Bioinspired Smart Materials for Directional Liquid Transport - Industrial

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Bioinspired smart materials for directional liquid transport Ying Cui, Dewen Li, and Hao Bai Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b00583 • Publication Date (Web): 30 Mar 2017 Downloaded from http://pubs.acs.org on April 2, 2017

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Industrial & Engineering Chemistry Research

Bioinspired smart materials for directional liquid transport Ying Cui1, Dewen Li1, and Hao Bai1* State Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China bioinspired materials, directional liquid transport, heat transfer, separation, microfluidic

Bioinspired materials capable of driving liquid in a directional manner have wide potential applications in many chemical engineering processes, such as heat transfer, separation, microfluidics and so on. Numerous natural materials and systems such as spider silk, cactus, shorebirds, desert beetles, butterfly wing, and Nepenthes alata have been serving as a rich source of inspirations in the area. During the last decades, great efforts have been devoted to design bioinspired smart materials for directional liquid transport. In this review, we will start from introducing several natural materials and systems with surface structural features contributing for their directional liquid transport property, followed by the basic concepts and theories about surface wettability, droplet motion and driving forces with different structural features. Then, we will summarize some typical applications of such bioinspired smart materials in industrial process and chemical engineering, particularly in heat transfer, separation, microfluidic systems.

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At the end, future perspectives of such bioinspired smart materials for directional liquid transport will be discussed.

1. INTRODUCTION Directional liquid transport is crucially important in many industrial applications and chemical engineering processes, such as heat transfer, separation, microfluidics and so on1. Many natural materials are capable of driving water directionally and spontaneously by controlling the interaction between their micro/nano surface structures with water, i.e., wettability. Inspired by such intriguing properties and designing principles, people have made great progress during the past decades in design and fabrication of smart materials for directional liquid transport. Those bioinspired materials with superwettability have aroused more and more attention from both basic scientific community and industry for their applications in energy-related areas, environmental protection, healthcare, and so on. In this review, we will start from introducing some natural materials and systems that have developed unique surface structural features for directional liquid transport. Then, we will try to understand those intriguing phenomena discovered in distinct natural systems based on the basic concepts and theories about surface wettability, droplet motion and driving force analysis. In the third section, we will summarize some typical applications of such bioinspired materials with directional liquid transport property, particularly in heat transfer, oil/water separation, and microfluidics. Besides the fundamental theories, we will summarize some typical applications of such bioinspired materials with directional liquid transport property, particularly in heat transfer, oil/water separation, and microfluidics, which are crucial in chemical engineering and industrial


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processes. In addition, perspectives and future research direction of bioinspired materials with directional liquid transport property will be discussed. Natural materials capable of transporting liquid directionally and usually spontaneously have been serving as a rich source of inspirations for developing bioinspired smart materials with special wettability, and designing efficient devices and systems. Some prime examples are as spider silk, cactus spine, shorebird’s beak, butterfly wing, and so on. Some spider silk (Figure 1a), after wetted in humid air, changes its morphology into a periodic spindle-knot structure covered with aligned nanofibers, which enables it to directionally drive and efficiently collect submillimeter water droplets from fog2. Similar in cactus (Figure 1b), the directional collection of condensed fog droplets is achieved through a system of well-distributed clusters of conical spines and trichomes on the cactus stem. The surface energy and cone-induced Laplace pressure gradients are believed to be the main driving forces for the directional liquid transport phenomena on these one-dimensional thread-shaped structures3. For shorebirds with long beaks (Figure 1c), water droplets containing nutrients have to be transported directionally in a relatively long way from the tip. They succeed in raising water droplets into their mouths by opening and closing their beaks successively4. Some butterflies such as Morpho aega (Figure 1d) have ratchet-like scales on their wings, which create an anisotropic superhydrophobic surface to directionally drive water droplets away from their body5. Beetles in Namib desert (Figure 1e) collect water from foggy wind on their backs with patterned wettability, where water droplets are directionally transport from the wax-covered hydrophobic region to the separated hydrophilic humps6. In carnivorous plant Nepenthes alata (Figure 1f), continuous, directional water transport is achieved on its peristome surface, resulted from its unique structural feature of two-order microgrooves with periodic duck-billed microcavities, which optimizes and enhances capillary



