of creature surface, the fabrication of superhydrophobic surfaces, and extending their potential functional applications in diverse fields. Chemical composition and surface roughness are the two main factors to govern surface wettability. According to biological inspiration (e.g., lotus effect) and the previous bionic research results, there are two ways to prepare superhydrophobic surfaces: one is to roughen the surface of low-surface-energy materials, that is, hydrophobic materials, and the other is to modify the rough surface with low-surface-energy materials. Clearly, it is focused on the effective construction of rough surface structures and low-surface-energy coating modification to prepare superhydrophobic surfaces. Many methods including physical, chemical, and the combination of physical and chemical methods have currently been developed to obtain superhydrophobic surfaces in a simple and environment-friendly way (Table 1). Herein, we will summerize the progress in the preparation of superhydrophobic surfaces and their related functions. 4.1. Physical Methods to Superhydrophobic Surfaces

4.1.1. Plasma Treatment. Plasma treatment is a simple and effective way to obtain rough surface structures and lowsurface-energy coatings; thus it has been widely used to prepare superhydrophobic surface. There are two main processes, plasma etching and plasma polymerization. Washo first introduced the oxygen plasma to fabricate superhydrophobic surfaces of poly(tetrafluoroethylene) (PTFE) with water CAs of 165°−170°.312 Later, a series of superhydrophobic coatings containing fluorine were fabricated via plasma treatment of the surfaces.66,68,313 Morra et al.66 proved that oxygen plasma treatment can produce porous PTFE coatings by an etching process (Figure 7a). The CAs reflects chemical modification of 8248

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optical properties related to strict requirements on surface roughness. For example, Poncin-Epaillard et al. fabricated transparent superhydrophobic surfaces even with a very low roughness (around 20 nm) through the plasma treatment.72 Teshima et al. reported a transparent and superhydrophobic PET substrate through the combination of oxygen plasma etching and surface chemical modification of various organosilane molecules via chemical vapor deposition (CVD).79 4.1.2. Phase Separation Methods. Phase separation approach is a simple method to construct superhydrophobic surfaces with rough structures by utilizing the instability of the multicomponent mixture, especially for polymers. When the unstable factor was aroused by two immiscible solvents, the phase separation approach also means a solvent−nonsolvent method. Erbil et al. reported a gel-like porous superhydrophobic film on various substrates (such as glass slides, Al foils, stainless steel sheets, etc.) via vacuum heating, utilizing PP as the raw material, and p-xylene as the solvent. MEK, cyclohexanone, and isopropyl alcohol were used as the nonsolvents, respectively.318 The results demonstrated that polymer concentration, temperature, and the nonsolvent have an effect on surface roughness (Figure 8a). Later, the phase separation method was also used to prepare superhydrophobic polymer surfaces.90,91 Han et al.90 prepared LDPE superhydrophobic surfaces with different structures through the adjustment of LDPE crystallization behavior, such as the crystallization time and nucleation rate. Solvent evaporation at low temperature resulted in the increase of the crystallization time and the nucleation rate, and thus the enhancement of the CA. Through the addition of nonsolvent (cyclohexane) and drying at room temperature, a floral structured LDPE superhydrophobic surface with water CA of 173.0° was fabricated (Figure 8b). In the same way, a superhydrophobic PVC film with micro-/nanoscale hierarchical structure was successfully coated onto glass substrates (Figure 8c).91 4.1.3. Template Methods. Various patterned structures can act as templates to produce surface roughness. In 1953, Bartell and Shepard fabricated a tetrahedral rough olefin surface by casting blocks of paraffin in molds constructed from heavy gauge aluminum foil.99 Surface roughness can be adjusted by ruling molds with different intervals, respectively. Bico et al. reported spike-like, shallow cavity-like, and stripe-like micropatterned surfaces by molding a tetramethylorthosilicate sol− gel between a bare silicon wafer and an elastomeric mold designed via a replicating approach.16 After further modification with fluoride, superhydrophobicity can be realized with a water CA of ∼167°. Recently, porous anodic aluminum oxide membrane has been used as template to prepare the aligned polyacrylonitrile

Figure 7. SEM images of PTFE coatings as oxygen plasma 2 min etching (a), and superhydrophobic nanospheres by continuous wave plasma polymerization (b). (a,b) Reproduced with permission from refs 66 and 73, respectively. Copyright 1989 and 2002 American Chemical Society.

the surface at short treatment time and will be controlled by surface roughness at longer etching times. Further, by using inductively coupled radio frequency argon plasma, McCarthy et al. have also prepared rough polypropylene (PP) surfaces in the presence of PTFE,72 with θA/θR as high as 172°/169°. Plasma etching can produce surface roughness on hydrophobic polymer surfaces, resulting in superhydrophobic surfaces. Plasma polymerization of hydrophobic monomer is the other plasma-based approach to fabricate superhydrophobic surfaces. McCarthy et al. prepared a superhydrophobic surface with water CAs of θA/θR = 174°/173° through the plasma polymerization of 2,2,3,3,4,4,4-heptafluorobutyl acrylate (HFBA) on a smooth poly(ethylene terephthalate) (PET) surface.314 Schreiber et al. reported a hexamethyldisiloxane film deposited by low temperature plasma polymerization with water CA of 180°.315 Matsumoto et al. reported a superhydrophobic plasma-polymerized fluorocarbon film using a mixture of CH4/C4F8 gas.316 Kang et al. fabricated a superhydrophobic film with a water CA of 174°/135° by the plasma polymerization of allypentafluorobenzene (APFB) on the plasma-pretreated polyimide (PI) films.70 Recently, Badyal et al. fabricated superhydrophobic nanospheres through the continuous wave plasma polymerization (Figure 7b).73 Moreover, they reported superhydrophobic surfaces by plasma fluorination of polybutadiene films.68 The combination between fluorine atoms and unsaturated alkene groups on the polymer surface as well as surface roughening by the electrical discharge are the main reasons for the superhydrophobicity. Favia et al. fabricated ribbon-like superhydrophobic films by fluorocarbons deposition on silicon with RF glow discharges plasma.317 Because surface roughness of the coatings prepared through the plasma treatment is easily designed and tuned, this method is versatile to design superhydrophobic coatings with unique

Figure 8. (a) SEM images of porous PP films on glass slides at 30 °C. Reproduced with permission from ref 318. Copyright 2003 American Association of the Advancement of Science. (b) SEM pictures of porous LDPE films. (c) SEM images of superhydrophobic PVC films. (b,c) Reproduced with permission from refs 90 and 91, respectively. Copyright 2004 and 2006 Wiley. 8249

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Figure 9. (a) SEM images of the aligned PAN nanofibers surface. (b) Water CA image of the PAN nanofiber surface. (a,b) Reproduced with permission from ref 319. Copyright 2002 Wiley. (c) SEM images of the aligned PVA nanofibers. Reproduced with permission from ref 320. Copyright 2003 Wiley. SEM images of the as-prepared PS nanopillars with different tip geometries: (d) flat, (e) concave shapes, and (f) nanotubes, respectively. Reproduced with permission from ref 104. Copyright 2010 American Chemical Society. (g) Schematic diagram of the experimental procedure used to fabricate PC nanopillar arrays. (h) Equipment scheme of preparation on a large scale. Reproduced with permission from ref 321. Copyright 2010 Wiley.

of hydrophobic SiO2 nanoparticles dispersed in PMMA resulted in the formation of superhydrophobic PMMA-SiO2. The PMMA-SiO2 nanocomposite films showed a static water contact angle of above 162°. They also prepared a superhydrophobic surface through spin coating of a mixture of micro- and nanosized calcium carbonate suspensions on a substrate and subsequent modification with stearic acid.106 The as-prepared coating exhibited a self-cleaning property with a water CA as high as 152.8° and a SA of 7.8°. In addition, through the combination of a wet-chemical process and spin coating, Gao et al. developed a new approach to fabricate a superhydrophobic CuI film without further low-free energy modification.95 Therefore, the spin-coating process provides a facile way to fabricate superhydrophobic surfaces on the flat substrate, especially for inorganic and polymer composite coatings. Yet it is not suitable to be applied on a curved surface. 4.1.5. Spraying Methods. The spraying deposition is one of the most well-developed approaches for painting, graphic arts, and industrial coating.98−102 Zhang et al. fabricated a superhydrophobic coating through spraying ethanol dispersion of metal alkylcarboxylate on any substrate.98 This dispersion was prepared through the reaction of metal salt and

nanofibers, and the prepared aligned nanofibers surface was superhydrophobic with the water CA as high as 173.8° (Figure 9a,b).319 This approach can be applied to various polymer precursors to achieve various nanostructures such as aligned amphililic PVA nanofibers,320 aligned PS nanorod and nanotube, and nanotube films (Figure 9c−f).103,104 Later, WHO developed a simple “template rolling press” method for constructing well-aligned PC nanopillar arrays at a large scale. The PC nanopillar arrays could be obtained by the following procedure (Figure 9g,h).321A column covered with an anodic alumina template was pressed on the film of PC, heated, stressed, and mechanically stripped off, obtaining the wellaligned polymer nanopillar arrays. 4.1.4. Spin-Coating Methods. As a traditional procedure to coat uniform thin films onto flat substrates, spin coating also has been widely used to produce superhydrophobic coatings with hierarchical micro- and nanostructures.97−105 By spincoating the suspension of hydrophobic SiO2 nanoparticles dispersed in PMMA solution, Zhang et al. prepared a superhydrophobic PMMA-SiO2 nanocomposite film with micro/nano hierarchical structures on glass slides in the absence of low surface-energy compounds.105 The introduction 8250

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nanoscale rough xerogel surfaces, which contributed to the formation of superhydrophobic surfaces. Shang et al. prepared rough structured films including nanoclusters and nanoparticles on glass plates through a sol−gel approach using different components of SiO2 sols acting as precursors.322 The resulting surface structures could be adjusted by controlling hydrolysis and condensation reactions. After further modification of selfassembled fluoroalkylsilane monolayer, the transparent superhydrophobic surface can be achieved. Usually, the abovementioned methods involved two steps, to create surface roughness via a sol−gel process and then to modify the surface with fluorides. Recently, several sol−gel methods were developed without further modification of fluorides. Rao et al. fabricated superhydrophobic silica aerogels using MTMS as the precursor.158 It was found that precursor molar ratios, catalyst concentrations, water, and solvent played important roles in the growth process of aerogels structures. The resultant aerogels could maintain their hydrophobicity even when heated to 480 °C. Another example is a sol−gel phase-separation process to produce superhydrophobic organo-silica foam developed by Shirtcliffe et al.159 A wettability transition from hydrophobic to hydrophilic was achieved when the film was heated to 400 °C or above. Cho et al. fabricated a superhydrophobic surface by a sol−gel process using a supramolecular organosilane with quadruple hydrogen bonds and low molecular weight PDMS.160 The results showed that a small amount of PDMS in the process could reduce the contact area because of intrinsic phase-separation. 4.2.2. Solvothermal Methods. Solvothermal synthesis, especially for hydrothermal synthesis, is a general method for the fabrication of micro-/nanostructured materials. By using hydrothermal synthesis, Jiang et al. have fabricated superhydrophobic aligned nanorod arrays such as ZnO and SnO2 and also prepared lotus-like TiO2 nanorod films (Figure 11).176−178,323 Zhang et al. fabricated superhydrophobic zeolite with rose-like microstructures by a direct in situ hydrothermal synthesis method.184 4.2.3. Electrochemical Methods. Electrochemical methods are usually used to fabricate micro-/nanostructured coatings of metals, metal oxides, and conducting polymers. By combining an electrochemical deposition and modification of fluoroalkylsilane, Li et al. prepared conductive superhydrophobic ZnO thin films with rough structures.195 Further study indicated that ZnO films showed superhydrophobicity after heating treatment even if without extra modification. Later, similar deposition methods have been employed to fabricate superhydrophobic surfaces of gold, silver, and copper.196,219,324 Importantly, the electrochemical method has one great advantage as compared to other methods because it does not depend on the shape and the size of substrate.200,325 There are also many other electrochemical methods developed to prepare superhydrophobic surfaces, such as electrochemical polymerization, anodically oxidized methods, nonelectric chemical plating, and galvanic cell reactions. Shi et al. obtained superhydrophobic polythiophene films with the CA of ∼154° by direct electropolymerization of thiophene in boron trifluoride−diethyl etherate (BFEE).221 Zhang et al. synthesized a stable superhydrophobic coating of silver clusters on p-silicon (100) wafer through galvanic cell reaction.326 Xia et al. reported a highly stable gold micro-/nanostructured coating through reaction of current exchange.327 The resultant surface exhibited

alkylcarboxyl acid in an ethanol solution. The resultant coating with flower-like hierarchical structures exhibited a stable superrepellent behavior for several oil liquids. Using the spraying process, they also fabricated the superhydrophobic perfluorooctanoic acid-modified TiO2/polystyrene nanocomposites coatings99 and superhydrophobic CNT nanocomposite films.16 Recently, Park et al. spin-coated a superhydrophobic surface using a novel statistical copolymer. The polymeric material is transparent, relatively inexpensive, easily prepared, solvent-processable, very simple, and applicable to rough substrates.106 This process is applicable for a variety of substrates and is not limited to small areas and flat substrates. The superhydrophobicity of those coatings can be easily repaired by spraying the dispersion again when the coating surfaces are damaged by using the cheap coating materials at any time and almost anywhere. 4.1.6. Electrohydrodynamics/Electrospinning. Electrohydrodynamics (EHD) technique is a conventional and feasible method to produce micro- and nanostructures, such as fibers and particles. Using the EHD technique, Jiang et al. fabricated a low-cost superhydrophobic PS film consisting of porous microspheres and nanofibers (Figure 10).116 The porous

Figure 10. SEM images and water CA photograph of the micro- and nanostructures composite PS films prepared by EHD technique. (a) SEM images of the PS films. (b) Water CA image of the film (CA = 160.4°). Adapted with permission from ref 116. Copyright 2004 Wiley.

microspheres and nanofibers contributed differently to the stable superhydrophobicity, that is, increasing surface roughness by microspheres and binding the porous micropheres by a 3D nanofibers network. 4.1.7. Ion-Assisted Deposition Method. Miller et al. reported superhydrophobic surfaces with a water CA of 150− 160° through controlling the surface roughness using the method of ion-plating by varying the voltage.133 By an ionassisted deposition, Yamashita and Anpo et al. prepared a TiO2 photocatalyst on the surfaces of superhydrophobic and porous Teflon sheets (PTS). UV light irradiation of TiO2 photocatalyst on PTS resulted in the photocatalytic degradation of organic pollutants, which could keep a good durability of the superhydrophobicity.134 4.2. Chemical Methods to Superhydrophobic Surfaces

4.2.1. Sol−Gel Methods. Sol−gel methods can easily tune the surface roughness by adjusting the composition of the starting materials and the operation process. Minani et al. fabricated flower-like porous alumina thin films by immersing porous alumina gel films in boiling water.153−155,297 The subsequent modification of fluoroalkylsilane created a superhydrophobic and transparent film. By aggregating perfluoroalkyl chain-containing organogelator, Nakano et al. fabricated superhydrophobic fibrous coatings on glass plates.156 The solvent evaporation from gels of organogelators resulted in 8251

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polyanion and polycation. Shiratori et al. have presented a superhydrophobic film by the LbL technique.328 They assembled a polyelectrolyte multilayer containing SiO2 nanoparticles, and then created a suitable surface morphology for superhydrophobicity by heating the multilayer film to 650 °C. However, hydrophobic modification is a necessary step in most fabrication processes of superhydrophobic coating via the LBL technique. For example, Rubner et al. prepared a honeycomb-like PAH/PAA multilayer film by the LBL technique, subsequently coated it with silica nanoparticles, and finally modified with semifluorinated silane by CVD.226 The honeycomb-like structure was fabricated by the suitable acid treatment of polyelectrolyte multilayer films. A single acid treatment produced small pores with diameter of 0.5−2 μm. In comparison, a combined acid treatment led to big pores with a diameter of 10 μm. Thus, they obtained stable superhydrophobic multilayer coating. Shen et al. prepared a superhydrophobic surface using a fluorinated exponentialgrowth multilayer.329 It was found that the enhanced exponential growth of multilayers induced the formation of micro/nano hierarchical structures, resulting in the lotus effect. After the CVD of (tridecafluoroctyl)-triethoxysilane on the multilayer film, the resultant surfaces exhibited superhydrophobicity with a CA as high as 172 ± 1.5°. The LBL technique can feasibly combine with other techniques together to control surface structures. Zhang et al. fabricated gold-cluster-covered polyelectrolyte multilayer films on indium tin oxide (ITO) by a combination of LBL technique and electrochemical deposition (Figure 12).219 The film behaved as a stable superhydrophobic after further modification of n-dodecanethiol. The CAs increased gradually with the electrochemical deposition time. 4.2.5. Self-Assembly Methods. Self-assembly methods can spontaneously organize molecular and nanoscaled units into ordered micro- and nanostructures by noncovalent interactions, which have been used as general approaches to fabricate superhydrophobic surfaces. Through the self-assembly of molecular units, Genzer et al. fabricated rough structures by combining self-assembly of surface grafting molecules with mechanical manipulation of the

