Design of Hierarchical Surfaces for Tuning Wetting Characteristics

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Design of hierarchical surfaces for tuning wetting characteristics Ariadna Fernández, Achille Francone, Lasse H. Thamdrup, Alicia Johansson, Brian Bilenberg, Theodor Nielsen, Markus Guttmann, Clivia M M. Sotomayor Torres, and Nikolaos Kehagias ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13615 • Publication Date (Web): 13 Jan 2017 Downloaded from http://pubs.acs.org on January 15, 2017

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Design of hierarchical surfaces for tuning wetting characteristics Ariadna Fernández1, Achille Francone1, Lasse H. Thamdrup2, Alicia Johansson2, Brian Bilenberg2, Theodor Nielsen2, Markus Guttmann3, Clivia M. Sotomayor Torres1, 4, Nikolaos Kehagias1 1 Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and The Barcelona Institute of Science and Technology, UAB Campus, 08193 Bellaterra, Barcelona, Spain, 2 NIL Technology ApS, Diplomvej 381, DK-2800 Kongens Lyngby, Denmark 3 Karlsruhe Institute of Technology (KIT), D-76344 Eggenstein Leopoldshafen, Germany 4 ICREA, Institució Catalana de Recerca i Estudis Avançats, 08010 Barcelona, Spain

E-mail: [email protected]

Abstract Patterned surfaces with tunable wetting properties are described. A hybrid hierarchical surface realized by combining two different materials exhibits different wetting states depending on the speed of water droplets impingement. Both “lotus” (High contact angle and low adhesion) and “petal” (high contact angle and high adhesion) states were observed on the same surface without the need of any modification of the surface. The great difference between the capillary pressures exerted by the micro and nanostructures was the key factor that allowed us to tailor effectively the adhesiveness of the water droplets. Having a low capillary pressure for the microstructures and a high capillary pressure for the nanostructures, we allow to the surface the possibility of being in a “lotus” state or in a “petal” state.

1. Introduction Effective control of interactions between liquids and solids surfaces, such as wettability and adhesion, is a major topic of research because of the great importance in various daily life applications1,2. In this regard, hierarchical periodic micro- and nanostructured surfaces with different wetting properties have been observed in a number of biological systems including lotus leaves 3, gecko feet 4 or rose petals 5-7. Such fascinating wetting and adhesion properties have provoked much research and have been attributed to a combination of the chemical nature of the surface and the hierarchical micro- and nanoscale surface topography 8-12. Superhydrophobic “lotus” surfaces that exhibit large water contact angles (CA, >150 ⁰), low water adhesion (Contact angle hysteresis (CAH), 150 ⁰) can pin water droplets, which can be useful for example to transport small volumes 1 ACS Paragon Plus Environment

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of aqueous solution without loss of liquid 5,6, for localized chemical reactions or microfluidic lab-on-a-chip devices 17. Rose plants exhibit a larger micropapillaes in the range from 15-20 µm and smaller nanofolds in the range from 700 to 900 nm. This configuration allow the water droplets to impregnate the microstructure formed by the large micropapillae, but not the nanofolds, giving rise to a droplet that is wetting the microstructures (Wenzel state), but not the nanostructures (Cassie state). This results to a droplet with large water contact angle and high adhesive force to water (Wenzel-Cassie’s state) 5,18. The droplets do not roll-off even if the petal is turned upside down. Superhydrophobic surfaces with controlled adhesion force have been demonstrated on polystyrene (PS) 19, polypropylene (PP) 20, poly(dimethyl siloxane) (PDMS) 21, carbon nanotubes 22, epoxy resin 23 and other materials 7,24-28. However, the controlled adhesion force on these structures is based on tunable surface morphology or surface chemistry. The ability to address simultaneously the adhesive properties, with high a low adhesion on the same surface, with no additional chemical or topographical modification, has not been reported yet 24. As the wetting behavior of these surfaces strongly depends on their micro and nano scale structures, fabrication techniques enabling precise control of the structures are required. Moreover, methods to fabricate these structures easily over large areas are required for successful applications of these functional materials. Nanoimprint Lithography (NIL) 29 is one of the most successful lithographic techniques due to its high resolution, high throughput, low cost and feasibility. The features on the patterned stamp can be replicated onto resist materials via different NIL variations, such as thermal nanoimprint (NIL) 30, ultraviolet assisted nanoimprint (UV-NIL) 31 and reverse nanoimprint lithography (RNIL) 32-33. In this paper we describe the fabrication of hybrid hierarchical patterned surfaces with tunable wetting properties. A hierarchical surface fabricated with a combination of two materials exhibit both “lotus” and “petal” effects depending on how the water droplet impinges on it, without additional chemical modification on the surface. The “petal” effect on these surfaces has been demonstrated to be a direct consequence of different factors: the hybrid nature of the surfaces and the controlled patterning of the nanostructures at the bottom of the microstructured surfaces, which determines the capillary pressures exerted by both the microstructures and the nanostructures.