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rise7. During the past decades, significant progress has been made in unravelling the secrets lying in the directional liquid transport property of more and more natural materials and systems. Inspired by those natural examples, great efforts have been also devoted to develop bioinspired smart materials for directional liquid transport (Figure 2). The key for driving droplet directionally is exerting unbalanced forces at its opposite side. Ideally, such gradients can also be created in situ by external stimuli, such as light8-11, temperature12, vibration13, and magnetic field14, 15, which work individually or collaboratively to transport liquid droplet. Bioinspired materials with directional liquid transport property have already been demonstrated beneficial for a wide range of applications (Figure 2). For example, smart surfaces, which favor spontaneous droplet removal by interacting with evaporative and condensate liquid, are highly desirable in efficient phase-change-based heat transfer. Superhydrophilic/superhydrophobic surfaces have exhibited significant enhancement in evaporation/condensation during the heat transfer process16, 17

. Another example goes to oil/water separation. Biomimetic surface with arrays of artificial

cactus spines has endowed efficient and continuous oil/water separation by collecting tiny oil droplets in a directional manner. Compared to traditional materials used for oil/water separation, such as membrane-based separating materials, bulk absorbing materials, and hygro-responsive membrane, the artificial cactus spine arrays are not only anti-fouling but also achieves high throughout separation18. Moreover, microfluidic system involving self-powered liquid transport would be more energy-favorable without aid of additional components as micro-pumps. Those microfluidic technology provide a more energy favorable and versatile way to precisely manipulate liquids in controlled direction with high speed and over a long-range9.

2. BASIC THEORIES


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In this section, we will go over some basic concepts and theories for directional liquid droplet motion. A liquid droplet on a solid surface tends to move if there is an unbalanced force acting on its opposite side. If the force is large enough to conquer the resistance during motion, the droplet will move. The equilibrium contact angle at the three-phase contact line (TCL) can be described by the Young’s equation, which is expressed as (Figure 3a) γcosθ = ߛௌீ − ߛௌ௅ ,

(1)

where γ represents the interface tension while subscript L, G, S denote the liquid, gas and solid respectively, and θ is the contact angle of the droplet on the solid surface. At the equilibrium state, there is no driving force for droplet motion. However, if the solid surface has a surface energy gradient, which could be introduced by a surface tension or roughness gradient, the droplet tends to move from higher surface energy side to the lower, or more wettable side (Figure 3b). In addition to surface energy gradient, the gradient of Laplace pressure could also lead to droplet motion. We know that there is a pressure difference on a curved liquid surface, which is the Laplace pressure, expressed as ∆P =

2γ R

(2)

where γ is the surface tension of liquid, and R is the radius of surface. If there is a droplet on a conical surface, there will be a driving pressure (Figure 3c), directing the droplet toward lower curvature, which can be expressed as19: ∆P = − න

௫ಲ

௫ಳ

2ߛ ߙ݀‫ݔ‬ ሺܴ + ܴ଴ ሻଶ

(3)



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where R is the local radius of the cone, R0 is the droplet radius, α is the half-apex angle and xA, xB are the position of the front and rear of the droplet respectively. If the droplet is inside the cone (Figure 3d), the driving pressure is different, expressed as9: ∆P = −ሺ