Figure 11. SEM images of the as-prepared superhydrophobic aligned nanorod arrays using hydrothermal synthesis, (a) ZnO and (b) TiO2. (a,b) Reproduced with permission from refs 176 and 177, respectively. Copyright 2004 American Chemical Society and 2005 Wiley. (c) SEM image of SnO2 and (d) the CA photograph. (c,d) Reproduced with permission from ref 323. Copyright 2006 The Royal Society of Chemistry.

superhydrophobicity after modification with n-dodecanethiol (CA of 165°). Cho et al. prepared superhydrophobic metal surfaces through the simple electrochemical reaction of Cu or Cu−Sn alloy plated on steel sheets with sulfur gas, and subsequent perfluorosilane treatment.225 The as-prepared metal surfaces exhibited water CAs over 160° with low CA hysteresis. Nonelectric chemical plating of Cu onto steel sheets resulted in the microstructures on these surfaces, while the nanotextures of copper sulfide on the microstructures were obtained via an electrochemical reaction of Cu in a sulfur-containing environment at 150 °C. 4.2.4. Layer-by-Layer Methods. The layer-by-layer (LbL) technique is easy and wide to tune various micro-/ nanostructures and fabricate superhydrophobic surfaces by electrostatic interactions between the different layers such as

Figure 12. SEM images of dendritic gold clusters formed on an ITO electrode modified with a polyelectrolyte multilayer by electrochemical deposition. Deposition time: (a) 2 s, (b) 50 s, (c) 200 s, (d) 800 s, respectively. (e) Dynamic water CA measurements on the surface of dendritic gold clusters as a function of the duration of electrochemical deposition at −200 mV (vs Ag/AgCl) by single potential time base mode. (f) Water CA photograph of the dendritic gold clusters surface, and after 40 min of exposure to ambient environment (g). Reproduced with permission from ref 219. Copyright 2004 American Chemical Society. 8252

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Figure 13. (a) Typical SEM image of as-prepared colloidal-crystal films made from latex spheres. (b) The relationship of assembly temperature and water CA of the as-prepared films assembled from latex spheres with varying ratios of nBA/St. Insets are the water CA photograph of the as-prepared superhydrophilic and superhydrophobic films. Reproduced with permission from ref 331. Copyright 2007 Wiley.

grafting points.330 They prepared HO-silicon substrates by treating the PDMS with UV light and O3. The graft of semifluorinated (SF) molecules on the resultant surface resulted in the long-lived superhydrophobicity. Xiao et al.222 reported a superhydrophobic organic film using stearic acid chemically adsorbed on the PEI coated aluminum wafer. First, a PEI coating was fabricated by putting the hydroxylation roughened aluminum substrates into polyethylenimine solution. Next, PEI-coated aluminum substrates were immersed into the STA and DCCD mixing solution. Arising from the chemical interaction between carboxyl groups in STA and amine groups in PEI, self-assembled STA monolayers were adsorbed on the coating, exhibiting the final superhydrophobicity. By using self-assembly of nanoscaled units, Whitesides et al.240 have fabricated superhydrophobic surfaces composed of aggregated gold half-shells pattern using template-assisted selfassembly. By modifying the gold half-shell with hexadecanethiolate self-assembled monolayers, a less adhesive superhydrophobic surface with static water CA equaling 163° was realized, and its slide angle was about 1°. Recently, Song and Jiang et al.331 developed a facile approach to regulate the wettability transition temperature of colloidal-crystal films between superhydrophobicity and superhydrophilicity through controlling the chemical composition of the latex spheres. The wettability transition of the colloidal-crystal films can be finely tuned at a desired temperature. The superhydrophobic films were fabricated from 40 to 90 °C by merely adjustment of the nBA/St ratios (Figure 13). This work provided an alternative approach to control wettability on colloidal-crystals films. 4.2.6. Bottom-Up Fabrication of Micro-/Nanostructure. The bottom-up methods have been widely applied in the building of larger, more composite structure materials such as composite micro-/nanostructure. Ming et al.294 prepared a superhydrophobic surface with hierarchical structures by using a unique epoxy-amine system (Figure 14). One layer of the particles contacted or embedded in the epoxy-based films via a covalent bond. Monoepoxy-end-capped PDMS was grafted onto the particles followed by hydrophobic modification, resulting in a superhydrophobic surface. Furthermore, a superhydrophobic composite thin film could also be fabricated by a method of appending colloidal silica particles242,295,332 or PTFE particles.333 Chen et al. reported the fabrication of Sn nanoflowers on a Ti substrate through the thermal-pyrolysis of a tin organometallic precursor.293 After the rheotaxial growth and thermal

Figure 14. Preparation of superhydrophobic films based on raspberrylike particles. Reproduced with permission from ref 294. Copyright 2005 American Chemical Society.

oxidation, the resultant 3D Sn nanoflowers were completely converted into 3D SnO2 nanoflowers possessing the original morphology. However, the SnO2 nanopetal porosity greatly increased in comparison with the as-synthesized Sn nanopetals. After further investigation of the wetting properties of Sn and SnO2 nanoflowers, it was found that the water CA of the Sn nanoflowers was about 90°, while that of SnO2 flowers was about 155° and superhydrophobic. 4.2.7. One-Step Methods. The fabrication methods of superhydrophobic coatings trended to be facile and cheap so as to realize practical applications. So to develop a one-step method has been attracting more attention to prepare superhydrophobic coatings. Xu and Dong et al. have prepared a micro- and nanoscale hierarchical structured superhydrophobic surface in one step from a micellar solution of PP-PMMA.334 The micelles solution of PP-PMMA was used as starting material to coat the substrates. During the solvent vaporization, micelles aggregated together to form microscale aggregate with diameter of 1−2 μm. On the microsphere surface, there are a lot of nanoscale micelle spheres. This structure was similar to that of the lotus leaf found in nature, resulting in a superhydrophobic surface with CA and SA of ∼160° and ∼9°, respectively. Recently, Wang et al. developed a universal approach to prepare a stable superhydrophobic coating on metal surfaces (such as copper, zinc, iron, etc.).242,335 For example, a microflower coating of copper carboxylate was successfully deposited onto the Cu surfaces by immersing a Cu plate into a fatty acids solution at proper concentration and chain length. With the increase of the immersion time, a few nanosheets and 8253

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small clusters first formed, became bigger and more dense, and finally completely covered substrates (Figure 15). This kind of superhydrophobic coating showed an excellent environmental stability and durability to solvents.

Figure 16. SEM images of the ACNTs films prepared by CVD method. Large area (a) and magnification (b) images of honeycomblike patterned ACNTs. (c) Island-like ACNTs. (d) Postlike ACNTs. Reproduced with permission from refs 337a and 337b. Copyright 2002 and 2003 American Chemical Society.

films with nanoasperities by a vacuum-deposited method, exhibiting the superhydrophobic property with a CA of ∼150°.341 Moreover, Amirfazli et al. reported a superhydrophobic surface with robust chemical stability from the low surface energy of n-hexatriacontane together with the randomly distributed micro-/nanostructures.262 4.3.2. Etching Methods. Etching method is widely used in the surface processing, including photolithography, wet chemical etching, and plasma etching (as described in the above section). Photolithography is one kind of the most effective methods to fabricate a rough surface. McCarthy et al. prepared a series of superhydrophobic arrayed silicon surfaces with different sizes and shapes by photolithography and silane chemistry (Figure 17).281 The results indicated that the surfaces exhibited superhydrophobicity with low adhesion when the square posts had X−Y dimensions of 2 and 32 μm and similar distances between the posts. Water droplets easily rolled off of these surfaces by tilting the substrates slightly. CAs had no relationship with the post height (20−140 μm). However, the water droplets were pinned on the surfaces when X−Y dimensions were of 64 and 128 μm and there were similar distances between the posts. The receding CAs increased with the increase of the distance between the posts, leading to the length of the three-phase contact line shortening. The change of the shape of the posts from square to staggered rhombus, star, or indented square increased the receding CAs. This study will provide some valuable parameters for design of superhydrophobic surface. By combining the nanosphere lithography and oxygen plasma treatment, Chen et al. prepared well-ordered PS nanobeads arrays.272 Subsequent gold deposition and ODT modification resulted in a superhydrophobic surface with a water CA of about 170°. Jiang et al. fabricated a superhydrophobic PDMS film through a laser etching method.273 The PDMS surface with heirarchical micro- and nanostructures showed a high water CA and a low SA, which could be adjusted by regulating the size of convexes (Figure 18). Moreover, Mazur et al. reported a femtosecond laser technology method to fabricate superhydrophobic silicon surface.274 Femtosecond laser was first used to irradiate the silicon surface to create micro-/nanostructures. Besides these physical etching methods above-mentioned, wet chemical etching is an inexpensive

Figure 15. SEM images of Cu(CH3(CH2)12COO)2 clusters formed on a copper plate by the solution-immersion process. The immersion times are (a) 4 h, (b) 16 h, and (c) 72 h. (d) A high-magnification image of a single cluster showing its unique flowerlike structure. Reproduced with permission from ref 242. Copyright 2006 Wiley.

In addition, 60Co irradiation-induced polymerization also provided a simple method to fabricating superhydrophobic surfaces, such as HFP/EMA vapor phase copolymerization under atmospheric pressure conditions.336 The resultant coralreef-like micro texture surface showed a water CA of 153°. This irradiation approach could easily be extended to prepare other superhydrophobic polymer surfaces. 4. 3. Combination of Physical and Chemical Methods

4.3.1. Vapor Deposition Methods. Vapor deposition methods including CVD and PVD can produce perfectly aligned nanostructures with controlled height and diameter. For example, Jiang et al. have prepared various kinds of ACNTs films on quartz-glass substrates by the method of CVD230,337,339 such as honeycomb-like, island-like, and postlike (Figure 16). The prepared films behave superhydrophobic with CA higher than 160° and SA lower than 5° due to the micro-/ nanostructure arrays of the surface. Wang et al. reported a superhydrophobic surface by depositing APTMS on silicon surface with a water CA larger than 153°.338 By applying CVD approaches, Lau et al. demonstrated a stable superhydrophobic ACNTs forests coated by thin PTFE.260 Hozumi et al. developed a microwave plasmaenhanced CVD method to prepare a superhydrophobic film, by using the mixture of TMS and FAS as starting materials.339 Since then, superhydrophobic transparent films with nanostructures were prepared by microwave plasma-enhanced CVD on glass, silicon, and PMMA substrates, respectively, by employing TMMOS (CH3)3Si(OCH3)) as starting materials.244,258,340,341 Furthermore, the mechanical properties and optical transparence of the films were improved using CO2 as an additive gas instead of Ar through the combination of microwave plasma-enhanced CVD and oxygen-plasma treatment. A single-step PVD has also been used to fabricate superhydrophobic surfaces. Miller et al. prepared PTFE thin 8254

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Figure 17. 2-D (X−Y) representations of surfaces containing different geometry posts (above, models; below, SEM images). Reproduced with permission from ref 281. Copyright 2000 American Chemical Society.

Figure 18. (a−c) Typical SEM images of the laser-etched PDMS surface. Photograph of a water droplet sliding behavior (d) on the rough PDMS surface. (e) The relationship between the width of microconvexes and the CA (left)/SA (right), respectively. Reproduced with permission from ref 273. Copyright 2005 Wiley.

ment of the superhydrophobic surfaces tends to be prepared simply, highly stable, environment-friendly, and multifunctional. Here, we will discuss multifunctional superhydrophobic surfaces. 4.4.1. Stable Superhydrophobic Surface. In practical application, the superhydrophobic surfaces generally need to be stable in the acid−base circumstance. A carbon nanofiber arrays was prepared through thermal pyrolysis of nanosturctured polyacrylonitrile (PAN) film.311 The aligned nanostructure carbon fiber without any modification of low surface energy material behaves as a superhydrophobic property with water CA larger than 150° for acidic and basic liquids; that is, the asprepared films showed superhydrophobic property in the whole pH range (Figure 19a,b). The structures of these carbon nanofibers were similar to that of graphite, which is responsible for the durability to the corrosion of acid and base. They have also fabricated stable bioinspired superhydrophobic surfaces by one-step solution-immersion process, and the superhydrophobic coating is very stable upon treatment with various solvents, as mentioned above.242 In addition, Yan et al. reported

method used to surface roughing. By utilizing nanoimprint lithography and wet chemical etching, Natali et al. reported superhydrophobic surfaces on silicon wafers.342 In addition to the above methods, with fast development of the field of superhydrophobic surfaces, some uncommon approaches have been involved in the fabrication of superhydrophobic surfaces, such as melting solidification,343 magnetron sputtering, stretching-controlled micromolding, wettingcompatible method, backside exposure method, nanocasting, supercritical carbon dioxide solution process, aluminuminduced crystallization, electron irradiation, and so on. Although they currently lack sufficient exploration, these uncommon approaches further enrich the database of fabrication methods, and more importantly they can solve problems of some special materials or special conditions in the fabricating process of superhydrophobic surfaces. 4.4. Functional Surfaces with Special Wettability

Recently, the study on superhydrophobic surfaces has aroused more interest for their various applications, and the develop8255

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Figure 19. (a) SEM image of the superhydrophobic nanostructured carbon fiber films. (b) The relationship between pH values and CA on the nanostructured carbon fiber film. (a and b) Reproduced with permission from ref 311. Copyright 2003 Wiley. (c) The bioinspired ribbed nanoneedle arrays of robust superhydrophobicity. (d) Force curves of the reversible compression of the water droplet between the prepared surfaces of ribbed nanoneedle arrays. (c,d) Reproduced with permission from ref 345. Copyright 2010 Wiley.

superhydrophobic needle-like poly(alkylpyrrole) films with environmental stability toward both temperature and organic solvents.344 Recently, Jiang et al. reported a bioinspired ribbed nanoneedle surface with robust superhydrophobicity (Figure 19c).345 The resultant surface demonstrated unique dynamic stability of superhydrophobicity. Squeezing and relaxing of a water droplet between the surfaces led to the fully reversible exploration of the superhydrophobic surface by the liquid. Water CAs reversibly decreased and increased in response to the force being applied or relaxed (Figure 19d). 4.4.2. Superhydrophobic Surfaces with Unique Optical Properties. The practical applications on glass coating strongly drove the development of a transparent superhydrophobic surface (Figure 20a,b).257 Surface roughness could enhance hydrophobicity but often reduced transparency because of light scattering. Therefore, it is key to find the limit range of surface roughness that enhances surface hydrophobicity but cannot reduce transparency. For example, on the basis of the CVD technique, a transparent superhydrophobic surface was prepared by finely tuning surface roughness (Figure 20a,b).257 Watanabe et al. developed a new process to prepare the nanoscale transparent superhydrophobic AlO(OH), SiO 2 , or TiO 2 films by using aluminum acetylacetonate (Al(C5H7O2)3) as a sublimation material and subsequent coating of fluoroalkylsilane.346 Antireflection is another vital optical property of coatings especially in the applications of solar cells. The functional coatings with superhydrophobicity and antireflection are becoming more attractive. Take SiO2 nanocoatings as an example, Xu et al. used methyl-modified SiO2 sol to prepare a superhydrophobic and antireflective coating with a CA of 165° and minimum reflectivity of 0.03%.348 Another example is monochromatic superhydrophobic coatings. Recently, a series of colloidal crystal films with superhydrophobicity were obtained by assembling monodispersed polymer latex spheres.241 These films showed brilliant and monochromatic colors covering the entire visible range tuned by varied diameters (Figure 20c,d).347

Figure 20. Superhydrophobic surfaces with special optical properties. (a,b) Photographs of a water drop placed on the transparency films prepared on glass and organic substrates, respectively. Reproduced with permission from ref 258. Copyright 2002 Wiley. (c) Photograph of as-prepared films deposited on glass substrates. (d) Reflectance spectra of the films with sphere diameters of 280, 270, 244, 216, 204, 191, and 173 nm. The corresponding peak positions are at 650, 622, 598, 534, 504, 490, and 444 nm, respectively, measured with a light incident along the normal surface ([111] direction). (c,d) Reproduced with permission from ref 347. Copyright 2006 Wiley.