2. Experimental Section 2.1 . Materials and Stamp preparation The polymers used were Ormocomp (Micro Resist Technology GmbH), poly(methylmethacrylate) (PMMA) with Mw= 75K and different viscosities (3.2 mPa·s and 5.6 mPa·s) in function of the patterned nanostructures. The substrate selected to carry out the imprints was silicon. To fabricate different types of hierarchical structures, three types of master moulds were fabricated. The master with the microstructures was fabricated by photolithography and reactive ion etching. The masters with the nanostructures were defined on a silicon wafer by deep-UV lithography followed by reactive ion etching. The master with the nano-spikes structures was created by dry etching. The Si moulds were treated with a

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fluorinated anti-adhesion layer (Optool DSX) in order to alter the surface hydrophobicity for easy mold releasing. The mold with the microstructures had a square array of cylindrical pillars, with a width, pitch (center to center distance between the structures) and height of 40, 115 and 40 μm, respectively. The mold with the nano pillars had a diameter, pitch and height of 500, 750 and 1500 nm, respectively. The mold with the nano spikes resulted in a random distribution of nanospikes with a diameter from 200 to 600 nm and height ranging from 1 to 3 µm

2.2. Fabrication of the Hierarchical Structures. Figure 1 illustrates the fabrication steps to realize the hybrid hierarchical structures. The moulds used were fabricated on PDMS (Sylgard 184, Dow Corning). The moulds were made by casting the PDMS prepolymer against the relief structure of the different silicon masters. The mixing ratio was 10:1 (precursor:curing agent). They were cured for 12h at 60 ⁰C. The PDMS moulds were peeled-off manually.

Figure 1: Schematic description of the fabrication process followed to realize 3D hierarchical surfaces. The microstructures were fabricated by UV-NIL followed by a RNIL process to transfer the nanostructures from the mold onto the microstructures. Finally, a PDMS stamp was fabricated to have a final mold with the 3D structures. In function of the imprint conditions two different configurations can be achieved. d.1) inking mode, with partial transfer and d.2) intact mode, with total transfer.

Ormocomp was selected to fabricate the microstructures due to its high thermal and mechanical stability. Ormocomp was spin-coated onto the Silicon wafer, and the PDMS mold was placed on the resist surface (Figure 1a). Then, pressure and UV light were applied in order to crosslink the resist (Figure1b). PMMA was selected as the material to transfer the second level containing nanostructures, following a modified NIL technique previously reported 33. In order to facilitate resist filling inside the PDMS stamp cavities, different PMMA viscosities were used depending on the topography of the stamp. PMMA viscosity was modified from 9.3 mPa·s to 3.2 mPa·s and 5.6 mPa·s with Anisole. After dilution, the resist was spin coated on the PDMS replica at 3000 rpm during 1 min. Subsequently, the coated PDMS was pre3 ACS Paragon Plus Environment