4ߛܿ‫ ߠݏ݋‬4ߛܿ‫ߠݏ݋‬ − ሻ ‫ݔ‬஻ ߙ ‫ݔ‬஺ ߙ

(4)

where parameters are defined in Figure 3d. γ is the liquid surface tension. θ is the contact angle between the liquid and the solid. α is the semi-apex angle of the cone. xA and xB are the distances of the liquid surface (side A and B) to the apex of the cone. The first item is the Laplace pressure of side B, directing toward the apex, while the second is the Laplace pressure of side A, directing away the apex. The difference of these two pressure forces drives the droplet to move toward higher curvature. There are many natural phenomena about directional droplet motion can be understood by the above mentioned basic theories. For example, droplets in a shorebird beak4 are similar as in a conical tube (Figure 4a), where water droplets could move into its mouth. A droplet on the superhydrophobic wings of butterfly Morpho aega (Figure 4b) easily rolls off along the radial outward (RO) direction of the central axis of the body, but gets pinned tightly in the reverse direction5. Such unique wettability results from the directional dependent arrangement of micro/nano ratchet-like structure. Along the RO direction, droplet on the oriented micro/nano ratchet-like structure forms discontinuous TCL, easier to roll off. While against the RO direction, the micro/nano ratchet-like structure take a close arrangement to form a quasi-continuous TCL, which pins the droplet on the surface. Besides shorebird and butterfly, some wetted spider silk is also capable of driving droplets directionally. The spindle-knot structure of wetted spider silk2 can be considered as two interconnected half-cones and is interweaved by highly random nanofibers with roughness


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gradient on surface (Figure 4c). Such unique surface structure generates both difference in Laplace pressure and surface energy gradient at the same time, which drives tiny water droplets from the joint to the spindle-knot. The similar principle has been observed in the fog collection system of some cactus spine3 (Figure 4d). Besides the conical shape of the spine, there are many microgrooves on the spine surface, sparser near the base than near the tip, enhancing the gradient of Laplace pressure, which drives the water droplets from the tip to the base. Another directional water transport phenomenon in nature could be observed on the peristome surface of Nepenthes alata7 (Figure 4e). Water droplet can be directionally transported against gravity from the inner side to the outer side of the peristome, but inhibited in the opposite direction, which is attributed to the arch-shaped micro-channels with a spacing of approximately 100 µm. From an energy point of view, Chen at el. get the total energy of the system of water in a gradient wedge by balancing between the interfacial potential and the gravitational potential. When minimizing the total energy with respect to the height of the water surface, the equilibrium height He(x) could be deduced as: ‫ܪ‬௘ ሺxሻ =

2ߛܿ‫ߙ ߠݏ݋‬ଵ − ߙଶ ଶ + ‫ܪ‬௘ ሺ‫ݔ‬ሻ ߩ݃‫ߙݔ‬ଵ ߙଵ ℎ

(5)

where parameters are defined in Figure 4f, assuming β1 ≈ β2 ≈ 90°. γ is the liquid surface tension. θ is the contact angle. H(x) is the height of the liquid surface at position x. ρ, g and h denote the liquid density, gravitational acceleration, and height of the intersecting plates. α1 and α2 are the opening angles at the bottom and the top of the micro-cavity model, respectively. β is the gradient opening angle. We could easily arrive at the relationship: ‫ܪ‬௘ ሺxሻ|ఈభ வఈమ > ‫ܪ‬௘ ሺxሻ|ఈభ ୀఈమ

(6)

It shows that a wedge with gradient wedge angle presents higher equilibrium height than that without gradient, which helps in the directional water transport on the peristome surface.



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3. APPLICATION 3.1 Heat Transfer Heat transfer is important for thermal management and energy saving in industrial applications. Phase-change-based heat transfer is the main choice in many chemical engineering processes, which mostly involves evaporation, condensation, liquid transport from the condenser section to the evaporator section. In the following, we will discuss some recent progresses in using bioinspired smart surfaces to improve the efficiency of phase-change-based heat transfer, including evaporation and condensation. Evaporation: The most common mode of evaporation in heat transfer process is pool boiling, in which the liquid is stationary and in contact with a heated surface. Boiling involves three processes, including nucleation, growth, and departure of vapor bubbles. During the conventional boiling process, the high density of vapor bubbles on the heated surface usually leads to vapor film formation, which acts as a thermal barrier hindering heat transfer. Ideally, superhydrophilic surface can minimize vapor film formation by promoting bubble departure and rewetting thereafter. As a result, nanostructured hydrophilic/superhydrophilic surfaces are widely studied to enhance boiling performance, which reduces the thermal conductivity and hence heat transfer efficiency. Various approaches have been developed to fabricate hydrophilic/superhydrophilic surfaces to improve the boiling efficiency20-46. Firstly, introducing nanostructure onto a hydrophilic surface can increase its wettability according to the Wenzel’s equation27, 32. For example, Peterson et al. used an oblique-angle deposition method to grow copper nanorods on smooth copper substrates, reducing the water contact angle from 55° to 38.5° for the smooth and nanostructured surface,