4.4.3. Superhydrophobic Surface with Conductive Properties. Conducting polymers and semiconductors are ideal candidates to obtain superhydrophobic and conductive surfaces. Recently, a conductive and transparent SnO2 nanorod film with superhydrophobicity was prepared by the hydrothermal synthesis.323 By using the polyaniline (PANI) and PS, Zhu et al. prepared a novel composite polymer film, which exhibited conductive and excellent superhydrophobic properties in a whole pH range by electrospinning method (Figure 21).349 A common superhydrophobic surface easily causes static-charge accumulation that brings the possibility of a fire or an explosion under dry conditions. The dual functional coating with excellent superhydrophobicity and conductivity will effectively solve this problem. 8256

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5. SUPERHYDROPHILIC SURFACES Superhydrophilic surfaces appeared as an opposite term to superhydrophobic surfaces. A general definition of superhydrophilic surfaces is the surfaces with CAs close to 0°. Similar to the superhydrophobic surfaces, surface roughness is a necessary feature of superhydrophilic surfaces, according to the basic principle developed by Wenzel2 and Cassie and Baxter.4 For a hydrophilic rough surface, the water can fill in the troughs on the rough surface, resulting in the decrease of apparent CA even to 0° (a superhydrophilic state). Although most efforts have been focused on the fabrication of superhydrophobic surfaces, the superhydrophilic surfaces have also received much attention because of their wide practical applications, such as antifogging, antifouling, and self-cleaning. Here, we briefly introduce the recent progress in the fabrication of superhydrophilic surfaces. In general, any kind of solid surface could be treated to superhydrophilic by chemical and physical approaches. One common approach is to coat hydrophilic inorganic meso- or nanoparticles onto solid surfaces. Because of the attractive selfcleaning capability, titanium oxide351 and zinc oxide352 are most studied among inorganic materials because of their photoinduced superhydrophilicity. Moreover, silica nanocoating351c is well studied due to its highly hydrophilicity and easy availability. Nanoparticles have been coated onto substrates through various methods such as inkjet printing,353 spin coating,354 sputtering,352a sol−gel techniques,351b solution growth,355 lithographic techniques,356 and electrochemical deposition.357 On the other hand, hydrophilic polymers act as a big group of coating materials for superhydrophilic surfaces. To make the hydrophilic polymer surface rough or to conjugate hydrophilic group onto rough polymer surface are the two main ways make polymer superhydrophilic. Many techniques, such as ion irradiation,358 electron beam,359 and plasma treatment,360 have been widely used to improve the hydrophilicity of polymer surfaces. It must be pointed out that several kinds of surfaces aroused much attention because of their capability to go from superhydrophobic to superhydrophilic states responding to external stimuli.361 For example, UV and ozone treatment made carbon nanotube films transform from superhydrophobic to superhydrophilic, and heating in a vacuum recovered the films.362 Zhang et al.363 showed that the superhydrophilic micro/nanostructured nylon 6,6 surface can be reversed to superhydrophobic after treatment with formic acid and ethanol. A reversible switch between superhydrophilicity and superhydrophobicity was tuned by controlling the adsorption/ desorption process of n-dodecanethiol associated with the lightinduced silver nanoparticles on WO3 nanostructured film.364

Figure 21. (a) SEM image of an electrospun PANI/PS composite film with lotus-leaf-like structure. (b) A water droplet on a PANI/PS composite film with CA of 166.5°. (c) Relationship between pH and the CA on a PANI/PS composite film. (d) The relationship between pH and conductivity of a PANI/PS composite film. Reproduced with permission from ref 349. Copyright 2006 Wiley.

4.4.4. Superhydrophobic Surfaces with Magnetism Properties. Little attention was paid to this branch of superhydrophobic surfaces with magnetism properties. It probably lies on their unclearly practical applications. Here, we introduce one example of a superhydrophobic and electromagnetic carbon nanofiber film containing the magnetic Fe3O4 nanoparticles (Figure 22).350 This functional film was

Figure 22. (a) SEM and (b) TEM images of Fe3O4-filled CNFs. Relationship between (c) the magnetization (Ms) or (d) CA of carbon nanofiber and the concentration of FeAc2 in the electron spin solution. Reproduced with permission from ref 350. Copyright 2006 Wiley.

6. RESPONSIVE SURFACES WITH SWITCHABLE WETTABILITY Stimuli-responsive materials can be applied to tune surface wettability because their surface free energy or morphology is sensitive to the change of external environments. The configuration of functional molecules on surface could be changed by external stimuli, and surface wettability would change correspondingly. However, the range of the wettability transition is usually very limited on a flat surface. Surface roughness can act as an effective way to amplify the transited range of surface wettability.365,445 For example, on smooth copper films, the CAs only can be adjusted in a relatively

prepared by electro-spinning method from the mixture solution of PVA/FeAc2 and high temperature treatment of 600 °C under argon atmosphere (Figure 22a). The hybrid degree of the functional film could be controlled by tuning the concentration of FeAc2 in electrical spinning solution (Figure 22b). Its magnetic properties and superhydrophobicity also can be tuned by the concentration of FeAc2 in electrical spinning solution (Figure 22c,d). 8257

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Figure 23. A summary of typical smart molecules and moieties that are sensitive to external stimuli, like temperature, light, pH, ion, sugar, solvent, stress, and electricity, and can respond in the way of wettability change.

narrow range between 68° and 113° by modifying the nalkanoic acids with varied chain length. Wang et al. realized the wettability from 0° to 160° by introducing micro-/nanostructured copper clusters.196 In the same way, a reversible transition between superhydrophilic and superhydrophobic can

be realized by combining responsive materials and surface roughness. Until now, many kinds of inorganic and organic molecules have been designed and synthesized with smart moieties that can responsd to the external stimuli, that is, temperature, light, pH, ion, sugar, solvent, stress, and electricity. 8258

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“super switch” due to the excellent responsive properties, reversibility, and stability and have promising applications in industry fields such as microfluidic, functional textiles, filters, and controllable drug release. 6.1.2. Photoresponsive Surfaces. As early as1997, Fujishima’s group reported their new discovery about the UV-generated superamphiphilicity on titanium dioxide surfaces.26 Water droplets and oil can quickly spread out on these surfaces after UV irradiation, and hydrophobicity will recover after long time storage in dark. This kind of surface was composed of tens of nanometers distribution of hydrophilic and oleophilic regions. The formation of a binary-cooperative surface of hydrophilic and oleophilic phases is considered to account for the unique feature.374 By UV illumination on the hydrophobic surface of TiO2, the partial Ti4+ sites will correspondingly convert to Ti3+ sites and form the hydrophilic domains. The discovery about the magical superamphiphilic TiO2 surfaces has attracted much interest on their various applications,375 such as antifogging and self-cleaning.376 Many efforts were devoted to fabricating TiO2 films with excellent performance.377 For example, Fujishima378 and Cohen379 et al. reported the multifunctional nanoporous TiO2 films, which exhibit antifogging and antireflection properties. Besides TiO2, other typical photosensitive semiconductor materials, such as ZnO, WO3, V2O5, SnO2, and Ga2O3, can switch their surface chemical state between two stable states (oxygen vacancies and hydroxyl groups).323,380 For example, Jiang’s group reported the micro-/nanoscale hierarchical structured TiO2 films380e and the aligned SnO2 nanorod films321 by low temperature hydrothermal method. A lightcontrolled surface switch between superhydrophobicity and superhydrophilicity was controlled by the alternation of UVirradiation and dark storage. In addition, they prepared a superhydrophobic/superhydrophilic reversible ZnO film by hydrothermal synthesis381 or the chemical vapor deposition method.352a A nanostructured tungsten oxide (WO3) film showed dual-responsive switching on wettability and photochromism by alternating UV irradiation and storage in dark.380b Moreover, Cho et al.380d fabricated photoresponsive V2O5 rosegarden-like films that reversibly switched between superhydrophobicity and superhydrophilicity. In addition to inorganic compounds, many organic compounds also have stimuli-responsive properties. The typical photoresponsive organic materials usually show a reversible photoinduced transformation between two conformations. For example, azobenzene often can transit reversibly between cis and trans isomers under ultraviolet and visible irradiation. A azobenzene monolayer on a laser etched rough substrate exhibited the transition from superhydrophobic to superhydrophilic by UV irradiation.382 When the azobenzene monolayer was modified on the surface of SiO2 inverse opal, the structural color and the response of wettability could be controlled through the size of the ordered monodisperse.383 A similar phemonemon also was observed by Picraux et al.384 on the photoinduced isomerization spiropyran-containing monolayer on the substrate of random silicon nanowire. Because of the unique feature of these photoresponsive surfaces, they can be widely used in antifogging, self-cleaning, lubricating resistance reduction, accelerating drying, increasing bioaffinity, and so on.385 For instance, amphiphilic TiO2 films can be coated on various substrates such as glass, ceramics, metals, and polymers, which have been widely applied to daily life and industry.

The typical organic smart molecules or moieties are summarized in Figure 23 according to the type of external stimulus. 6.1. Single Stimuli-Responsive Surfaces

6.1.1. Thermoresponsive Surfaces. Temperature-responsive polymers have been widely used to prepare thermoresponsive surfaces. Polymer chains often exhibit a conformation change in response to external stimuli because of their flexible property.366 As a classic thermoresponsive polymer, poly isopropylacrylamide (PNIPAAm) thin film was grafted onto smooth silicon substrate via surface-initiated atom-transfer radical polymerization (SI-ATRP).373 The surface performed a hydrophilic state with a CA of 63.5° when temperature was controlled at 25 °C, and the surface became hydrophobic with a CA of 93.2° at 40 °C (Figure 24).

Figure 24. Mechanism of thermal-responsive wettability changing of PNIPAAm grafted rough substrate. Reproduced with permission from ref 367. Copyright 2004 Wiley.

On a rough silicon surface modified by PNIPAAm, a reversible switching between superhydrophilicity (about 0°) and superhydrophobicity (about 150°) can be realized in a narrow temperature range of about 10 °C. In addition, a fast reversibility and excellent stability of thermally switching was observed by López et al.368 on PNIPAAm-grafted porous anodic aluminum oxide (AAO) membranes. In the past decades, many other kinds of temperatureresponsive polymers were also used to fabricate thermoresponsive superhydrophobic/superhydrophilic surfaces. For example, Zareie et al.369 presented a reversible switch of surface wettability on self-assembled monolayers of oligo(ethylene glycol) tethered on gold substrates. In 2008, Jiang et al. prepared thermoresponsive poly(N-isopropylacrylamide)/ PS composite films by a facile electrospinning technique.370 They also obtained a surface just by coating poly(εcaprolactone) on a rough substrate, to reversibly switch between superhydrophobic and superhydrophilic with temperature stimuli.371 Furthermore, a PNIPAAm-modified micro-/ nanostructured copper mesh film was used for thermocontrolled water permeation.372 Recently, a gradual or even linear wettability change on surfaces was generated by precisely controlling the transition temperature.373 On the basis of the above research, such switchable surfaces can be considered as a 8259

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6.1.3. pH-Responsive Surfaces. In recent years, pHresponsive surfaces with superwettability have attracted a lot of attention as they could be applied to many fields such as drug delivery, separation, and biosensors.386 Minko et al. reported a thin polyelectrolyte brush film with a change-gradient composition across the sample.387 The gradient of surface charge density consequently create a gradient of surface wettability. The pH change can reversibly switch the wetting behavior of this gradient film. Zhang et al. fabricated the micro-/nanostructured rough gold surface and then modified with a mixed monolayer of HS(CH2)9CH3 and HS(CH2)10COOH.388 The surface was superhydrophobic for acidic droplets with pH = 1 and superhydrophilic for basic water droplets with pH = 13. The deprotonation of the surface carboxylic acid groups endows the as-prepared monolayer a pH-responsive wetting behavior. Furthermore, they prepared a rough gold thread surface modified by pH-responsive 2-(11-mercaptoundecanamido)benzoic acid (MUABA).389 This surface exhibited a pHresponsive behavior with a large change in CA from nearly superhydrophobic to superhydrophilic. The spherical water droplet kept well on the gold thread with a large CA, indicating it is nearly superhydrophobic at low pH (Figure 25). If a water

both polymers are present in the top layer. The switching behavior can be amplified by surface roughness. Thus, the surface wettability of the fabric could be switched from superhydrophilic to superhydrophobic. These solvent-sensitive surfaces can find diverse applications from reversible patterning, sensors, to microfluidics. Further, they fabricated self-adaptive surfaces (SAS), which were composed of microscaled needlelike structures and the nanoscaled mixed self-assembled polymer brush irreversibly grafted onto the needles.391b By exposing the surface to different solvents that were sensitive to one of the components of the brush, reversible switching between the superhydrophobicity and the superhydrophilicity could be observed. It was superhydrophobic with a CA of 160° when the surface was exposed to toluene, and a water droplet could roll easily on the surface. After another immersion in an acid (pH = 3) water for few minutes, however, a drop of water can spread on the surface. 6.1.5. Electricity-Responsive Surfaces. As a simple and convenient way, electricity has been paid special attention to control surface wettability. Lahann et al.392 reported a reversibly switchable surface by depositing a low-density self-assembled monolayer of (16-mercapto)hexadecanoic acid (MHA) on a gold surface. The electrical potential can trigger the conformational transition of the monolayer and induce surface wettability switchable. However, the extent of CA change on surface was too small (20°−30°). Electrowetting provides a feasible way to control the wettability of superhydrophobic surfaces without changing their chemical composition. Krupenkin et al.393 reported a dynamic nanostructured surface for the electrical control of the wetting behavior of liquids, ranging from the superhydrophobic state to nearly complete wetting at low voltage of 22 V. They further studied different wetting states of liquid droplets including high-surface-tension liquids (such as water and molten salt) and lower-surface-tension liquids (such as 2propanol and methanol). When no voltage was applied, a droplet of molten salt kept a spherical shape with a CA of nearly 180° on the substrate (Figure 26a); a droplet of cyclopentanol showed an immobile droplet on the substrate (Figure 26c). The droplet with lower surface tensions could completely wet the substrate. The four frames of a liquid droplet on the nanostructured substrate showed a demonstration of an electrically induced transition between different wetting states. The droplet of molten salt underwent a sharp transition to the immobile droplet state when the voltage was about 22 V (Figure 26b). With the voltage of about 50 V, the CA of the cyclopentanol droplet decreased (Figure 26d). An electrically controlled reversible wetting/dewetting transition then was demonstrated on superhydrophobic nanostructured surfaces.394 A short pulse of electrical current was transported through the substrate to reverse the transition and make the immobile droplet go back to the rolling ball state. 6.1.6. Stress-Responsive Surfaces. The surface wettability of a solid substrate is mainly governed by chemical composition and surface structures. As mentioned above, we summarized reversibly control of surface wettability by thermal treatment, light illumination, pH or solvent inducement, and electrical-field stimulus. On the other hand, the surface wettability can be also reversibly tuned by manipulating the geometric structures of the solid surface. PTFE is one of the common-used hydrophobic elastic materials. When the density of the PTFE crystals was changed, like extended axially with an extension ratio from 0% to 190%,

Figure 25. Water droplet contacting gold threads with the pHresponsive coatings, (a) pH = 1.1 and (b) pH = 9.2. Reproduced with permission from ref 389. Copyright 2009 American Chemical Society.

droplet of pH 9.2 is used, the water droplet spreads onto the gold thread surface immediately. The pH-responsive wettability on the gold thread can provide tunable supporting forces that influence its floatation. Besides small molecules, polymers containing pH-functional groups also show a pH-sensitive change of surface wettability. For example, a superhydrophobic PANI-PAN coaxial nanofiber film was prepared by a combination of electrospinning and polymerization.390 By simply tuning the acid−base and redox properties of probe solution, this nanofiber film exhibited a pHreversible conversion between superhydrophobicity and superhydrophilicity quickly. 6.1.4. Solvent-Responsive Surfaces. Minko et al. reported the solvent-responsive wettability on mixed brushlike layers on polyamide substrates.391 The polymer brushes consisted of an assembly of polymer chains of two incompatible polymers, which were sensitive to toluene, ethanol, and water. The preferred polymers preferentially take over the top of the surface in selective solvents; while in nonselective solvents, 8260