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baked and the solvents present in the liquid PMMA were removed. The imprint process was performed using a desktop CNI tool from NIL Technology ApS. The coated PDMS replica was placed on the pre-patterned surface at a temperature of 120 ⁰C (Figure 1c). Depending on the imprint pressure and time, two different transfers can be realized. Figure 1d.1 shows the inking mode, where the applied pressure is low and the transfer takes place only on the elevated parts of the microstructures. Figure 1d.2 shows the intact mode, where the applied pressure is high and the transfer take place both on the top and on the bottom of the micropattern. These two different configurations present different dynamic wetting behaviors, as it will be explained in the following sections.

2.3. Characterization of the Multiscale Hierarchical Structures. The apparent contact angle (CA) of water was measured by a contact angle analyzer (Kruss EasyDrop Standard). The CA was measured by gently placing a water droplet (5 μl) on the surface. The sliding angle was calculated by a planar surface with a tilting option, the angle at which the droplet starts to roll-off was considered as the sliding angle. The hysteresis was measured as the difference between advanced and receding contact angle, which were measured by increasing and decreasing the volume of the droplet, respectively. The presented values were averaged over at least five points on each sample. A high-speed camera (at recording rate of 1200fps) was used to capture the drop impact and rebound sequence. The resolution of the acquired images was 440x144 pixels. The impact experiments were performed by varying the velocity of the droplets at increments of 0.05 m/s. This was the accuracy obtained by analyzing the records of the impacting droplet obtained with the camera. SEM images were taken using an environmental scanning electron microscope FEI Quanta 650 FEG at and acceleration voltage of 5 kV and a working distance of 10 mm. Samples were coated with a 10 nm Pt layer prior to imaging.

Results and Discussion 3.1. Morphology control and Wettability of Hierarchical Surfaces The selected geometrical parameters used to define the micro and nano structures in this work are given in Table I.

Surface Structure

Geometrical parameters Diameter Pitch Height

r

f

Microstructures 40 µm 115 µm 40 µm 1.5 0.12 Nano-pillars 500 nm 750 nm 700 nm 3.25 0.40 Nano spikes 200-600 nm Random 1-3 µm Table I: Dimensions, surface roughness (r) and area fraction (f) of the different structures used in this work.

The width, pitch and height of the structures described in the experimental section were designed to minimize the contact area between the water and the solid surface. According to the Wenzel and the Cassie-Baxter equations, the roughness factor and the solid-liquid contact fraction for a given geometry can be calculated. In the case of a square array of cylindrical pillars, these factors are given by: 4 ACS Paragon Plus Environment


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rMicropillars  1  

f SL -M icropillars

dh a2

d    a

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(1) 2

(2)

where d is the diameter of the pillar, a is the pitch and h is the height. Taking d=40 μm, a=115 μm and h=40 μm, the calculated roughness factor is 1.48 and the solid-liquid contact fraction is 0.12. In the case of a hexagonal array of nano pillars, the roughness factor and the solidliquid contact fraction are given by the following equations,

rNano  Pillars  1 

f SL Nano Pillars 

2dh 3a 2  d 

(3) 2

  2 3a

(4)

where d is the diameter of the pillar, a is the pitch and h the height of the pillars. Taking d = 500 nm, a = 750 nm and h = 700 nm, the obtained roughness factor is 3.25 and solid-liquid contact fraction is 0.40. For the case of nano-spikes, due to the random distribution of the pillars it was not meaningful to make an estimation of the roughness factor and the solidliquid contact fraction. The selection of the structures is a key parameter in this work, since, they need to yield a high roughness and a low solid-liquid contact fraction. At the same time, as it will be explained, the contribution of the microstructures and the nanostructures needs to produce different dynamic behaviors (capillary pressures) when analyzing the dynamic effects of the droplets on the hierarchical surfaces. To fabricate the structures, two different fabrication modes were used. Figure 2 shows SEM images of the fabricated samples. The first set of images (a.1, a.2 and a.3) depict the micro and nanostructures selected for the fabrication of the hybrid hierarchical structures. The second set of images (b.1, b.2, b.3 and b.4) depict the hierarchical fabricated surfaces. The intact mode (Figure 2b.1 and 2b.2) corresponds to a high pressure imprint, obtaining transfer of the nanostructures both on the top and on the bottom of the micro array. The inking mode (Figure 2b.3 and 2b.4) corresponds to a low pressure imprint, obtaining transfer of the structures only on the elevated parts of the micro array.