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respectively27. Besides, surfaces with hierarchical structures that combine micro- and nanostructures can also decrease the contact angle and improve boiling efficiency42. Condensation: Condensation is another important phase-change-based heat transfer process. Condensation-based heat transfer involves two processes, including droplets condensation on the surface and the removal of condensed droplets. During the conventional condensation process, the condensed droplets usually form an insulating liquid film that sticks on the surface, which has low thermal conductivity and hinders further heat transfer. Owing to their large contact angle, hydrophobic/superhydrophobic surfaces appear to be an ideal solution to prevent the formation of liquid film and accelerate the removal of condensed droplets. Such removal can promote continuous nucleation and growth of new droplets. Hence, signiﬁcant interest and great efforts have been devoted to design hydrophobic/superhydrophobic surfaces to improve the condensation-based heat transfer efficiency16, 17, 47-70. Daniel et al. used a diffusion-controlled silanization method to fabricate a surface with wettability gradient. They showed that such surface could be used as the heat exchanger to eliminate the insulating liquid film that sticks on the surface and accelerate heat transfer process51. When a small droplet of silane was put in the center of a silicon surface, the silane droplet evaporated and diffused radically on the silicon surface. The center of the silicon surface was thus hydrophobic and the edge was hydrophilic, driving condensed droplets move rapidly from the center towards the edge. The driving force for the spontaneous droplets removal came from the surface wettability gradient. Miljkovic et al. fabricated a silanized copper oxide (CuO) superhydrophobic surface with nanostructures and studied the phenomenon of droplets coalesce during condensation on such surface. The condensed droplets can jump from the surface due to the release of excess surface



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energy58 (Figure 5a). Furthermore, these nanostructured superhydrophobic surfaces can not only promote removal of micrometer sized droplets but also improve heat transfer efficiency. They experimentally demonstrated a 25% higher overall heat flux and 30% higher condensation heat transfer efficiency compared to state-of-the-art hydrophobic condensing surfaces. This work applied a low cost and scalable approach to significantly enhance condensation-based heat transfer performance. Chen et al. prepared a surface with hierarchical structures that combine micro- and nanostructures and obtained the Cassie state under the condensation condition by mimicking the hierarchical surface structure of lotus leaf48 (Figure 5b). On such hierarchically structured superhydrophobic surfaces, silicon micropillars were covered by short carbon nanotubes, both of which were modified with a hexadecanethiol coating. It was found that the fabricated surface was robust enough to maintain superhydrophobicity during droplet condensation process. Such hierarchically structured superhydrophobic surfaces can accelerate the removal of condensed droplets and thus significantly improve the heat transfer efficiency. Wang et al. prepared a hierarchical nanograss micropyramidal surface with hybrid wettabilities to meet the requirements that superhydrophilic surface promotes droplet nucleation and the superhydrophobic surface promotes droplet departure during a continuous droplet condensation process49 (Figure 5c). The hybrid wettabilities surface was designed by mimicking the desert beetles,

which

can

collect

water

from

humid

air

by

their

unique

superhydrophilic/superhydrophobic patterned surface. The locally superhydrophilic nucleation sites facilitated droplet growth and coalescence, resulting an approximately 65% increase in the droplet density. On the other hand, the global superhydrophobicity of the surface promotes removal of the condensed droplets, approximately 450% increase.