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coated fabric become superhydrophilic. Thus, a reversible wettability of the PANI-fabric was observed when it was doped with PFSEA and dedoped with ammonium gas. The cooperative effect of the porous structures and reversible doping/dedoping features of PANI resulted in the reversible wettability of the PANI-coated fabric from superhydrophobicity to superhydrophilicity.398 Recently, Jiang’s group reported an ammonia responsive surface wettability switch on indium hydroxide films, which were prepared by hydrothermal synthesis.399 The films are superhydrophobic in air. When exposed to an ammonia hydroxide atmosphere for 20 h, the water droplet can spread on the films, indicating their superhydrophilicity. By heating the superhydrophilic film in the air at 90 °C for 1 week, the superhydrophilic film returned to be superhydrophobic again. The surface wettability switch shows a good reversibility. This kind of smart surface is promising in exploring new types of ammonia sensors simply by valuing surface wettability. 6.1.8. Enthalpy-Driven Surfaces. So far nearly all of those smart surfaces switching in surface wettability are limited to entropy-driven processes. However, enthalpy-driven behaviors are very common in various important life phenomena and molecular recognition behaviors. Wang et al. reported a enthalpy-driven smart surface that switches between superhydrophilicity and superhydrophobicity by using i-motif DNA strands with a fluoride-containing hydrophobic group and immobilizing them onto a gold surface through Au−S bonds (Figure 28).400 Under basic conditions, the i-motif structure of DNA molecules on the surface (superhydrophilic state, state I) converts into the stretched single-stranded structure (unstably superhydrophobic state, state II). In the presence of complementary stands (Y), the duplex structure (stable superhydrophobic state, state III) will form. Further adding acid can recover the original state of the DNA. During this transition process, only conversion from state I to II is entropydriven, but the other transformations are enthalpy-driven processes. This macroscopic switching of surface wettability originates from the cooperative effect of surface microstructure and collective nanoscale motion of DNA nanomotors. 6.1.9. Ion-Responsive Surfaces. Ion-pairing interaction was reported to have capability to induce the transition from superhydrophobicity to superhydrophilicity.407 Ions can reversibly exchange between cationic or anionic electrolytes and their complexes. This reversible process of ion exchange can help to design smart surfaces that are suitable to the physiological conditions. Cho et al.401 fabricated ion-responsive surfaces by grafting poly[2-(methacryloyloxy)ethyltrimethylammonium chloride] (PMETAC) brushes with quaternary

Figure 26. Four frames of a liquid droplet on the nanostructured substrate from the video recording proving electrically induced transitions between different wetting states. The voltage was applied between the substrate and the droplet (contacted through the Pt wire). Reproduced with permission from ref 393. Copyright 2004 American Chemical Society.

the water CA increased to 165° from 108° on its flat surface.395 Recently, this method of biaxially extending and unloading was used to generate reversible wettability on an elastic polyamide film.396 Figure 27a shows that the elastic polyamide film is superhydrophobic (water CA is 151°) before biaxial extension. When biaxially extending the elastic polyamide films to an extension ratio larger than 120%, the water droplet spreads out on the films (Figure 27b). After unloading, the surface wettability returns to superhydrophobicity with the recovery of the surface microstructures to their original state. The change of the triangular net-like structure upon biaxial extension and unloading is believed to be responsible for the reversible switching between superhydrophobicity and superhydrophilicity. Moverover, the shape-memory materials also can change their surface morphology reversibly responding to external stimuli, which could be another kind of smart material with tunable surface wettability. 6.1.7. Gas-Responsive Surfaces. In 2007, a gas-responsive surface of PANI-coated fabric was proved to reversibly switch between superhydrophobicity and superhydrophilicity triggered by ammonia gas.397 A superhydrophobic PANI-coated fabric was prepared by in situ doping polymerization in the presence of perfluorosebacic acid (PFSEA), with a static CA as high as 160.9°. A 1 min exposure to dry ammonia gas made the PANI-

Figure 27. Reversibly change in structure and wettability of the triangular polyamide film during extension and unloading. Reproduced with permission from ref 396. Copyright 2005 Wiley. 8261

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Figure 29. CAs reversibly change when the pH and/or temperature is varied. Reproduced with permission from ref 402. Copyright 2006 Wiley. Figure 28. Enthalpy-driven responsive surface of DNA nanomotor. The hydrophobic group Bodipy 493/503 (green circle) acts as the functional group grafted onto oligonucleotide strand X and the i-motif structure. The complementary strand Y is a nonthiolated strand for forming duplex structures. At low pH, the DNA adopts an i-motif conformation (superhydrophilic state, state I). Raising the pH destabilizes the i-motif to produce an unstably superhydrophobic state (state II) or a stably superhydrophobic state (state III, when a complementary strand is present). This cycle transition process is reversible. Reproduced with permission from ref 400. Copyright 2007 Wiley.

red) and the others are almost smaller than 20° (the background color is blue). If the pH is fixed, the film is hydrophilic at low temperatures and hydrophobic at high temperatures. If the temperature is fixed, the film is hydrophobic at low pH and hydrophilic at high pH. This dual-responsive property mainly depends on the competition between intermolecular and intramolecular hydrogen bonding among NIPAAm, AAc, and water. Further adjusting the ratio between hydrophobic and hydrophilic moieties can tune LCST close to body temperature, which could be important for drug delivery applications. In 2009, a dual-responsive rough surface coated by polypeptide was proved to switch between superhydrophilic and superhydrophobic states.403 The macroscopic phenomenon of surface originates from the unfolding/aggregation of the poly-L-lysine (PLL) in response to pH and temperature, and micro-/nanocomposite structure as well. At pH lower than the pKa of PLL (∼11.0), there was a superhydrophilic state on the rough surface resulting from a PLL random coil conformation. At pH higher than the pKa, the surface became hydrophilic with the changing from random to R-helix conformation. Further increasing the temperature, PLL gradually adopted aggregated β-sheet structures and led to the superhydrophobic state. To decrease pH and temperature simultaneously can cause a reversible transition from superhydrophobic to superhydrophilic states (Figure 30). As shown in Figure 31a, the CA of a water droplet on the smooth surface in state I (pH 6.5) was 49.4° ± 1.6°, increased to 56.5° ± 0.8° (state II,) and further increased to 67.2° ± 2.1° (state III). In comparison, a large change of CA was observed on the rough substrate in Figure 31b. Moreover, considering the complex conditions in the human body, multiresponsive surfaces have been developed that involved some biomolecules like glucose. For example, the copolymer films of PNIPAAm and poly(acrylamido phenylboronic acid) (PPBA) formed on flat and rough silicon substrates using SI-ATRP.404 The films can reversibly switch between superhydrophilicity and superhydrophobicity in response to glucose, temperature, and pH. The multiresponsive surface may have an important impact in biomedical fields.

ammonium groups onto the rough gold surfaces. The reversible switch between superhydrophobicity and superhydrophilicity was achieved by direct anion exchange. The as-prepared substrate exhibited superhydrophilic properties. However, when immersed in bis(trifluoromethane)sulfonamide (TFSI) solution for a certain time, the substrate exhibited superhydrophobicity, indicated by the water CA of 171° ± 3° and the SA less than 5°. Further exposed to SCN− solution, the wettability of the substrate switched from superhydrophobic to superhydrophilic. In addition, the switching behavior can keep good reversibility with little damage even after many switching cycles. 6.2. Dual- and Multiresponsive Switchable Surfaces

As shown in the above section, many smart surfaces that can switch between superhydrophilicity and superhydrophobicity have been fabricated successfully with the combination of responsive materials and surface roughness. However, most of these surfaces are responsive to only one kind of external stimuli, restricting their applications in complex practical environments that need multiple-responsibility. Dual-responsive surfaces were recently reported that simultaneously exhibit temperature- and pH-sensitive wettability. These surfaces requires two types of stimuli-responsive materials. Such surfaces were realized by grafting poly(Nisopropylacrylamide-co-acrylic acid)(P(NIPAAm-co-AAc)) copolymer thin films on rough silicon substrates.402 Both a narrow temperature range of about 10 °C and a relatively broad pH range of about 9 can induce the transition between superhydrophilicity and superhydrophobicity. Figure 29 shows the mechanism of the dual-responsive surfaces under the external stimuli (temperature and pH). The general trend in the change of wettability is that one-half of the CAs of the water profiles are almost larger than 130° (the background color is 8262

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chemical reaction at interface. This section is mainly focused on the understanding of the solid−liquid adhesion, artificial interfacial materials, and their potential applications. 7.1. Theoretical Understanding of Liquid/Solid Adhesion

These examples brought a significant question about how liquid/solid adhesion is influenced when the water CA are at superhydrophobic states. Adhesion is the interaction between different objects, which is the proportion of effective contact areas. In this manner, the actual contact area between liquid and solid is the most significant parameter to take into account. In the Wenzel state, the valley parts of the substrate are filled by the liquid, forming a continuous triple-phase liquid/air/solid contact line (TCL) and leading to maximum contact between liquid and solid; thus the adhesion between liquid and solid is maximum. In the Cassie wetting state, air was trapped into the interface between the droplet and the substrate, forming a composite liquid/solid and liquid/air contact model and noncontinuous TCL. Here, two possibilities could influence the adhesion. If the air trapped into the interface was connected to the atmosphere, the liquid/air contact could largely reduce the adhesion as compared to the liquid/solid contact, resulting in a “lotus model” and low adhesion. On the contrary, if the air was sealed between the water and the substrate structures, the negative pressure caused by pulling of the water droplet will provide a sufficient adhesion force to the water droplet, thus preventing the water droplet from rolling off the surface.32 This led to the high adhesion “gecko model”. The Wenzel state and Cassie state are two ideal situations. In many cases, a water droplet may partially wet the top of a superhydrophobic surface, leaving air partially trapped in the valley parts of the substrate. This case is obviously an intermediate state between the Wenzel and Cassie states, which can be referred to as a metastable state.28,405 The liquid/solid adhesion can be enhanced to a certain extent as compared to the pure Cassie state, regarding the chemical composition and topography characteristics of the substrate. This case is the typical “rose petal model”. It should be noted that even under different wetting states, the apparent water CA can be comparably as high as 150°, showing “superhydrophobicity” to the observers. In addition, the metastable wetting state can change to either the Wenzel or the Cassie state if the energy barrier can be overcome under certain circumstances, such as pressure, vibration, heat, magnetic force, etc. Therefore, it can be promising to utilize different stimuli to intelligently control the liquid/solid adhesion and develop further potential applications.

Figure 30. At low pH, PLL mainly adopts a random coil conformation (state I). Raising the pH allows the dominant existence of R-helix conformation (state II). Heating the surface destabilizes the R-helix conformation to produce the aggregated β-sheet structures (state III). Lowering the pH and temperature induces a reversible conversion process from state III to state I. Reproduced with permission from ref 403. Copyright 2005 American Chemical Society.

Figure 31. Profiles of a water droplet on (a) smooth and (b) rough substrates showing the different wettability of states I, II, and III. Reproduced with permission from ref 403. Copyright 2005 American Chemical Society.

7. ADHESION-CONTROLLED LIQUID/SOLID SURFACES Recently, the interesting phenomena regarding the adhesion between solid surfaces and liquid are beginning to arouse people’s attention, especially at hydrophobic and superhydrophobic surfaces. Surfaces with the same apparent CAs can vary significantly different in liquid adhesions. For instance, a surface that shows superhydrophobicity can easily cause a water droplet to roll off; that is what lotus leaves have told us.7 However, it can be puzzling that a surface that shows water CA as high as that of lotus leaves may still have strong adhesion; water droplets will pin on a surface even if the surface is placed upside down, such as rose petals in nature. Moreover, various surfaces, natural or artificial, with different liquid solid adhesions have been discovered or fabricated, including low adhesion surfaces, high adhesion surfaces, anisotropic adhesion surfaces, adhesion tunable surfaces, stimuli-responsive adhesion surfaces, and novel oil adhesion surfaces in water medium. These findings and materials can greatly expand the knowledge of surface wettability in a different way, and may also have many applications in print, bioseparation, patterning, assay, and

7.2. High Adhesive Surfaces

The natural examples of superhydrophobic surfaces with high adhesion are rare, except for the rose petals. However, the artificial surfaces with high adhesion are more developed. The key point for designing this kind of surface is choosing suitable geometries. For instance, inspired by the gecko-feet effect, Jin et al. prepared the superhydrophobic PS nanotube array with high adhesion and proposed the possible mechanism based on negative pressure (Figure 32a,b).28 Three kinds of superhydrophobic nanostructures, such as nanopore array, nanotube array, and nanovesuvianite structures, then were prepared by Gao et al. (Figure 32c−e).406 When a water droplet contacts the solid surface, only the nanovesuvianite surface could form open air pockets, which are connected and cannot form a negative pressure when the liquid is pulling away from the surface, leading to very low solid−liquid adhesion and very low 8263

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Choi et al. report a gecko-inspired, hairy poly(dimethylsiloxane) (h-PDMS) surface (Figure 34), replicated

Figure 32. SEM images of superhydrophobic surfaces with different adhesions and mechanisms: (a) the aligned PS nanotube layer; inset, the shape of water on the PS surface when it is turned upside down. (b) Schematic illustration of adhesion forces caused by negative pressure when a water droplet is pulled away. (c−e) Top view images of a superhydrophobic TiO2: (c) nanopore array, (d) nanotube array, and (e) nanovesuvianite structures. Reproduced with permission from refs 34 and 413. Copyright 2005 and 2009 Wiley.

Figure 34. SEM images of h-PDMS replicas of AAO templates anodized at the second anodization for (a) 10 min and (b) 20 min. The bottom shows a static water CA on each replica surface. The scale bar is 500 nm. Shapes of a water droplet on the fabricated, hairy hPDMS nanopillar surface with different tilt angles: (c) 90° and (d) 180°. Reproduced with permission from ref 408. Copyright 2008 Wiley.

by utilizing an anodic aluminum oxide (AAO) membrane as a template.415 The resulting hairy h-PDMS-coated glass shows the static water CA of 150.5° ± 0.4° and high adhesion to water droplet. The densely packed h-PDMS nanopillars contribute to the high adhesion. The adhesive surface can catch a water droplet on a tilted self-cleaning surface.

SAs. In the contrary, sealed air pockets could be formed in the nanopore array and nanotube array surfaces, thus enhancing surface adhesion. Superhydrophobic surfaces with high adhesion provide one effective solution to transport liquid droplets with small volume. For example, the adhesive superhydrophobic PS nanotube film was used as a “mechanical hand” to transport a superparamagnetic microdroplet (M-droplet).407 As shown in Figure 33, the PS nanotube film can suck the M-droplet when the upper magnet is applied. When the magnet force is reversed, the M-droplet will leave the adhesive PS film to the ordinary superhydrophobic surface. Importantly, there is no lost volume in this process of droplet transport.

7.3. Adhesion-Tunable Surfaces

With modification of the original surface with chemical composition or surface topography, the adhesion between the surface and liquid may vary a lot. Besides, the fabrication parameters of certain surfaces could play a dominant effect in deciding the surface adhesion to liquid. In this part, we will discuss some examples of adhesion-tunable surfaces. 7.3.1. Chemical Composition. By tuning the chemical composition of surface coatings, the superhydrophobic TiO2 surfaces showed different adhesion to water droplets.409After modification with 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PTES), the sponge-like nanostructure TiO2 surface (Figure 35a) showed superhydrophobic behavior, and water droplets could easily roll off on this surface (Figure 35b). If introducing nitrocellulose (NC) to PTES during surface modification, the water droplet adheres firmly to the surface even when the film was placed vertically (Figure 35c). The NC could lead to a disruption of the densely packed hydrophobic PTES molecules to some extent (Figure 35d), resulting in the formation of hydrogen bonds between water and nitro groups. Therefore, the introduction of amphiphilic materials may be an effective way to tune the adhesive property of superhydrophobic surfaces. 7.3.2. Surface Topography. Huang et al. showed a good example by comparing two kinds of patterned surfaces consisting of dense arrays of microscaled PDMS lenses (lock, Figure 36a) and bowls (key, Figure 36c) to demonstrate how surface topography affects the liquid/solid adhesion.410 The PDMS microlens-array substrate is very adhesive, and a water droplet did not slide away even if the substrate was turned upside down (Figure 36b). In contrast, the microbowl-array surface exhibited very low adhesion to a water droplet (Figure 36d). The water droplet had a wet-contact with the microlensarray, thus resulting in a high adhesion (Figure 36e). However, air trapped in the microbowls (Figure 36f) greatly decreased the adhesion.