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Figure 2. Tilted SEM images of a) Micro (a.1) and nanostructures (a.2,3) selected for the fabrication of hybrid hierarchical surfaces and b) cylindrical micropillars decorated with nanostructures resulting from two different fabrication modes (b.1) intact mode of the nanospikes on micropillars, b.2) intact mode of the nanopillars on micropillars, b.3) inking mode of the nanospikes on micropillars and b.4) inking mode of the nanopillars on micropillars). The insets correspond to their WCA images.

Table II lists the apparent and dynamic contact angle measurements and their standard deviation on the different surfaces. The measured WCA values of all samples are larger than 150 ⁰, clearly indicating superhydrophobic properties. The surfaces exhibit a clear “lotus” state, as can be observed on the sliding and contact angle hysteresis measurements, confirming the low adhesion between the droplet and the surfaces. Interestingly, the wettability of the surfaces is similar in the two different surface configurations. This “lotus” state is observed when the droplet is resting only on the elevated parts of the surface, and the pattern at the base of the elevated parts does not influence the wetting state.

Surface Structure

Contact angles [⁰] Apparent Sliding Hysteresis

Intact mode 167± 3 7±4 6±3 (Nanospikes-Micropillars) Inking mode 162± 4 10±3 9±4 (Nanospikes– Micropillars) Intact mode 168± 2 6± 2 4±2 (Nanopillars– Micropillars) Inking mode 168± 2 9± 3 10±5 (Nanopillars– Micropillars) Table II: Experimental contact angles of the PMMA/Ormocomp hierarchical structures fabricated with PMMA/Ormocomp.

As it will be demonstrated in the following section, these surface configurations do not affect to the normal wettability of the structures, (i.e. gently droplet deposition to analyses the common parameters, such as contact angle, hysteresis or sliding angle). However, when dynamic effects are analyzed, the behavior changes dramatically, and the effect of having 6 ACS Paragon Plus Environment


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nanostructures on the base of the array becomes crucial to have an effective control of the adhesion over the superhydrophobic surface.

3.2. Dynamic Behavior Dynamic effects on a superhydrophobic surface, such as impingement 34,35 or vibration 36,37 are of great importance to probe energy barriers responsible for wetting transitions. Usually, these transitions are directly from a composite to a homogeneous state, a Cassie to Wenzel transition 38,39. In this regard, hierarchical surfaces open a pathway to intermediate transitions, potentially useful if one can get a precise control over them. In this section, an intermediate state in the transition from pure Cassie to pure Wenzel will be demonstrated. This intermediate state corresponds to the so-called “petal” effect, where the water penetrates the microstructures, but not the nanostructures, leading to an effective high contact angle and a high adhesion force. Drop impact and rebound characteristics from superhydrophobic surfaces are governed primarily by its impact velocity and inherent liquid properties, such as density, viscosity and surface tension 40. When a droplet impinges on a superhydrophobic surface, it deforms and stores kinetic energy, which will make the droplet subsequently recoil. The shape and extent of the deformation are balanced by the surface tension and the velocity of the drop and can be quantified with the dimensionless Weber number, which is the ratio of kinetic to surface energy. After impingement, the inertial energy of the water drop is dissipated through viscous forces. This viscous resistance affects the drop rebound characteristics and can be quantified by the Reynolds number, defined as the ratio of the inertial to viscous force 41. These two dimensionless numbers can be used to characterize the impact dynamics. The Weber number, We, is defined as:

dV 2 We  

( 5)

where ρ is the density, d is the diameter and σ is the surface tension of the droplet. The Reynolds number, Re, is defined as: Re 

dV 

(6)

where η is the dynamic viscosity of the liquid. As the impact velocity V increases, a different dynamic behavior was found: for relatively low impact velocities, regular rebound of the droplets and air trapping were observed on all the structures. However, for intermediate impact velocities, the dynamics of the droplets were different for the intact and inking modes of fabrication. For the inking mode samples, a pinning of the droplet is observed, with a droplet on the Wenzel state, indicating penetration both in the microstructures and in the nanostructures. For the intact mode samples, pinning was also observed, but with a high contact angle, indicating a “petal” state, where the droplet penetrates the microstructures but not the nanostructures. For high impact velocities, Wenzel droplets were observed on all the samples.


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Figure 3 shows the impact diagrams for the intact mode samples. For the sample with nanopillars-micropillars, the “petal region” is present when the droplets impact with velocities from 0.81 to 1.71 m/s. At lower velocities the “lotus” state appears and at higher velocities the Wenzel state appears. For the sample decorated with nanospikes-micropillars, the “petal region” is present when the droplets impact with velocities from 0.71 to 0.98 m/s. At lower velocities the “lotus” state appears and at higher velocities only the Wenzel state appears.

Figure 3. Dynamic behavior of droplet on a) nanopillars-micropillars surface and b) nanospikesmicropillars surface.

For the samples fabricated with the inking mode a velocity of 0.80 m/s was needed to provoke a direct Cassie-to-Wenzel transition. Figure 4 shows a schematic of all the wetting states observed in samples fabricated with the intact mode. Three different states are clearly differentiated. For the case of the nanospikes-micropillars surface, first the droplet rests in a “lotus” state, with a CA of 167 ⁰. If the droplet impacts the surface at a velocity in the range between 0,7 and 0,9 m/s (28 < We < 47, 2169 < Re < 367 ), the “petal” state appears, with a CA of 151 ⁰ and a high adhesion force, as evidenced in the contact angle hysteresis value (30°). If the droplet impacts the surface at a velocity higher than 0.9 m/s, the Wenzel state appear, with a CA of 117 ⁰ and a CAH of 48°. The same situation is observed with the nanopillarsmicropillars sample, but the threshold velocities to produces the transitions to “petal” state are higher, being in the range between 0.8 and 1.7 m/s (36 < We < 162, 3218 < Re < 6837). When the droplet impacts at a velocity higher than 1.7 m/s, only the Wenzel transition is observed. It should be stressed that, although the Cassie-Wenzel transition at high Weber numbers is a common trend that has been widely studied 34-40, the appearance of an intermediate state at moderate Weber number has not been yet observed.



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Figure 4. Schematics of the different wetting states observed on the hierarchical surfaces made with the intact mode. a) Transitions for the nanospikes-micropillars configuration and b) transitions for the nanopillars-micropillars configuration.

Figure 5 shows the process of a droplet impinging on the nanopillars-micropillars surface fabricated with the intact mode, impacting at different velocities. In case I, the droplet is impacting at 0.6 m/s. The droplet hits the surface and is deformed after 3 ms (Case Ib). After 8 ms the droplet recoils and after 10 ms bounce-off from the surface. After 34 ms the droplet impinges again the surfaces. Several rebounds later on, at 87 ms the droplet stabilizes in a “lotus” state. In Case II, the droplet impacts at a velocity of 0.8 m/s. The droplet hits the surface and it is deformed after 2 ms (Case IIb). After 10 ms the droplet recoils while still attached to the surface. Then, the droplet starts to stabilize and after 65ms a droplet in a “petal” state is observed. For Case III, the droplet is impacting with a velocity of 1.8 m/s. The droplet hits the surface and it is deformed after 2 ms (Case IIIb). After 10 ms the droplet recoils while still attached to the surface. Then, the droplet starts to stabilize and after 56ms a droplet in a Wenzel state.