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Varanasi et al. fabricated a superhydrophobic surface by sputtering a thin layer (200~350 nm) of ceria onto nanograss-covered silicon microposts47 (Figure 5d). Owing to the high contact angle and low hysteresis of such surface coated with rare earth oxides, the rapid removal of condensed droplets was realized for excellent heat transfer performance.

3.2 Separation The oil spill accidents happened in the Sea Island (1991) and the Gulf of Mexico (2010) have drawn attention from researchers in multidisplinary fields as oil spills have long-term effects on the environment. To tackle this problem, many cleanup techniques have been suggested, for example, in-situ burning, oil skimmer, utilization of absorbents, and so on. However, there are still many intrinsic limitations in those methods, such as high cost, poor recyclability and low flux. Bioinspired smart materials, usually membranes, allow only one liquid to pass through while block the other. For example, the membrane that is superhydrophobic and superolephilic, allows only oil pass through while block water, capable of separating the oil/water mixture. Feng et al.71 first proposed a superhydrophobic and superoleophilic mesh film (Figure 6a) for oil/water separation by a facile and inexpensive spray-and-dry method. But such superoleophilic mesh is easy to be contaminated and blocked. Xue et al. applied a superhydrophilic and underwater superolephobic PAM hydrogel-coated mesh to separate oil/water mixture72, which showed both high selectivity and efficiency (>99%). More importantly, it is anti-fouling for continuous separation as it is an oil removal approach. Khosravi et al.73 also reported a superhydrophobic and superoleophilic steel mesh by depositing carbon soot nanospheres on the mesh followed by polypyrrole vapor phase deposition, which could collect organic solvents from



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water surface. They demonstrated the facile and rapid collection of oil by shaping the material as a miniature boat (Figure 6b), the separation efficiency is still higher than 99% after 50 cycles. In practical applications, oil/water mixtures always contain many micron-sized oil droplets, which cannot be separated by phase separation. Li et al.18 fabricated a device for separating micron-sized oil droplets from water (Figure 6c), which is difficult for the most existing methods. Inspired by the collection of water droplets on conical cactus spines, they developed an oleophilic PDMS conical needle array for the collection of micron-sized oil droplets. Arun K. Kota et al.74 also reported membranes with hygro-responsive surfaces through surface reconfiguration (Figure 6d), which are superhydrophilic in air and superoleophobic under water, respectively. This hygro-responsive membrane can separate a range of different oil–water mixtures from free water and oil to emulsions in a single-unit operation by capillary force-based separation with >99.9% separation efficiency. In emulsion separation,Shi et al.75 also fabricated an ultrathin single-walled carbon nanotube (SWCNT) network film on a ceramic membrane (Figure 6e), with a tunable thickness of the tens of nanometer scale. This film could effectively separate both micrometer and nanometer-sized surfactant-free and surfactant-stabilized water-in-oil emulsions with flux 2–3 orders of magnitude higher than commercial membranes, and a high separation efficiency (> 99.95%). Besides separation, membranes with special wettability are also multifunctional. For example, adopting a strategy of wettability asymmetry, Li et al.76 fabricated a Janus mesh for oil/water separation by integrating superoleophobic and superoleophilic single-layer copper mesh, through which oil droplets could penetrate unidirectionally from the superoleophobic to the superoleophilic side but are jammed from the reverse direction underwater (Figure 6f). This


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unidirectional liquid transportation has important applications in the field of wastewater purification and liquid manipulation.