Figure 33. No lost transport processes of a superparamagnetic microdroplet in alternating magnetic fields. (a) A M-droplet was placed on a low-adhesion superhydrophobic PS nanotube film. (b) When magnet A was switched on, the M-droplet was magnetized and attracted to fly upward. (c) M-droplet was stuck onto the surface of PS nanotubes. (d) When magnet B was switched on, the direction of magnetic force was reversed and the M-droplet fell down. (e) When both magnets were switched off, transport was stopped. (f) The Mdroplet can be reversibly transported upward and downward by alternating magnetic fields. Reproduced with permission from ref 407. Copyright 2007 American Chemical Society. 8264

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Figure 35. Adhesion-controlled surface by changing chemical composition. (a) SEM image of the sponge-like structure TiO2 film. The inset shows the shape of a water droplet placed onto the TiO2 film. (b) Sliding behavior of a water droplet on the PTES-modified TiO2 surface. (c) A water droplet trapped on the PTES and NC-modified TiO2 surface. (d) Scheme of surface chemical composition changes. Reproduced with permission from ref 409. Copyright 2008 American Chemical Society.

Figure 37. Morphology of Si nanowire arrays and their corresponding water adhesion behaviors. (a) Nanowires from the metal-assisted chemical etching of Si with Au nanoparticles at a shorter-glancing angle deposition (GLAD) duration and (b) at longer-GLAD duration. The insets are top-view SEM images of the respective samples (scale bar: 10 μm). CA and wetting behaviors of the two different nanowire samples. (c) A 6 μL drop on water on surface (a). A 4 μL droplet of water on the SNS at a tilting angle of 0° (d) and 180° (e) for sample (b). Reproduced with permission from ref 411. Copyright 2011 American Chemical Society.

Figure 36. Patterned PDMS surfaces with different adhesion to water. (a) SEM image of the fabricated PDMS microlens arrays. (b) Behavior of a water droplet on the microlens-array surface. (c) SEM image of PDMS microbowl arrays. The inset shows a high-magnification SEM image of the bowl-shaped structure. The scale bar is 2 μm. (d) Shape of a water droplet on the microbowl-array surface. (e,f) Schematic image of the possible static and dynamic behavior of a liquid droplet at microlens-array and microbowl-array interfaces. Reproduced with permission from ref 410. Copyright 2009 Wiley.

contrast, the surfaces with leaf-shaped spherulites that possess sharp edges displayed a low adhesion surface. 7.4. Stimuli-Responsive Surfaces with Tunable Adhesion

As is known, surfaces that can respond to environmental stimuli, such as pH, temperature, ions, and electricity, are of great interest to many applications. Utilizing stimuli-responsive materials, scientists have fabricated many “smart” surfaces that could change their adhesion to liquid by changing the environmental stimuli. In this part, we will have detailed discussions on this issue. 7.4.1. Mechanical-Responsive Adhesion. Recently, a PMDS surface with regular pillar array has been demonstrated to switch between low and high adhesive states of superhydrophobicity driven by curvature change.413 The adhesion force and the SA strongly depend on the surface curvature. With increasing surface curvature, the adhesion gradually decreased from high to low and vice versa, as shown in Figure 39. This unique switching can be used as a “mechanical hand” to transport water droplet without any loss. 7.4.2. Thermal-Responsive Adhesion. Recently, Jiang’s group reported a thermal-responsive adhesion switching of water droplets on superhydrophobic surfaces modified by

Choi et al. tuned the selective adhesion of superhydrophobic surfaces by controlling simply the morphologies of Si nanowire arrays (Figure 37),411 that is, “clumped” nanowire surface and “straight” nanowire surface. The clumped nanowire surface showed a superhydrophobic state with low adhesion, while the straighter nanowires exhibited a superhydrophobic state with high adhesion. By adjusting the parameters during phase separation and crystallization of polymers, Han et al. controlled porous structures of isotactic poly(propylene) (PP) coatings with superhydrophobicity.412 Different porous PP network structures can cause different adhesion properties. As shown in Figure 38, the surface composed of connected 3.5−6 μm size microspherulites showed high adhesion for water droplets. In 8265

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Figure 38. Adhesion-controlled surfaces by phase separation and crystallization. (a) SEM images of superhydrophobic surfaces and two related SAs. (b) Schematic diagrams of different morphologies formed, which are related to different adhesion properties (white, air; gray, water; black, polymer). Left: Wenzel’s state with high adhesion. Right: Cassie’s state with low adhesion. Reproduced with permission from ref 412. Copyright 2006 Wiley.

Figure 40. Temperature-responsive surface for liquid/solid adhesion. (a) The proposed conformation rearrangement of a side-chain liquid crystal polymer upon phase transition. (b) The reversible switching of a water droplet from rolling to pinned, following the temperature transition from 23 °C (left) to 75 °C (right, the inset shows that the droplet sticks to the surface, even when the substrate turns upside down). (c) At 23 °C, the water droplet is in Cassie’s state; at 75 °C, it is in Wenzel’s state. Reproduced with permission from ref 414. Copyright 2009 Wiley.

alternating process of magnetizing and demagnetizing (Figure 41c,d). The wide range of adhesive force from low to high could be effectively tuned under different magnetic fields (Figure 41e). 7.4.4. Photoelectrical-Responsive Adhesion. A photoelectrical cooperative effect was applied to transit the adhesive states from low to high on a superhydrophobic aligned composite nanorod array (ACNA) surface.416 When the voltage was applied below the threshold value of electrowetting, the red ink droplet adopted the Cassie state on the ACNA surface. When the ACNA surface was illuminated through a photomask with an “H” pattern, a photoassistant electrowetting occurred on the illuminating area. Accordingly, a wetting state of the patterned area transferred from the Cassie state to the Wenzel state because of the electrocappillary pressure, and the red ink subsequently entered into the ditches (Figure 42). In this way, a simple pattern of “H” was produced on the surface. This photoelectrical-responsive surface gives a possibility for modern printing. 7.4.5. Heat, pH, and Ion Multiresponsive Adhesion. A typical example is a stimuli-responsive polymer coating that PNIPAM and poly(dimethylamino)ethyl methacrylate (PDMAEMA) were cografted from initiator-modified anodized alumina substrates with irregular micro-/nanoscale surface structures.417 As shown in Figure 43, the as-prepared copolymer coatings exhibit highly unusual superhydrophobicity. The changes of temperature, pH, and electrolytes can reversibly switch low and high adhesive states for spherical water/acid/ alkali/salt droplets. The multiresponsive properties originate from the cooperative effect of a mixed responsive polymer

Figure 39. (a) Scheme for water-droplet transport using curvaturedriven switching between the pinned state and roll-down state. (b) Process for capturing and releasing a water droplet. Reproduced with permission from ref 413. Copyright 2011 Wiley.

liquid-crystal polymer (PDMS-4OCB) with thermal-responsive side chains.414 The wettability switch was attributed to the phase transition of liquid crystal from the smectic A (SmA) phase to the isotropic phase (Figure 40a). The water droplets could be switched from the rolling to the pinning state on an optimized rough silicon surface (Figure 40b), while maintaining a superhydrophobic state. The observation of Cassie’s state at 23 °C and Wenzel’s state at 75 °C (Figure 40c) provided direct evidence that the temperature can cause an adhesive transition of superhydrophobic surfaces. 7.4.3. Magnetic-Responsive Adhesion. By applying external magnetic fields, an adhesion switching on a superhydrophobic iron surface was achieved.415 As shown in Figure 41a,b, reversible switching between the low adhesive state and the high adhesive state for a M-droplet could be realized by the 8266

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Figure 41. Scheme of magnetic-responsive surface for liquid/solid adhesion. Before and after the superhydrophobic iron surface was magnetized, a M-droplet can (a) roll and (b) be pinned on the surface, respectively. (c,d) Schematic illustration of the interactions between the superhydrophobic iron surface and the M-droplet. (e) Force− distance curves recorded before and after the M-droplet contacted the as-prepared iron surface. Four lines with different symbols represent the superhydrophobic iron surface that was magnetized in a magnetic field with different intensities. Reproduced with permission from ref 415. Copyright 2008 Wiley.

Figure 42. Schematic diagrams of the reprography process. (a) Surface wettability locally transits from Cassie to Wenzel states with the patterned light “H” illuminating to the ACNA below the electrowetting threshold voltage. (b) After the light and the voltage are turned off, an ink-patterned “H” is left with the removal of redundant ink. (c) When the ink pattern is transferred onto a printing paper, the desired image “H” is obtained. Reproduced with permission from ref 416. Copyright 2009 Wiley.

coating, a permanently hydrophobic substrate, and surface roughness.

liquids, especially for those with low surface tension (γLV < 30 mN/m). Tsujii et al. reported in 1997 the first superoleophobic surfaces for low surface tension liquids.419 They fabricated an anodically oxidized aluminum surface with a fractal structure. After modification of perfluorodecylphosphate or perfluorododecyl phosphate, the surface displayed a CA of 150° with rapeseed oil (γLV = 35.7 mN/m), and oil droplets rolled around even if the surface was only slightly tilted.420 Fujii et al.421 prepared a superoleophobic aluminum−niobium alloy with a dual scale pillared structure by combining oblique angle magnetron sputtering deposition (OAD) and anodic oxidation. Upon being modified with fluoroalkyl phosphate (FAP), the CAs of 156° and 151° and CAH 2° and 6° were observed for rapeseed oil and hexadecane, respectively. Because of the commercial availability and the ease of the silanization process, perfluorinated silanes have been one of the most common choices with low surface energy to modify a surface. Cao et al.422 fabricated hierarchically structured porous silicon films by gold-assisted electroless etching. After being silanized using perfluorooctyl trichlorosilane, the films displayed a CA of 151° for hexadecane. Hierarchically textured cotton textiles coated by positively/negatively charged silica microparticles423 displayed an apparent CA as high as 152° and a TA of 9° for hexadecane after being silanized with perfluorodecyl trichlorosilane. Wu et al.424 reported alumina nanowire forests by electrochemically etching aluminum foils and subsequently anodizing the activated aluminums. After

8. SUPEROLEOPHOBIC SURFACES 8.1. In-Air Superoleophobic Surfaces

Although there are significantly fewer reports on superoleophobic surfaces than on superhydrophobic surfaces, the number of reports on superoleophobic surfaces is rapidly increasing every year as more researchers recognizing the importance of re-entrant structures in designing superoleophobic surfaces. Two criteria related to characterizing the superoleophobic surfaces are sometimes neglected. On one hand, the superoleophobic surfaces display apparent CAs θ > 150° for both high and low surface tension liquids as well as low CAH Δθ.418 Just reporting the static CA θ or the apparent θadv > 150° for liquids does not adequately describe superoleophobicity. Obtaining a low CAH (Δθ < 5°) and low TAs (α < 5°) for liquids with low surface tension is important for many applications, such as oil−water separation. On the other hand, it often puzzles people which liquids should be used in monitoring superoleophobicity. Thus, it is sometimes difficult to compare across different superoleophobic surfaces prepared from different groups, because liquids with various surface tensions ranging from heptane (γLV = 20.1 mN/m) to diiodomethane (γLV = 50.8 mN/m) have been used as probe oils. Only using one or two liquids such as diiodomethane and glycerol is not sufficient to claim superoleophobicity of a solid surface. Therefore, it is important to measure the CAs and CAH for a variety of polar and nonpolar 8267

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fluorodecyl POSS blends onto stainless steel wire meshes, Tutija et al.119,431 fabricated hierarchically structured superoleophobic surfaces with re-entrant texture at both the coarser and the finer length scales. The ultralow CAH on the hierarchically structured surface is a direct consequence of the reduced liquid/solid contact area, allowing ∼2 μL droplets of heptane, as well as liquids possessing a higher surface tension, to easily roll off (α ≤ 2°) and bounce on the surfaces. The development of such surfaces extends the application fields to different transparent substrates such as windows, phones, tablets, and computer screens. By spraying PDMSfluorodecyl POSS microparticles onto transparent PDMS pillars, the heirarchically textured, transparent surfaces can repel a range of high and low surface tension liquids and display extremely low CAH. In this manner, a wide range of materials from metals to polymers and a wide range of surface coatings from perfluoralkyl silanes to fluorinated precursors have been used for the fabrication of superoleophobic surfaces. These results pave the way for further research on developing durable superoleophobic surfaces that are critical for many practical applications. Recently, surfaces with controllable superoleophobicity and switchable oil adhesion were fabricated on engineered functional titanium for the first time (Figure 44) .432 After the twoFigure 43. (a) Schematic illustration of reversible adhesion of water droplets induced by macromolecular anchors on hydrophobic molecule-modified Al2O3 substrate. (b) Thermal-induced switchable adhesion of water droplets. Results of measurements of CA and maximum adhesion volume/SA on PNIPAM grafted anodized alumina surfaces from different ratios of initiator/PFOTS modified surface: sticky state at 25 °C and rollable state at 40 °C. (c) pH-induced switchable adhesion of water droplets. Digital images of droplets with different pH on PDMAEMA-grafted Al2O3 substrate. Reproduced with permission from ref 417. Copyright 2010 American Chemical Society.

silanization with perfluorooctadecyl trichlorosilane, the surfaces displayed CAs of 155° and 153°, and TAs of 5° and 3° for rapeseed oil and hexadecane, respectively. After similar surface modification, Wang et al.425 displayed a superoleophobic texture consisting of well-aligned titanium dioxide nanotubes fabricated by anodization on micropillars of titanium. The main chemicals used for surface modification include fluorinated precursors, fluorinated monomers, or fluorinated polymers. Ahuja et al.426 fabricated silicon nanonail with reentrant texture using reactive ion etching. By plasma-assisted chemical vapor deposition of C4F8, which acted as the precursor, these surfaces displayed θadv of 155° with a series of alcohols from methanol, ethanol, to 1-decanol. Hsieh et al.427 prepared stacks of silica spheres with a two-tier hierarchical texture by using a two-stage spin-coating technique. When coated with a perfluoroalkyl methacrylic copolymer, the surfaces displayed θadv* of 150° for hexadecane. A nanoporous film of poly(3,4-ethylenedioxypyrrole) on textured micropillar surface displayed θadv* of 153° and TA of 27° for 6 μL hexadecane droplets.428 However, the same surface displayed a much higher CA of 155° and a lower TA of 3° for 6 μL sunflower oil droplets (γLV = 31 mN/m). There are also several reports on preparing superoleophobic surfaces of polymer composites.429 Steele et al.430 spray-casted composites of a perfluoroalkyl methacrylic copolymer and zinc oxide nanoparticles onto glass slides. The resulting hierarchically structured surfaces displayed CA of 154° and Δθ of 6° for hexadecane. By electrospinning microbeads of PMMA-

Figure 44. (a−g) Tunable CA of micro-/nanostructured TiO2/Ti under UV illumination with different times from 0 to 90 min and the XPS analysis. (h,i) The reversible performance after storing in the dark for 2 weeks. The left drop is hexadecane, and the right drop is glycerol. Reproduced with permission from ref 432. Copyright 2010 American Chemical Society.

step of anodization, a thin and well-aligned TiO2 nanotube array formed on the surface of the microstructure of Ti. By the postmodification of hydrophobic materials, the surfaces showed a superoleophobic with CA = 156° for hexadecane. A reversible surface switching between sliding superoleophobicity and sticky superoleophobicity was achieved by UV irradiation and annealing of titanium surfaces. Furthermore, a magnetic oil droplet was manipulated to move on the surface by an external magnetic field. 8.2. Underwater Superoleophobic Surfaces

The wetting behavior of liquid droplets on the solid surface is not an apparent or simple contact between two phases but one among three phases. Recently, the authors systemically investigated the self-cleaning effect of fish scales and found that superhydrophilic surfaces in the water/air/solid system 8268

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when applied with a positive voltage. In reverse, the CA increased and the oil droplet started to roll under the influence of gravity. When a positive voltage was applied again, the oil droplet could be stopped, implying that the adhesive force increased to conquer the influence of the gravity. The oil droplet could start to roll again when a negative voltage was applied. The ionic changes during the oxidation and reduction of the PPy film facilitate the reversible transition from the Wenzel to the Cassie state in aqueous medium. 8.2.2. Thermal Control of Oil/Solid Adhesion on Underwater Superoleophobic Surfaces. A thermal-responsive poly(N-isopropylacrylamide) (PNIPAM) hydrogel surface exhibit a reversible wettability and adhesion to oil at water/solid interface.435 Below the PNIPAM lower critical solution temperature (LCST, 32 °C), the hydrogel surface showed an underwater superoleophobicity with low adhesion, while above its LCST it becomes an oleophobic and high adhesive state. This responsive property can be further tuned by changing ordered surface microstructures. The oil droplet could easily roll off of the hydrogel surface at low temperature, indicating low adhesion to oil. At 40 °C, the oil droplet can adhere firmly on the hydrogel surface even (Figure 46a). More interestingly,

show superoleophobic properties in an oil/water/solid system by replacing the air phase with a water phase.33 To understand the unique wetting property in the oil/water/ solid system, smooth, microstructured, and micro-/nanostructured silicon surfaces were selected as model structures. The smooth silicon surface is oleophobic with a CA of 134.8°, while the microstructured and micro-/nanostructured silicon surfaces show superoleophobic with CAs larger than 150°. Their adhesive behaviors are significantly different. As compared to the smooth silicon surface, which exhibits remarkable adhesion to an oil droplet, the microstructured silicon surface showed a relative smaller adhesive force (about 10.2 μN). The adhesive force is smaller than 1 μN for the micro-/nanostructured silicon surface. The adhesive property in the oil/water/solid system is comparable with that of a water/ air/solid system. When the surfaces contact with oil droplets, water molecules can be trapped in the micro-/nanostructures, which forms an oil/water/solid interface. These trapped water molecules that play a role similar to that of air will vastly decrease the adhesive force between the solid surface and the oil droplet. Inspired by the oil-repellent nature of the composite surface of fish, Jiang et al. prepared robust underwater superoleophobic coatings by fabricating hierarchical macromolecule-nanoclay hydrogels (Figure 45).433 The robust superoleophobicity of

Figure 45. Macromolecule-nanoclay composite for low oil−solid adhesion in water medium. (a) Bioinspired hierarchical surface with microprotuberances and nanometer-scale roughness (635.3 nm rms roughness) shows superoleophobicity. (b) Nonpatterned smooth surface (338.9 nm rms roughness) shows oleophobicity. (c) Dynamic underwater-oil adhesion measurements on hydrogel surfaces with different preloads (3 μL oil droplet). Reproduced with permission from ref 433. Copyright 2010 Wiley.