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Figure 5. Snapshots depicting the dynamics of a 10 µl water droplet for the three cases studied in this wok: “lotus” state, “petal” state and Wenzel state.

The phenomenon of partial pinning exhibited by the surface when is in the “petal” range, can be explained by the balance between wetting and anti-wetting pressures that govern the impact dynamics of the droplets 40,42-44. A larger Pwetting than Pantiwetting causes the droplet to wet the surface. There are two contributions to the Pwetting, which involves the droplet kinetic energy and the shock-wave resulting from the droplet surface impact. One is the Bernouilli pressure, also denoted as dynamic pressure (PD) 34 and the water hammer pressure (PWH) 42. The dynamic pressure, which is due to the velocity of the droplet during impingement, is given by:

PD 

1 V 2 2

(7)

where ρ is the density of the impinging liquid and V the impact velocity. The water hammer pressure corresponds to the shock-waver produced at the moment of impact, which can be sufficiently strong to cause a wetting transition 42, is given by: PWH  kCV

(8)

where C is the speed of the sound in water. The coefficient k refers to a collision factor describing the elasticity of the collision. A wide range of experimentally determined values of k is available in the literature, ranging from 0.2 to 0.001 44-46, as a function of the impact velocity or the droplet volume. Experiments on droplet impingement typically use a droplet speed of the order of m/s, for which the water hammer coefficient is typically approximated to 0.2 42,43. We will assume 0.2 as an accepted value, taking into account that this values could be an overestimate of the water hammer pressure acting on the surface. The antiwetting pressure (Pantiwetting) is associated with the capillary pressure. It denotes the force per unit area offered to a water droplet as it transitions from a Cassie state to a Wenzel state. It is defined by 40,44: PC   cos A

LC AC

(9)

where σ is the surface tension, θA is the advancing contact angle of the flat surface, LC is the capillary perimeter and AC is the capillary area. As observed in the equation 9, both the selection of the material and the topography of the structures have a great importance for the calculation of the capillary pressures. Since the superhydrophobic surfaces have a hierarchical structure, there will be two different capillary pressures 47: the capillary pressure due to the microstructures (PCM) and the one caused by the nanostructures (PCN). In the case of the microstructures, we have a square lattice of circular pillars, and its corresponding capillary pressure will be:

PCM   cos A

D  P2  D2 4

(10)



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where D is the diameter of the pillar and P the pitch. For the case of the microstructures, we have a hexagonal lattice of circular pillars, and its corresponding capillary pressure will be:

 PCN   cos A

2

D

3 2  2 P  D 4 8

(11)

By using two different surface structures and two different material we are imposing great differences in the corresponding capillary pressures. The corresponding capillary pressure exerted by the micropillars will be PCM= 0.21 kPa for θA= 105 ⁰, D = 40 µm and P = 115 µm. The corresponding capillary pressure exerted by the nanopillars will be P CN= 0.28 MPa for θA= 58 ⁰, D = 500 nm and P = 750 nm. This large difference between the two exerted capillary pressures will be the factor that allows the “petal” state, where the water is wetting the microstructures, but no the nanostructures. For the case of the nanopillars-micropillars structures, with an impact velocity of 0.8 m/s (the necessary velocity of obtain the “petal” state), the water hammer pressure is about PWH = 0.24 MPa, and the corresponding dynamic pressure will be PD = 0.34 kPa. In this case, the wetting pressure is higher than the capillary pressure exerted by the microstructures, but lower than the capillary pressure exerted by the nanostructures. Thus, the droplets will penetrate into the microstructures but not into the nanostructures, since PCM < PWetting < PCN. It is important to highlight that this value for the water hammer pressure is an overestimation, since we are in the impact velocity range below 1 m/s, there is a controversy on the possible value of the k coefficient for low impact velocities, ranging from 0,001 to 0,1 45 46. For higher impact velocities, V > 1.7 m/s, the water hammer pressure is 0.51 MPa and the dynamic pressure is 1.49 kPa, which exceeds both exerted capillary pressures, thus the droplet is able to penetrate into both microstructures and nanostructures. Figure 6 depicts the schematics of the three possible wetting states existing on our hierarchical surfaces and their corresponding pressure balances. For impact velocities below 0.8 m/s, the wetting pressure is lower than the antiwetting pressures, thus the droplet remains in the “lotus” state. For impact velocities in the range of 0,8 m/s < V < 1.7 m/s, the PWetting is between the two possible antiwetting pressures, thus the droplet remains in the “petal” state. For impact velocities above 1.71 m/s the wetting pressure is higher than the anti-wetting pressures, thus the droplet penetrates in the structures, allowing the Wenzel state.