3.3 Bioinspired microfluidic devices Transport liquid in a controllable direction is important for microfluidic systems in both fundamental research and real applications. Microfluidic devices usually include microfluidic chips and many pumps and valves, which not only occupy large space, but also require additional external energy supply. Bioinspired microfluidic devices involving natural structures may help addressing the above-mentioned issues, which would be more energy-favorable without any aid of additional components in driving liquids. In the following, we will discuss some recent progresses on the directional liquid transportation in some bioinspired microfluidic devices like microactuator, and microfiber, micro ratchet-like surfaces that are potentially applicable for microfluidics. Firstly, cone-like tubular which can generate capillary force to propel liquid transportation have been applied to microfluidic devices. Recently, Yu and co-workers reported a new strategy for utilizing photo-induced asymmetric deformation of tubular microactuator to manipulate the transportation of liquid slug (Figure 7a)9. Inspired by the lamellar structure of artery wall, they designed a linear liquid crystal polymer (LLCP) and fabricated a tubular microactuator through a facile solution process. The asymmetric deformation of tubular microactuator arises from light-induced reorientation of azobenzene mesogens, resulting in not only the decrease of the thickness of the wall but also the increase of cross-sectional area of tubular microactuator. Because the cross-sectional area of tubular microactuator increases with the higher light intensity, the tubular microactuator become an asymmetric cone-like geometry under



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illumination, which generates capillary force to propel liquid slug towards the light attenuated direction. In addition, owing to the ordered lamellar structure and the high molecular weight, the tubular microactuator is strong and tough enough to be changed into various shapes, such as free-standing straight, serpentine and helical. These microactuators provide a straightforward and versatile solution to precisely manipulate long-range liquid transport with desirable velocity and pre-defined direction. Furthermore, tubular microactuator can also work as a micromixer and shows considerable potential in micro-pumps without any aid of additional components. This innovative strategy has a wide range of applications including liquid mixing, coalescence operation, and even capture and convey of the microsphere at microscale. In addition, bioinspired spindle-knots ﬁbers with directional liquid transport property have great potential application in energy-favorable microfluidic devices15, 77-91. By mimicking the silk-spinning mechanism and apparatus of spiders, Lee et al. (Figure 7b) used a microfluidic chip and a digital controller to fabricate bioinspired fibers with periodic spindle-knots89-91. Both the chemical composition and structure of the fiber can be varied by independently controlling the fluid volume within each inlet channel by the valve operation. Besides microfluidic technology, dip-coating78, fluid-coating77 and coaxial electrospinning85 can also be applied

to fabricate

spindle-knot fibers for controlling directional liquid transport. Such silk-mimetic fibers are potential in building up fiber-based open microfluidics79. Futhermore, bioinspired micro ratchet-like surfaces with directional liquid transport property also provide an alternative solution to make microfluidic devices more energy-favorable92-95. Liu et al. (Figure 7c) reported the wings of Morpho deidamia butterflies can directionally transport fog droplets in both static and dynamic state, resulting from their micro/nano ratchet-like surface structure95. Inspired by such unique structure, they fabricated a surface with tilted nanowires,


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which was superhydrophilic at ambient temperature, but superhydrophobic at 350°C. By subtly tuning the length of tilted nanowires and chemical characteristic of the surface, directional liquid transportation at high temperature could be realized. This work opened a door for designing two-dimensional surfaces for directional liquid transport in energy-favorable microfluidic devices.

4. PERSPECTIVE As discussed in the previous sections, great achievements have been made during the past decades in discovering more types of natural materials and systems with directional liquid transport property. Besides, the principles and strategies unraveled in the natural examples have been successfully transferred to synthetic materials and devices not only mimicking their structural features but also unique wettability. More importantly, such bioinspired smart materials have shown great potential in many industrial applications and chemical engineering processes, such as heat transfer, oil/water separation, and bioinspired microfluidic devices, as we summarized and emphasized in the previous sections. Despite all these achievements, there still remain many challenges in materials design and fabrication, such as (1) controlling extremely small droplets (micrometer scale), which is difficult owing to the contact angle hysteresis, (2) precisely and/or programmably control droplet moving direction, and (3) combine more driving forces to drive droplets faster and in a longer distance. Another main challenge is to broaden the applications of such bioinspired smart materials with directional liquid transport property in many other industrial processes including desalination, aerosol filtration, and anti-icing. We believe that with increasing knowledge obtained from nature, more and more bioinspired smart materials will be developed and applied



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in many strategic fields like energy-saving techniques, healthcare products, and environmental protection.