Figure 46. Thermal-responsive adhesion between the hydrogel surface and oil droplets. (a) The oil droplet is easy to roll off the surface at 23 °C, while at 40 °C it pins onto the surface even if upside down. (b) A n-hexane oil droplet could spontaneously “fly away” from the PHS while decreasing the temperature from 40 to 23 °C. (c) Adhesive force measurements using oil droplets as detecting probes. The adhesive force is 5.8 ± 1.8 μN at 23 °C and 23.1 ± 1.9 μN at 40 °C. Reproduced with permission from ref 435. Copyright 2010 The Royal Society of Chemistry.

hybrid hydrogels mainly originates from the synergetic effect of micro-/nanostructures and the mechanical strength of hybrid hydrogels. Through a photoinitiated polymerization, hierarchical structures on the hydrogel surfaces can be patterned by a design-guided molding process with micro-/nanoscaled topographical templates. The as-prepared hydrogel showed a largely enhanced mechanical strength and excellent superoleophobicity in water, which shows promising applications in antifouling coatings for ships. 8.2.1. Electrochemical Control of Oil/Solid Adhesion on Underwater Superoleophobic Surfaces. Jiang et al. developed an in situ wettability switch of conducting polypyrrole (PPy) films based on an oil/water/solid system.434 By using a simple redox reaction of PPy, a smart switch between adhesive Wenzel and nonadhesive Cassie state of superoleophobicity can be easily realized to oil droplets underwater. The oil droplet can adhere onto the PPy surface

if we dropped an oil droplet of low density (such as n-hexane, density: 0.660 g/cm3)436 on the surface at 40 °C in the beginning, and then decreased the environmental temperature from 40 to 23 °C, the n-hexane droplet could “fly away” from the hydrogel surface spontaneously during this process (1−4 of Figure 46b). The breaking of the buoyant force of the oil droplet and adhesion force at lowing temperature enabled it to gradually detach from the surface. This interesting property could provide promising methods for applications such as droplet manipulation, smart optimization of microfluidic devices, and so on. Although liquid/solid adhesion is in its infant stage, it has shown many exciting and promising results for antifouling coatings, no-loss small volume liquid transportation, and localized chemical reaction. To put the discoveries into real application, many questions remain to be solved. For instance, 8269

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what kind of surface geometry is most efficient to obtain a high CA as well as a high liquid−solid adhesion? Is it possible to find a way to generally treat any surface and turn them into superhydrophobic and high liquid−solid adhesion at the same time? What is the fundamental mechanism of high CA as well as high adhesion? Can we prove it through solid evidence? Solving these questions is urgent and important for scientists to move forward. We believe this new and fascinating field of wettability will attract more attention in the next decades in the field of surface science and engineering.

9. APPLICATIONS During the past few decades, many different synthesis strategies have been developed to fabricate functional surfaces with superwettability. Scientists and engineers have begun to investigate the utility of these special wetting surfaces in different fields.42b,e,43a−h,437 Nowadays, a great number of superwetting surfaces have been commercialized. Given space constraints, we could not discuss all of the interesting and significant work concerning the applications of TiO2 surfaces with superwettability. Therefore, in this section, we will mainly address the applications of the special wetting surfaces (such as superhydrophilic, superhydrophobic, and superoleophobic) in anticorrosion, antifogging, anti-icing, drag reduction, medicine, oil−water separation, self-cleaning, water harvesting, and other fields (Figure 47).

Figure 47. Representative applications of superwetting surfaces. (i) For anticorrosion, air trapped between the superhydrophobic coating and solution can provide an effective barrier to prevent the migration of corrosive ions. (ii) For antifogging, the superhydrophilic coatings can spread the condensed water droplets quickly to form a thin sheetlike film and dramatically suppress the fogging behavior. Reproduced with permission from ref 467c. Copyright 2010 The American Physical Society. (iii) For anti-icing, the superhydrophobic surfaces can delay and reduce the accumulation/adhesion of wet snow, ice, and frost. Reproduced with permission from ref 486. Copyright 2009 American Chemical Society. (iv) For drag reduction, the superhydrophobic coatings can keep a layer of air between the solid substrate and the liquid as a lubricant film to reduce the drag. Reproduced with permission from ref 509. Copyright 2011 The American Physical Society. (v) For oil−water separation, the combination of superhydrophobicity and superoleophilicity can lead to functional coatings with capability of separating oil/water mixture. Reproduced with permission from ref 526. Copyright 2011 Wiley. (vi) For print and reprography, using the green printing plate-making technology colorful images could also be obtained through overprinting with fine resolution. Reproduced with permission from ref 541b. Copyright 2013 The Royal Society of Chemistry. (vii) For textile, the product of superamphiphobic textiles shows an excellent self-cleaning property. Reproduced with permission from ref 544b. Copyright 2011 Wiley. (viii) For water harvesting, spider silk possesses directional water collection ability due to its periodic spindle-knot structure, which will bring a new clue to design functional materials of water harvesting. Reproduced with permission from ref 551a. Copyright 2010 Wiley.

9.1. Anticorrosion

Corrosion has been recognized as one of the most serious problems in our society, and the resultant losses are in the hundreds of billions of dollars each year. Corrosion resistance has received increased attention in recent years. Currently, one of the strategies for corrosion protection in industrial practices is the surface treatment using chromium-containing compounds, which have a negative impact on health and environment. Recently, superhydrophobic coatings have been widely used on a variety of engineering material surfaces, such as Al, Cu, Fe, Ti, Zn, alloy, and steel, to improve their corrosion resistance (Table 2). The direct fabrication of superhydrophobic coatings on metal surfaces should be a promising solution for enhancing their anticorrosion performance.438 The corrosion resistance mechanism of superhydrophobic surfaces could be ascribed to the presence of the air layer between the substrate and solution, which provides an effective barrier to prevent the migration of corrosive ions. Jiang et al. fabricated Mg alloy surfaces with stable superhydrophobicity and enhanced corrosion resistance utilizing a solution immersion method and postmodification with fluoroalkylsilane.439 Since then, some research groups focused on the investigation of the corrosion resistance of Mg alloys, the lightest metal structure materials among the practical metals. For example, Xu et al. demonstrated the fabrication of superhydrophobic Mg alloy surfaces with corrosion resistance through an electrochemical machining method followed by fluoroalkylsilane modification.440 Ishizaki et al. fabricated a series of superhydrophobic Mg alloys through the microwave plasma-enhanced chemical vapor deposition and immersion process.441 Corrosion resistance and chemical stability of the resultant superhydrophobic Mg alloys have been systematically investigated. They also reported the preparation of color-tuned superhydrophobic Mg alloy with corrosion resistance and damping capacity using the chemical-free immersion approach in ultrapure water.442 The color can be switched from silver

with metallic luster to specific colors such as orange, green, and orchid, which is dependent on the immersion time. Duan et al. reported the corrosion resistance of superhydrophobic layered double hydroxide films on a porous anodic alumina/aluminum substrate.443 The superhydrophobic nature of the film provided an effective barrier to aggressive species. Boukherroub et al. fabricated superhydrophobic coatings on the Zn substrate as effective corrosion barriers through a solution immersion approach.444 The superhydrophobic film exhibited effective corrosion resistance after immersing in acidic, alkaline, and saline aqueous solutions. Yeh et al. prepared anticorrosion electroactive epoxy materials through replicating fresh Xanthosoma sagittifolium leaves.445 These coatings on steel substrates presented a synergistic effect of superhydrophobicity 8270

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Table 2. Superhydrophobic Metal/Alloy Surfaces Used in Corrosion Resistance substrate

method

ref

Al alloy Al alloy Al sheet Al sheet Al sheet Al sheet bulk metallic glass Cu, Fe Cu plate Cu plate Cu plate Cu plate Cu plate Cu plate Cu plate Cu plate Cu plate Cu plate Cu mesh Mg alloy Mg alloy Mg alloy Mg alloy Mg alloy Mg alloy stainless steel steel Ti foil Zn electrode Zn foil Zn foil

self-assembly ultrasound irradiation anion exchange anodization electrospinning sol−gel solution immersion solution immersion in situ crystallization dip-coating electrodeposition electrolysis potentiostatic electrolysis sol−gel deposition solution immersion solution immersion surface graft polymerization surface oxidation spraying chemical-free immersion chemical vapor deposition electrochemical machining solution immersion solution immersion solution immersion curing process nanocasting solution immersion electrosynthesis replacement deposition solution immersion

446 447 443 448 449 450 451 250 452 453 454 455 456 457 458 459 151 460 461 442 441a 440 441b 441c 439 306 445 462 463 464 444

Table 3. Typical Surfaces with Special Wettability Used in Antifogging material

method

fluorosurfactantmacromer f-PEG

grafting to

f-PEG

grafting to

glass PDMS PEMA/PVA polysaccharide porous TiO2 polymer

alkali corrosion soft lithography spin coating layer-by-layer sol−gel RAFT

PUA silica cone arrays

ion etching reactive ion etching layer-by-layer dip-coating dip-coating template layer-by-layer layer-by-layer spin coating sol−gel sol−gel multilayer assembly sol−gel sol−gel spin coating FSP layer-by-layer

silicate/PDDA SiO2 SiO2/La(OH)3 SiO2 nanoparticles SiO2/PAH SiO2/PDDA SiO2/polymer SiO2/PS TiO2 TiO2/PEG TiO2/SiO2 TiO2/SiO2 TiO2/WO3 TiO2/SiO2 TiO2/SiO2

spin-casting

wetting property hydrophilic/ oleophobic hydrophilic/ oleophobic hydrophilic/ oleophobic superhydrophilic superhydrophobic hydrophilic superhydrophilic superhydrophilic hydrophilic/ oleophobic superhydrophilic superhydrophilic

ref 472 471a 471b 473 47 474 468b 467c 471c 478 465

superhydrophilic superhydrophilic superhydrophilic superhydrophilic superhydrophilic superhydrophilic superhydrophilic superhydrophilic superhydrophilic superhydrophilic

475 476 477 469b 469a 469c 478 479 466 467b

superhydrophilic superhydrophilic superhydrophilic superhydrophilic superhydrophilic

480 481 482 483 467a

structures or the introduction of other functional components, superhydrophilic TiO2-based materials without photocatalytic activation have been fabricated for antifogging by several research teams.467 Currently, superhydrophilicity-induced antifogging coatings for automobile mirrors have been commercialized by several companies. The second path involves the use of hydrophilic polymers to obtain superhydrophilic surfaces and antifogging.468 The third case is the design of textured or porous surfaces to significantly enhance the surface wettability by introducing roughness at the right length scale.469 To date, lotus leaves-inspired superhydrophobic materials with low CA hystersis have been used as self-cleaning coatings and stain-resistant textiles. However, Cheng et al. found lotus leaves become wet when water is condensed onto the leaves, where fog drops in microscale could locate in the interspaces between micropapillae.470 Therefore, lotus-like superhydrophobic surfaces should be unsuitable for the antifogging application. Recently, Jiang et al. found the mosquito compound eyes possess ideal superhydrophobicity, which evolves an effective protective mechanism for maintaining clear vision in a humid habitat.47 Inspired by mosquito compound eyes, artificial compound eyes with superhydrophobic and antifogging properties were fabricated through a soft lithography approach followed by low-surface-energy fluoroalkylsilane modification. Arising from their high surface energy, hydrophilic surfaces are susceptible to contaminations that are difficult to remove.

and redox catalytic capability. The combination of anodic protection and the water barrier arising from the superrepellent property resulted in the improvement of the corrosion protection of superhydrophobic electroactive epoxy on the cold-rolled steel electrode. 9.2. Antifogging

Fogging is ubiquitous as it frequently occurs on mirrors, glass, and other substrates, which are a safety hazard and a hindrance both in daily life and in many technological applications. This problem could be effectively solved through controlling the interaction between substrates and liquids (Table 3). The antifogging strategies can be briefly categorized following three approaches: (i) the superhydrophilic approach (the water CA 0°), (ii) the superhydrophobic approach (the water CA > 150° and low SAs), and (iii) the hydrophilic/oleophobic approach (simultaneous hydrophilicity and oleophobicity for antifogging and contamination resistance, respectively). Superhydrophilic coatings can dramatically restrain the fogging behavior, where the condensed water droplets will instantaneously spread flat to form a thin water film on the substrate rather than to form dispersed droplets.465 In the case of superhydrophilic surfaces, three different approaches have been developed to achieve antifogging. The first involves the utilization of TiO2-based functional materials due to their photoinduced superhydrophilicity of TiO2. Watanabe et al. reported TiO2-coated glass with antifogging under UV illumination.466 Through the rational design of surface 8271

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could suppress the ice formation upon impact of supercooled water both in laboratory conditions and in natural environments. The anti-icing capability of these superhydrophobic surfaces is dependent not only on their super water repellence but on the particle size exposed on the substrate. This implies that surface roughness of superhydrophobic surfaces can affect the efficacy of anti-icing materials. Poulikakos et al. further investigated the effect of surface roughness on icephobicity using untreated and coated surfaces ranging from hydrophilicity to superhydrophobicity.489 Although hydrophobic surfaces exhibited higher icing resistance than hydrophilic rough surfaces, hydrophilic surfaces with nanoscale roughness values close to the critical nucleus radius present higher icephobicity as compared to typical hierarchically rough superhydrophobic surfaces. In practice, the presence of frost will affect the capability of anti-icing materials.487,493,494 Song et al. fabricated hydrophobic and superhydrophobic isotactic polypropylene (iPP) coatings to evaluate their different effects on the frost formation.493 They found frost is hindered on the superhydrophobic i-PP surface. The metastable three-phase line on superhydrophobic surfaces resulted in the retardation of the solidification of the liquid and the formation of frost, which provides an effective avenue for the fabrication of antifrost materials. Icing of water on superhydrophobic substrates is a complex phenomenon. Currently, anti-icing properties of superwetting surfaces are topics that are appearing in the scientific literature with increasing frequency.484 These recent research works extended our understanding for the icing phenomenon and shed light on some promising approaches for the design of antiicing surfaces using special wetting materials. However, to construct commercially viable and durable icephobic surfaces using the superhydrophobic approach, further fundamental experimental and theoretical research is still necessary to investigate the effect of wettability, surface roughness, surface texture, and environmental conditions on anti-icing performance.496 A deeper understanding of these features is essential for a rational design and the reproducible construction of antiicing materials for practical applications. For example, the clarification of icing mechanism of supercooled water freezing on surfaces is a fundamental issue. Recently, Poulikakos et al. reported environmental conditions, such as humidity and gas flow, could fundamentally alter the ice crystallization mechanism and drastically affect the icephobic behavior and relevance on supercooled superhydrophobic surfaces.490 These findings shed light on the explication of surface and environmental conditions on the supercooled droplet freezing, which should be helpful for us to optimize the current superhydrophobic anti-icing materials through considering the effect of environmental condition, surface structure, and wettability.