Figure 6. Schematics descriptions of three different wetting states in function of the wetting and antiwetting pressures.

The same behavior was observed on the surface with the nanospikes-micropillars, obtaining a similar needed wetting pressure to wet the microstructures (PWH = 0,21 MPa and PD = 0,26 11 ACS Paragon Plus Environment

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kPa). However, when it exceeded 0.29 MPa the droplet was able to wet the nanostructures, which means that the capillary pressure due to the nanospikes is lower. However, due to the random distribution of the structures it was not possible to calculate this pressure theoretically. Due to the difference observed between the water hammer pressure calculated here and the experimentally observed transition, we propose a model based on the energy barriers needed to provoke the different transitions. The transition between the different observed wetting states was explained in terms of the energy barrier between the different states by Patankar 48. The energy barrier is understood as the energy required to wet the sides of the surface elements of a superhydrophobic surface. We can interpret the energy barrier in terms of work done by capillary force, which is the product of the capillary pressure and the liquid-air interfacial area, to displace the air gaps through a height h within the roughness elements. Thus, the energy barrier between of different wetting states corresponding to one unit cell is:

EState1  Estate2 unit cell  PC A C h

(12)

The total energy barrier between the two states of a droplet of radius R may be determined by multiplying the energy of the unit cell by the total number of pillars (n) beneath the droplet: n

R 2

(13)

A

where A is the area of the unit cell. As we have two different wetting transitions and two different capillary pressures, we will have two different energy barriers leading two different transitions. Thus, the total energy required to produce the transition to “petal” state will be: E Lotus  E Petal  PCM hR 2  hR 2 cos  A

D 

P  2

4

(14) D

2

On the other hand, the total energy required to produce the transition to Wenzel state will be:

E Lotus  E wenzel  PCN hR 2  hR 2 cos  A

 D 2 3 2  2 P  D 4 8

(15)

Thus, the energy required to produce the transition to the “petal” state corresponds to Epetal = 2.3 mJ, and the energy required to produce the transition to Wenzel state corresponds to EWenzel = 2.7 J. Again, this large difference between the two energy barriers evidences the possibility of having intermediate wetting states. With these considerations, it is important to highlight the importance of the distribution of the nanostructures on hierarchical structures for effective control on the water adhesion. If the nanostructures are only on the elevated parts of the micropillars, the effect of hierarchy is negligible for dynamic effects, since for the impacting droplets it is important to consider the partial penetration in the cavities of the pillars structures. Furthermore, our inking mode 12 ACS Paragon Plus Environment


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fabricated surfaces require a wetting pressure of 0.23 MPa to produce a direct Wenzel transition. We demonstrate an application of these surfaces in micro-droplet transport. Figure 7a depicts the water droplet on a “petal” state on our surface. The droplet is brought in contact with a hydrophobic surface, showing deformation of the droplet. Then, when the droplet is detached from the hydrophobic surface, it remains on the surface with the “petal” state. In figure 7b, the opposite situation is shown; the droplet is brought in contact with a hydrophilic surface, showing how the droplet is perfectly transferred to the hydrophilic surface without loss of mass.