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Figure 1. Natural materials for directional liquid transport. a) Spider silk: periodic spindle-knots and joints. Scale bar represents 50 µm, 2 µm and 500 nm, respectively. b) Cactus spine: conical spines and trichomes on the cactus stem. Scale bar represents 5 cm, 100 µm, 20 µm, 20 µm, 20 µm, 2 µm, respectively. c) Shorebird: droplet trapped and transported along its beaks. d) Butterfly wing: ratchet-like oriented scales. Scale bar represents 100 µm. e) Desert beetles: hydrophilic humps surrounded by wax-covered hydrophobic region. Scale bar represents 10 mm and 10 µm, respectively. f) Nepenthes peristome: second-order microgrooves containing periodic duck-billed microcavities with arch-shaped edges.



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Figure 2. Typical applications of bioinspired materials with directional liquid transport property.


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Figure 3. Basic theories about directional transport of liquid droplet. a) Force analysis on the triple-phase contact line (TCL) based on Young’s equation. b) A droplet on a gradient surface tends to move toward more wettable direction. c) A droplet on a cone tends to move to the direction with lower curvature. d) A droplet in a cone tends to move to the direction with higher curvature.



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Figure 4. Principles of directional liquid transport on natural materials. a) Shorebird directionally transports water to its mouth by periodically opening and closing its beaks. b) A droplet easily rolls off along the radial outward (RO) direction on the wing of butterfly Morpho aega, but is pinned tightly against the RO direction. c) A water droplet moves from the side to the center of a spindle-knot on spider silk owing to the cooperative effect of both surface energy and Laplace pressure gradients. d) Cactus efficiently collects fog by driving tiny water droplets directionally from the tip to the base along its spine. e) Water droplet can be directionally transported against gravity from the inner side to the outer side of the peristome on the surface of Nepenthes alata, but not in the opposite direction. f) Capillary rise in a gradient wedge.


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Figure 5. Dropwise condensation enhanced by superhydrophobic surfaces with different micro/nano structures: nanostructures, micropillars, micropyramids, microposts. a) Copper oxide nanostructures modified with silane. b) A structured Si substrate deposited with carbon nanotube. c) A nanograss-decorated Si substrate with hybrid wettability. d) Nanograss-covered cubical microposts.



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Figure 6. Bioinspired materials with directional liquid transport property for oil/water separation. a) Superhydrophobic stainless steel mesh as an early attempt to apply superwettability for oil-water separation. b) A superhydrophobic and superoleophilic miniature boat remains high separation efficiency above 99% even after 50 cycles, showing its durability. c) A device for separating micron-sized oil droplets from water by conical oleophilic arrays inspired by cactus spine. d) Membranes with hygro-responsive surfaces through surface reconfiguration, which can separate a range of different oil–water mixtures from free water and oil to emulsions in a single-unit operation. e) An ultrathin single-walled carbon nanotube (SWCNT) network film on a ceramic membrane, which could effectively separate both micrometer and nanometer-sized surfactant-free and surfactant-stabilized water-in-oil emulsions. f) A functional separation mesh with Janus superwettability where oil droplets can only pass unidirectionally.


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Figure 7. Microfluidic systems involving directional droplet motion. a) Light-driven manipulation of liquid in tubular microactuator. b) Directional liquid transport on bioinspired fibers. c) Directional liquid transport based on butterfly-mimetic microratchet structure.



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AUTHOR INFORMATION Corresponding Author Hao Bai TEL: +86-0571-87951689 E-mail: [email protected] Present Addresses State Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources Notes Any additional relevant notes should be placed here. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (No. 21674098), State Key Laboratory of Chemical Engineering (No. SKL-ChE-16T02) and the ‘1000 Youth Talents Plan’ of China. ABBREVIATIONS TCL, three-phase contact line; LLCP, linear liquid crystal polymer.


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Bioinspired Smart Materials for Directional Liquid Transport - Industrial

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