Therefore, advanced solutions for antifogging should combine hydrophilicity and oleophobicity necessary for antifogging and contamination resistance, respectively. Recently, functional polymers with simultaneous hydrophilicity and oleophobicity were fabricated by two research teams, exhibiting unique antifogging and oil-repellent properties.471 These research works provided a new platform for the next generation of antifogging surfaces. 9.3. Anti-icing

Ice formation and accretion on exposed substrates will hinder the operational performance of highways, airplanes, ships, power lines, telecommunications equipment, and others.484 Most current deicing approaches include either physical or chemical removal of ice, resulting in both energy and resource dissipation. Within the past few years, inspired by biomaterial surfaces with superwettability, artificial superhydrophobic surfaces with high static CAs and low CA hysteresis have been fabricated by using different synthesis strategies. Recently, superhydrophobic surfaces were proposed to be used in the field of anti-icing (Table 4). Because of the outstanding water Table 4. Typical Superhydrophobic Surfaces Used in Antiicing substrate

method

ref

Al alloy Al plates Al plates Al plates Al plates Al plates, Ti alloy Al, Si Cu plates i-PP PDMS steel, polymer

spin coating spin coating spin coating spin/dip coating spray coating spin/dip coating vapor deposition EGD phase separation template spin coating

485 486 487 488 489 490 491 492 493 494 495

repellency, these superhydrophobic surfaces have an extra advantage in delaying and reducing the accumulation/adhesion of wet snow, ice, or frost. Quéré et al. demonstrated that freezing could be significantly delayed when depositing water on the cold microtextured superhydrophobic surfaces.492 If the microtextured superhydrophobic substrate was tilted, droplets were removed without freezing and accumulating. This can be attributed to the presence of air sublayers acting as thermal barriers between the superhydrophobic substrate and the liquid. Cohen et al. studied the relationships between advancing/receding water CAs and ice adhesion strength on smooth bare and coated steel substrates.495 It was found that maximizing the receding water CA could minimize ice adhesion. Therefore, the icephobicity of nominally smooth substrates can be predicted simply by measuring the receding water CA. Aizenberg et al. reported the fabrication of ice-free nanostructure surfaces originating from the repulsion of impacting water droplets.491 The behaviors of dynamic droplets impacting supercooled nano- and microstructure surfaces were investigated. Highly ordered superhydrophobic micro- or nanostructures could remain entirely ice-free down to ca. −25 to −30 °C in the case of water droplet impact, arising from their abilities to repel impacting water before ice nucleation occurs. Gao et al. investigated the antiicing property of superhydrophobic nanoparticle−polymer composite surfaces.486 These superhydrophobic composites

9.4. Drag Reduction

Drag is one of the main hindrances for aircrafts, ships, submarines, and microfluidic devices. In nature, biological materials have evolved different solutions to achieve optimal drag reduction.42e,43g,497 Inspired by biomaterials with special drag reduction, many different synthesis strategies have been developed to fabricate antidrag surfaces. Generally, dragreducing coatings can be constructed through the following two approaches: (i) to design shark skin-inspired surfaces with ribbed textures,498 and (ii) to design lotus leaf-inspired superhydrophobic surfaces with a low CA hysteresis.499 For 8272

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the superhydrophobic surface, the fluidic drag reduction can be mainly ascribed to the water repellency of the surface, which generates a lubricating gas film to establish an air/water new boundary condition and dramatically reduce the interaction between the solid substrate and water.500 Here, we will mainly focus on superhydrophobicity-induced drag reduction. In 1999, Watanabe et al. first found a drag reduction phenomena for Newtonian fluids when water passed through a 16 mm-diameter pipe with superhydrophobic walls.501 Since then, a number of efforts were devoted to theoretical and experimental studies of the fluidic drag reduction of superhydrophobic coatings. Theoretical simulation indicated that a superhydrophobic coating can be effective for drag reduction not only in laminar flows but also in turbulent flows.502 Daniello et al. demonstrated drag reduction in turbulent flows by micropatterned superhydrophobic surfaces, which were previously noted for their effective ability in laminar flow drag reduction.503 Bhushan et al. investigated the drag reduction efficiency on bioinspired superhydrophobic structures by the pressure drop measurement in the channels using turbulent and laminar flows.504 It was found that superhydrophobicity can induce drag reduction in water flow, where that drag reduction in turbulent flow was higher than that in laminar flow. Zhang et al. fabricated superhydrophobic gold threads and demonstrated for the first time that the superhydrophobic coating can effectively reduce the fluidic drag for objects moving in water.505 Under the same propulsion, the velocity of a superhydrophobic gold thread is about 1.7 times that of a normal hydrophobic gold thread. Hwang et al. prepared tens of centimeter-scale flexible superhydrophobic nanofiber structures by the curing approach, demonstrating drag reduction (up to 28.5%) and no aging degradation.506 Recently, carbon nanotubes deposited on different substrates were proved to possess superhydrophobicity and drag reduction by the chemical vapor deposition method.507 Although superhydrophobic surfaces have exhibited promise in drag reduction due to the existence of a lubricating gas film between the solid surface and the liquid, superhydrophobic surfaces may become wet and lose their slip effect under liquid pressure or at surface defects. Recently, two research teams developed different approaches to achieve a stable gas layer on solid surfaces. Kim et al. fabricated superhydrophobic surfaces possessing underwater restoration and retention of gases for drag reduction.508 To restore a stable gas blanket, superhydrophobic multiscale structures with a certain geometric criterion were constructed. Electrolysis was developed to achieve the self-controlled gas-generation. The superhydrophobicity can be maintained even under high liquid pressures and in the presence of surface defects. The other approach was proposed by Vakarelski et al. through the Leidenfrost effect, which was observed and understood over 250 years ago.509 The Leidenfrost effect was proved to be versatile for the creation of arobust and continuous lubricating vapor layer on the surface of a heated solid sphere moving in a liquid. It was found that Leidenfrost effect-induced vapor layers can reduce the hydrodynamic drag by over 85%. These pioneering works should be helpful for the implementation of superhydrophobic surfaces for drag reduction.

tunable drug release through the displacement of air to control delivery rates.510 3D superhydrophobic poly(ε-caprolactone) electrospun meshes were fabricated, which contain poly(glycerol monostearate-co-ε-caprolactone) as a hydrophobic polymer dopant. The entrapped air layer within superhydrophobic meshes exhibited long-term stability in the presence of serum, demonstrating efficacy against cancer cells in vitro for >60 days. Schmuki et al. fabricated high-aspect-ratio TiO2 nano test tubes, which can be applied as a self-cleaning platform for high-sensitive immunoassays by immobilizing an antibody for diagnosis of a target antigen.511 The TiO2-based immunosensor possesses reusable and self-cleaning characteristics arising from the photocatalytic property of TiO2. Bormashenko et al. demonstrated that superhydrophobic surfaces could be used for smart droplet manipulation, that is, cutting of droplets, even nonstick droplets with a two-sided superhydrophobic scalpel, which has a potential for biomedical applications.512 Gristina et al. investigated Saos2 cell adhesion on superhydrophobic nanotextured slippery substrates.513 Besides the nanoscale topographical features, superhydrophobicity of the textured surfaces also played an important role in the inhibition of cell adhesion. TiO2 has a promising antibacterial effect due to its special photocatalysis and self-cleaning properties.514 Sulfur-doped titania thin films with self-cleaning were prepared by Parkin et al. through the atmospheric pressure chemical vapor deposition.515 The photocatalytic activity was improved due to the incorporation of sulfur into TiO2. In comparison with commercial products, the sulfur-doped TiO2 showed superior photocatalysis and photoinduced superhydrophilicity. These films were found to be effective agents for killing the bacterium Escherichia coli using light sources commonly found in UK hospitals. Antibacterial Escherichia coli materials with photoinduced self-cleaning properties were also fabricated by Wang et al. using 2,6-anthraquinone sulfonate by a layer-by-layer assembly approach.516 In addition to the above-mentioned TiO2-based antibacterial surfaces, superhydrophobic surfaces with antibacterial properties were also developed by several research teams. For example, Lee et al. fabricated silverperfluorodecanethiolate complexes with superhydrophobic, antibacterial, and antifouling properties through a reaction of silver nitrate with perfluorodecanethiol.517 Under UV irradiation, silver nanoparticles were formed on the wire surface and exhibited antibacterial properties. Schoenfisch et al. reported the fabrication of fluorinated silica colloid surfaces with antibacterial and superhydrophobic properties. The combination of multiscale features from silica colloids and a low surface energy fluorinated silane xerogel reduced the adhesion of highly pathogenic Staphylococcus aureus and Pseudomonas aeruginosa, demonstrating the possible candidates for the next generation of medical devices.518 9.6. Oil/Water Separation

Separation of water and oil mixture is considered a worldwide challenge due to frequent oil spill accidents and the increasing industrial oily wastewater. For example, the flood of oil in the Gulf of Mexico in 2010 is one of the most serious pollution accidents of the last decades. Recently, functional surfaces integrated with both superhydrophobicity and superoleophilicity were proved to be effective in the separation of oil and water mixture. Utilizing different synthesis strategies, a series of superhydrophobic/superoleophilic materials (such as metal mesh, carbon nanotube sponge, polymer film, etc.) have been

9.5. Medicine

Recent research work demonstrated that special wetting surfaces can be used in the field of medicine. Grinstaff et al. demonstrated superhydrophobic materials can be used in 8273

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to large strains repeatedly in air or liquids without collapse, exhibiting high recyclability, selectivity, and absorption capacity to a variety of solvents. Stellacci et al. reported cryptomelanetype manganese oxide nanowire membranes with paper-like structures.523 The resultant free-standing membranes can selectively absorb oils up to 20 times the membranes’ weight in preference to water, arising from the cooperation of superhydrophobicity and capillary action. Furthermore, the obtained nanowires could be resuspended in solutions and subsequently reform the original paper-like structure for many cycles, demonstrating the recyclable characteristic. Liu et al. fabricated multifunctional polyurethane foam with simultaneous superhydrophobicity and superoleophilicity.524 The resultant foam presented oil/water separation, self-cleaning, and super-repellency toward corrosive liquids. Recently, through the fusion of different optimized biological solutions from the lotus leaf, the water strider leg, and the bird bone, multiscale metallic foams with low adhesive superhydrophobicity were fabricated by Liu et al., which showed oil/water separation, striking loading capacity, self-cleaning, and superior corrosion resistance.525 Until now, the research field of oil−water separation materials has mainly focused on the synthesis of superhydrophobic and oleophilic surfaces. Recently, Jiang et al. fabricated a novel hydrogel-coated mesh with superhydrophilicity and underwater superoleophobicity for oil/water separation, which exhibited completely opposite wettability toward conventional hydrophobic and oleophilic structues.526 This novel material can selectively separate water from oil/ water mixtures such as gasoline, diesel, vegetable oil, and even crude oil/water mixtures effectively, possessing recyclability and resistance to oil fouling.527 This is a new attempt for the design of next-generation functional materials used in oil−water separation. The introduction of stimuli-responsive polymers to achieve intelligent materials for controllable oil−water separation should be an important issue in the near future. For the traditional separation technology, it is difficult to separate micrometer-scale oil droplets from water. Inspired by the micrometer-scale water droplets collection on conical cactus spines, Jiang et al. also prepared an oleophilic array of conical needle structures, which can be used for the collection of micrometer-size oil droplets from water.534 Underwater, the resultant structured cone arrays could capture micrometer-size oil droplets and transport them toward the base of conical needles, showing high continuity and high throughput. Recently, Yu et al. first fabricated zeolite-coated mesh films with superhydrophilic and underwater superoleophobic properties, exhibiting gravity-driven oil−water separation.528 Water can permeate through the film, whereas oils are retained on the film. Furthermore, the resultant films showed stable corrosion resistance in the corrosive media, which is impossible for their practical application in industrial fields.

fabricated and used in oil and water mixture separation (Table 5), which provide an important avenue for the development of advanced separation techniques. Table 5. Typical Surfaces with Superwettability Applied to Oil−Water Separation substrate

method

wetting property

ref 522 529 530

PIP

hydrophobic/oleophilic hydrophobic/oleophilic superhydrophobic/ superoleophilic superhydrophobic/ superoleophilic superhydrophobic/ superoleophilic superhydrophobic superhydrophobic/ superoleophilic superhydrophobic/ superoleophilic superhydrophobic/ superoleophilic superhydrophobic/ superoleophilic superhydrophobic/ superoleophilic superhydrophobic/ superoleophilic superhydrophobic/ superoleophilic superhydrophobic/ superoleophilic underwater superoleophobic

steel

sol−gel

hydrophobic/oleophilic

539

steel

spray-and-dry

superhydrophobic/ superoleophilic superhydrophobic/ superoleophilic

519

CNT sponge CNT sponge copper mesh

CVD CVD ECD

copper mesh

cryptomelane filter paper

solutionimmersion solutionimmersion self-assembly dip-coating

PDVB

solvothermal

polyester textile

CVD

PS

airbrush

PU−PS

self-assembly

PU sponges steel

solutionimmersion CVD

steel

dip-coating

steel

copper mesh

stainless mesh stainless mesh stainless mesh stainless mesh stainless mesh TPU

electrospinning

531 532 523 533 520 521 534 535 536 537 538 526

540

The first work concerning superhydrophobic/superoleophilic materials for oil−water separation was reported by Jiang et al.519 Analogous to the surface structure of the lotus leaf, Teflon-coated stainless steel meshes with both superhydrophobic and superoleophilic properties were fabricated by a spray-and-dry method. Mixtures of water and diesel oil can be successfully separated using this superwetting mesh film. On the basis of this strategy, industrial machines with high oil− water separation efficacy have been developed, which are now commercialized and equipped on sea ships. Xiao et al. fabricated nanoporous polydivinylbenzene materials with both superhydrophobicity and superoleophilicity through a solvothermal approach.520 These nanoporous polymers demonstrated preferential selectivity for various organic compounds, as compared to conventional activated carbon adsorbent, which can be reused again after a simple low-pressure distillation. Seeger et al. reported the fabrication of superhydrophobic and superoleophilic polyester textile through chemical vapor deposition.521 Because of the superwetting properties and flexibility of the textile, the resultant polyester materials exhibited high oil/water separation efficiency, selective oil absorption capacity, and reusability. Wu et al. synthesized hydrophobic/oleophilic carbon nanotube sponges with threedimensionally interconnected framework.522 These obtained sponges can be deformed into differen shapes and compressed

9.7. Printing and Reprography

With increasing demands for an environmentally friendly, lowcost, high-resolution patterning technique, extensive efforts have been devoted to improving or replacing the traditional printing and reprography technology. Because surface wettability plays an important role in printing and reprography, a great number of studies have been focused on the precise control of liquid pattern on solid surfaces with superwettability. Currently, offset printing is the dominating technology for printing of magazines, advertising leaflets, and books. This 8274

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desired self-cleaning function. Therefore, TiO2 has a wide variety of practical applications in self-cleaning and related fields. Since the late 1990s, a great number of superhydrophilic self-cleaning coatings have been commercialized, including glass, tiles, plastic films, cement, tent materials, and others.43b,437a,b In China, TiO2 nanoparticles-coated selfcleaning glass has been used in the National Opera Hall.543 In Japan, self-cleaning tiles and windows have been utilized in “eco-life”-type houses since 2003.437b The story of self-cleaning surfaces begins in nature with the sacred lotus, which has been a symbol of purity in Asia. Water droplets on lotus leaves showed the spherical shape and can roll freely in all directions, where dirt particles are easily removed. The cooperation of surface hydrophobic epicuticular wax and multiscale structures results in a high static water CA and a small SA, which is essential for the final formation of superhydrophobicity-induced self-cleaning. Up to now, many different approaches have been developed to prepare superhydrophobic self-cleaning surfaces by creating optimal surface geometrical structures and other specific components. In the past decade, a great number of commercial superhydrophobic self-cleaning products have been available to the consumer. For example, Jiang et al. developed self-cleaning nanoties with both water-repellent and oil-repellent functions.544

technology is based on anodized aluminum plates, where the hydrophobic and hydrophilic areas are wetted selectively by oilbased ink and water, respectively. For color printing, it requires usually 3−4 plates that cannot be recovered. Recently, a new type of offset-printing plate was developed by Fujishima et al. using superhydrophobic−superhydrophilic patterns on Ti substrates.541 The resolution for color printing can reach 150 lines per inch. TiO2-based printing plates were reusable after elimination of the patterns through photocatalytic decomposition under UV irradiation, demonstrating a new, resourcesaving, environmentally friendly printing technology. Recently, Jiang et al. reported patterned wettability transition through photoelectric cooperative and anisotropic wettability for liquid reprography.542 Utilizing a photoelectric cooperative wetting process, the patterned wettability can be changed from the Cassie state to the Wenzel state on a superhydrophobic aligned ZnO-nanorod array surface. Liquid reprography can be achieved through the patterned wettability transition. To improve the mechanical strength of nanostructures and the controllability of patterned wettability, Jiang et al. also fabricated superhydrophobic aligned-nanopore arrays using TiO2-coated nanoporous anodic aluminum oxide films. The aligned nanopore array provided a photoelectric cooperativeinduced patterned wettability for reusable and robust printing, demonstrating a novel avenue for liquid reprography. On the basis of nanomaterials with superhydrophilic/ superoleophilic properties, Song et al. developed a green printing plate-making technology. By tuning the surface energy of the imaging materials (oleophilic and hydrophobic) and the micro-/nanoscaled structure of the plate substrate (superhydrophilic), the spreading of the printed microdroplets can be well controlled arising from the surface energy contrast between the ink solution and the plate substrate. The printing plate can be used to obtain colorful images with fine resolution for thousands of times, exhibiting good durability. Comparing with the traditional laser typesetting plate-making and computer to plate technologies, the direct inkjet plate technique can efficiently decrease the waste of the plating materials and thus make the process economical and environmentally friendly, which have promising applications as green printing processes in the printing industry.