Figure 7. Snapshots of the sequence showing a droplet in contact with a) a hydrophobic surface and b) a hydrophilic surface.

3.3. Effect of the Surface Inclination Experiments of droplet impact at oblique conditions were performed in order to evaluate the possible wetting transitions that can take place in this configuration. Since the drop impacts a surface at an angle, it is necessary to re-define the Weber and the Reynolds numbers in terms of normal impact velocity (VN) and tangential impact velocity (VT) as 49,

dVN 2 dVT 2 We N  ; WeT    Re N 

dVN dVT ; ReT   

( 16)

(17)

It is reasonable to expect that the normal component of the impact velocity causes the water penetration rather than the total speed. We thus use the We number that is dependent on this component (WeN). For these tests, droplets with a range of normal Weber numbers from 2 to 244 and Reynolds number from 525 to 8398 were used. Room-temperature (25 ⁰C) droplets were used at inclinations of 20, 30 and 45 ⁰. Although the rebound is predicted at high Weber numbers, drops often rebound only partially when their Weber number exceeds a certain threshold value. This partial rebound regime occurs when the meniscus of impacting water droplets can penetrate between the features of the textured surfaces and disrupt the air pockets 42. For this reason, different velocities were tested. A low impact velocity, below the threshold to produce the “petal” effect on 0⁰ tilt surfaces, to observe the impact and rebound of the droplet. An intermediate impact 13 ACS Paragon Plus Environment

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velocity, to analyses if “petal” droplets form under oblique conditions, and thirdly, high impact velocities. Complete rebounds were observed at low and moderate impact velocities and Weber numbers as the air layers below the droplet remain intact, while pinning is observed at high velocities, when the air pockets are disrupted by the wetting pressure of the impinging droplet. The transition observed in these experiments is a direct Cassie-to-Wenzel transition, thus, petal state is not observed on oblique impacts. Interestingly, these transitions take place under the same conditions of the Wenzel at 0 ⁰ tilt surface. Table III shows a summary of the transitions to Wenzel state for each inclination. It can be observed that the transitions occur at different droplet height release, but the normal components of the velocity and the Weber number remain rather constant. These results indicate that the wetting pressure exerted by the normal velocity is responsible for the disruption of the air pockets. Inclination

0⁰

20 ⁰

30 ⁰

45 ⁰

Transition Height (m)

0.15

0.17

0.21

0.30

Impact velocity (m/s)

1.71

1.83

2.03

2.42

We number

164

186

229

329

Normal impact velocity (VN) (m/s)

1.71

1.70

1.76

1.71

Normal We number (WeN)

164

163

172

162

Table III: Summary of results for each of the tilt angles tested in this study. The drop diameters are 4 mm.

The experimental data allows to build and design an effective control over the water dropletsurface dynamics, where the desired wetting state is a function of the impact velocity of the droplet and the inclination angle.

3. Conclusions We describe the fabrication of hybrid hierarchical patterned surfaces with tunable wetting properties. A hierarchical surface fabricated with a combination of two different materials exhibit both “lotus” and “petal” effect by varying the deposition conditions of the water droplets, without the need of chemical modification of the surface. The large difference between the capillary pressures exerted by the microstructures and by the nanostructures is the key factor to tailor effectively the adhesion of the water droplets. This study contributes to a better understating of multifunctional surfaces.

Acknowledgment This work was funded by the European Commission (FP7-2012-NMP-ICT-FoF) project PLAST-4-FUTURE (www.plast4future.eu), under Grant Agreement number 314345. The work at ICN2 was partly supported by the Severo Ochoa program (Grant SEV-2013-0295). The PHENTOM project FIS201570862 is also acknowledged. 14 ACS Paragon Plus Environment


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Design of Hierarchical Surfaces for Tuning Wetting Characteristics

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