9.9. Water Harvesting

Water is essential for all living creatures. Currently, water shortage and scarcity has become a global problem. For example, about one billion human beings live without access to clean water sources in Asian, African, and Latin American countries. In nature, some biomaterials have evolved different solutions to achieve versatile and effective abilities for water collection, such as Stenocara beetle, cribellate spider, and Cotula fallax plant.63,64,545 Learning from nature, harvesting water directly from the atmosphere should be an alternative approach to solve the problem of water shortage, especially in dry and fog-laden regions. As discussed in section 3.12, Stenocara beetles in the Namib Desert collect drinking water from fog using hydrophilic and hydrophobic patterns on their backs.63 Inspired by the water harvesting surface of the Namib Desert beetle, Cohen et al. fabricated hydrophilic patterns on superhydrophobic surfaces with water collection characteristics, which is similar to the Stenocara beetle.546 Badyal et al. prepared plasmachemical patterned superhydrophobic−superhydrophilic surfaces to mimic the Stenocara beetle’s back for water collection.547 It was found that the optimal hydrophilic pixel size/center-tocenter distance compares favorably with the hydrophilic/ hydrophobic patterned back of the Stenocara beetle. Mimicking the back of the Stenocara beetle, Rühe et al. prepared various superhydrophobic surfaces patterned with hydrophilic to hydrophobic domains.548 They found the droplets will dewet from the hydrophilic area once a critical volume has been reached due to the gravity effect. This indicated, for a given bump, the pinning force was constant and was independent of the drop volume. Ju et al. demonstrated that hydrophilic dots on patterned hydrophobic surfaces can be used as a flexible gas barrier.549 This concept could be effective to fabricate gas barrier films with enhanced performances. Recently, Harris et al. developed a facile and scalable method to fabricate Stenocara beetle-inspired surfaces for atmospheric water capture.550 These coatings were prepared by dewetting thin polymer films consisting of raised hydrophilic bumps on a hydrophobic

9.8. Self-Cleaning

Self-cleaning coatings have a variety of promising practical applications in daily life, agriculture, industry, and military. In the past few decades, a great number of synthesis strategies have been developed to prepare self-cleaning functional materials.42b,e,43a−g,437 Nowadays, many self-cleaning coatings have been commercialized ranging from glass, cement, tile, to textiles. As a commercial product, their potential is huge and their market truly global. Currently, self-cleaning materials can be classified into the following four categories: (i) TiO2-based superhydrophilic self-cleaning; (ii) the lotus effect-inspired superhydrophobic self-cleaning; (iii) gecko setae-inspired dry self-cleaning; and (iv) marine organisms (such as shark skin and pilot whale skin)-inspired antifouling self-cleaning. Here, we will mainly discuss superwettability-induced self-cleaning. Superhydrophilic surfaces are ubiquitous in nature, which endows biomaterials with self-cleaning. Titania (TiO2) is one of the most promising functional materials arising from its unique physical and chemical characteristics.26 It is well-known that TiO2 possesses photocatalysis and photoinduced superhydrophilicity. These special photoinduced properties resulted in the 8275

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substrate. The resultant polymer films with improved water droplet detachment could collect more atmospheric water than a corresponding flat film. In addition to the outstanding mechanical property, spider silk is revealed to possess the directional water collection ability due to its periodic spindle-knot structure.64 The water collection mechanism of spider silk is different from that of Stenocara beetle. Inspired by the capture silk of the cribellate spider, a series of artificial spider silks with spindle-knots have been fabricated by Jiang et al.64,551 Through optimizing the curvature, roughness, and chemical gradients, tiny water drops can be driven with controllable direction (“toward” or “away from” the knot) on the fiber surfaces.551a This work should pave the way for the design and construction of smart materials and devices to drive tiny water drops with a controllable manner. To reveal the effect of experimental conditions on the geometric parameters of the spindle-knots and hence water collection efficiency, a series of bioinspired artificial spider silks were fabricated.551b It was found that the solution surface tension and viscosity as well as the fiber drawing-out velocity are crucial experimental parameters to obtain optimal periodic spindle-knots with different sizes. Artificial spider silks with larger spindle-knots could collect more water than those with smaller spindle-knots. To prepare durable and inexpensive bioinspired fibers for directional water collection on a large scale, a fluid-coating method was developed to continuously fabricate periodic spindle-knots on nylon fibers.551c These bioinspired fibers showed directional water collection in the foggy environment.

such as re-entrant surface curvature, also brings some questions. For self-cleaning superhydrophobic surfaces, several main aspects need further exploration: to synthesize low-cost materials, to develop large-scale preparation, and to realize important multifunction. Moreover, the mechanical durability of superhydrophobic surfaces remains a great challenge today for their practical applications. The organic and inorganic hybrid may be a possible solution for pure polymeric materials. The solution of these problems will pave the way for superhydrophobic surfaces to practical applications. Moreover, it must be noted that the liquid or gas also occupies the equal important place to the solid at the surface wettability. The liquid/liquid/solid interfaces, such as the water/oil/solid interfaces, have been emerging as a new branch in this field. Very recently, inspired by pitcher, a kind of omniphobic surface reported by Aizenberg’s group gave us a new vision of superwettability from practical applications and fundamental sciences, far beyond only one example. Beyond two-dimensional surfaces with superwettability, the study on one-dimensional surfaces like fibers or channels just starts at their early stage. Accordingly, their artificial surfaces will become one of the recent focuses in surface science and nanotechnology because of their wide and important applications like collecting fog or collecting oil microdroplets from water waste. Especially, the complex practical conditions require the environmentally robust or smart surfaces with special wettability. Learning from nature is our constant principle for surface design and fabrications. There are numerous mysterious wetting properties in nature that will give us inspiration to understand the principal mechanism behind these unique wetting phenomena and to develop novel interface materials. It is self-evident that these studies on bioinspired surfaces with superwettability accelerate the drastically developing areas of surface chemistry and surface coating technology. We must point out that those surfaces with superwettability can also highly impact other fields, such as surface pattering, microreactor, biosensor, condensation, heat transfer, and so on. To take superhydrophobic surfaces as an example, a high adhesive superhydrophobic surface was recently demonstrated as a platform for surface patterning of various materials from inorganic, organic, to biological species, such as macromolecules, nanoparticles, nanowires, and cells as well. Another case is to condense the trace amount of biomolecules on a droplet on a superhydrophobic surface, because a few, even one, biomolecules will condense onto a very tiny point after the evaporation of the droplet.

9.10. Other Applications

In addition to the above-mentioned applications, special wetting surfaces also demonstrated important applications in microfluid, shielding, solar cell, sensor, and other fields. Low adhesive superhydrophobic surfaces are one suitable candidate for droplet-based microfluidic systems due to the high mobility of liquids on such surfaces. Sikorski et al. fabricated copperbased superhydrophobic surfaces, which can be used in wireguided droplet microfluidic systems.552 Large-area polymerbased superhydrophobic carbon nanofiber coatings were prepared by Megaridis et al., which showed tunable electromagnetic interference shielding and attenuation in the terahertz frequency region.553 Choi et al. reported an ordered microshell array on the flexible and transparent polydimethylsiloxane elastomer surface for solar cell applications.554 It was found the degradation of solar cell efficiency by dust can be reduced through the construction of superhydrophobic coatings on the solar cell module.

10. CONCLUSION AND PERSPECTIVE In this Review, we have concluded the recent progress on bioinspired surfaces with superwettability, from unique biological examples to new understanding on its theory, from artificial surface with superwettability to dynamical wettability switches, and from proof-of-concept to practical applications as well. However, surface wettability is a complex scientific problem involving many parameters on surfaces. Especially some emerging new phenomena of surface supper-wetting bring new challenges to the traditional theory and concept; therefore lots of experimental explorations are needed for practical applications. For example, because 65° probably is the new limit between hydrophilicity and hydrophobicity, how will Wenzel’s equation be changed? Unique surface topography,

AUTHOR INFORMATION Corresponding Author

*Fax: +86-10-82621396. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 8276

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Biographies

Xi Yao received his Ph.D. under the supervision of Prof. Lei Jiang at ICCAS in 2011. He then did his postdoctoral training with Prof. Joanna Aizenberg at the School of Engineering and Applied Sciences and Wyss Institute of Bioinspired Engineering at Harvard University. He joined City University of Hong Kong as a tenure-track assistant professor in September 2014. His research interests focus on natural phenomena at multiscale surfaces and interfaces, such as self-cleaning lotus leaves, water walker:water striders, and slippery leaves of pitcher plant.

Shutao Wang received his Bachelor’s degree (2000) and Master’s degree (2003) from Northeast Normal University under the supervision of Prof. Enbo Wang, and his Ph.D. in 2007 from the Institute of Chemistry Chinese Academy of Sciences (ICCAS) under the supervision of Prof. Lei Jiang. He then worked in the Department of Molecular & Medical Pharmacology and California NanoSystem Institute at the University of California at Los Angeles, as a postdoctoral researcher in Hsian-Rong Tseng’s group (2007−2010). He was appointed as a full Professor of Chemistry in 2010 at ICCAS. Currently he is a full Professor of Chemistry at the Technical Institute of Physics and Chemistry Chinese Academy of Sciences (TIPCCAS). His research interests include the design and synthesis of bioinspired multiscaled materials with special adhesion and their applications at the nanobiointerface.

Lei Jiang received his Ph.D. degree (1994) from Jilin University in China (Tiejin Li’s group). He then worked as a postdoctoral fellow in Prof. Akira Fujishima’s group at Tokyo University. In 1996, he worked as a senior researcher in the Kanagawa Academy of Science and Technology under Prof. Kazuhito Hashimoto. In1999, he joined ICCAS as part of the Hundred Talents Program. In 2009, he was elected academician of the Chinese Academy of Science. He is currently a professor at ICCAS, TIPCCAS, and Dean of the School of Chemistry and Environment, Beihang University. His research interest focuses on bioinspired interfacial materials with superwettability.

ACKNOWLEDGMENTS We are grateful for financial support by the National Research Fund for Fundamental Key Projects (2012CB933800, 2013CB933000, 2011CB935700, 2012CB933200, 2012CB34100), the National Natural Science Foundation (21434009, 21121001, 21127025, 21175140, 21273016, 21071148, 20920102036), the Program for New Century Excellent Talents in University, Beijing Natural Science Foundation (2122035), and the Key Research Program of the Chinese Academy of Sciences (KJZD-EW-M01).

Kesong Liu received his Ph.D. degree (2006) in materials science at Harbin Engineering University under the supervision of Prof. Honggang Fu and Prof. Milin Zhang. He then worked as a postdoctoral fellow (2006−2008) in Prof. Lei Jiang’s group. He is currently an associate professor at the School of Chemistry and Environment, Beihang University. His research interests are focused on the design, construction, and application of bioinspired materials

ABBREVIATIONS AACA aluminum acetylacetonate

with multifunction integration. 8277

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Chemical Reviews AAO ACNA ACNTs AKD APFB APTMS BFEE CA CNT CTPEDT CVD DAK DCCD EA ECD EGD EHD EISA FAS f-PEG FSP GLAD HFBA HFCVD HFP/EMA HNA i-PP ITO LBL LCST LDPE MAMs MDA MEK MHA Ms MTMS MUABA NAF NC Hcp ODT OEG PAA PAH PAN PANI PC PCB PCL PDDA PDMAEMA PDMS PDVB PECVD PEI PEMA PET

Review

PFSEA PI PIP PLL PMETAC

perfluorosebacic acid polyimide photoinitiated polymerization poly-L-lysine poly[2-(methacryloyloxy)ethyltrimethylammonium chloride] PNIPAAm poly(N-isopropylacrylamide) PNIPAAm-co-AAc poly N-isopropylacrylamide-co-acrylic acid PNIPAM poly N-isopropylacrylamide PP polypropylene PPBA poly(3-acrylamidophenylboronic acid) PPy polypyrrole PMMA poly(methyl methacrylate) PS polystyrene PSS poly(sodium-p-styrenesulfonate) PS−PVP poly(styrene-co-2-vinylpyridine) PTES 1H,1H,2H,2H-perfluorooctyltriethoxysilane PTFE polytetrafluoroethylene PTBA poly(tert-butyl acrylate) PTS Teflon sheets PU polyurethane PUA polyurethane acrylate PVA poly(vinyl alcohol) PVC poly vinyl chloride PVD physical vapor deposition method PVP poly(2-vinylpyridine) RAFT reversible addition−fragmentation chain transfer polymerization RO radial outward SA sliding angle SAS self-adaptive surfaces SCLCP side chain liquid-crystal polymer SF semifluorinated SI-ATRP surface-initiated atom-transfer radical polymerization SmA smectic A SQUID superconducting interference device SRGP simultaneous radiation-induced graft polymerization STA stearic acid TCL three-phase solid/liquid/gas contact line TFSI bis trifluoromethane sulfonamide TMMOS trimethylmethoxysilane TMS tetramethylsilane TPU thermoplastic polyurethane XRD X-ray diffraction

anodic aluminum oxide aligned composite nanorod array aligned carbon nanotubes alkylketene dimmer allypentafluorobenzene aminopropyltrimethoxysilane boron trifluoride−diethyl etherate contact angles carbon nanotube carboxyl-terminated polyaryl ether dendron thiol chemical vapor deposition dialkylketone N,N′-dicyclohexylcarbodiimide electrostatic attraction electrochemical deposition electroless galvanic deposition electrohydrodynamics evaporation-induced self-assembly fluoro-alkyl silane perfluorinated end-capped polyethylene glycol flame spray pyrolysis glancing angle deposition 2,2,3,3,4,4,4-heptafluorobutyl acrylate hot filament chemical vapor deposition hexafluoropropylene/ethyl methacrylate hydrofluoric-nitric−acetic acid isotactic polypropylene indium tin oxide layer-by-layer lower critical solution temperature low density polyethylene mechanically assembled monolayers 11-mercaptoundecanoic acid methyl ethyl ketone (16-mercapto)hexadecanoic acid magnetization methyltrimethoxysilane 2-(11-mercaptoundecanamido)benzoic acid normal adhesive force nitrocellulose hexagonally close-packed octadecanethiol oligo(ethylene glycol) poly(acrylic acid) poly(allylamine hydrochloride) polyacrylonitrile polyaniline polycarbonate polycarbobetaines polycaprolactone poly(diallyldimethylammonium chloride) poly(2-(dimethylamino)ethyl methacrylate) poly(dimethylsiloxane) polydivinylbenzene plasma enhanced chemical vapor deposition polyethylenimine poly ethylene-maleic anhydride poly ethylene terephthalate

REFERENCES (1) Young, T. An Essay on the Cohesion of Fluid. Philos. Trans. R. Soc. London 1805, 95, 65−87. (2) Wenzel, R. N. Resistance of solid surfaces to wetting by water. Ind. Eng. Chem. 1936, 28, 988−994. (3) Wenzel, R. N. Surface Roughness and Contact Angle. J. Phys. Colloid Chem. 1949, 53 (9), 1466−1467. (4) Cassie, A. B. D.; Baxter, S. Wettability of porous surfaces. Trans. Faraday Soc. 1944, 40, 546−550. (5) (a) Bartell, F. E.; Shepard, J. W. Surface Roughness as Related to Hysteresis of Contact Angles 0.2. The Systems Paraffin-3 Molar Calcium Chloride Solution-Air and Paraffin-Glycerol-Air. J. Phys. Chem. 1953, 57 (4), 455−458. (b) Fowkes, F. M. Attractive Forces at Interfaces. Ind. Eng. Chem. 1964, 56 (12), 40. (c) Timmons, C. O.; Zisman, W. A. Effect of Liquid Structure on Contact Angle Hysteresis. J. Colloid Interface Sci. 1966, 22 (2), 165. 8278

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