Fabrication of micro-textured surfaces for a high hydrophobicity by evaporative patterning using screen mesh templates

Applied Surface Science 400 (2017) 64–70

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Fabrication of micro-textured surfaces for a high hydrophobicity by evaporative patterning using screen mesh templates Hideo Tokuhisa ∗ , Shiho Tsukamoto, Satoko Morita, Shogo Ise, Mitsuru Tomita, Naoki Shirakawa National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan

a r t i c l e

i n f o

Article history: Received 11 April 2016 Received in revised form 21 November 2016 Accepted 27 November 2016 Available online 2 December 2016 Keywords: Hydrophobicity Micro-textured surface Evaporative patterning Screen mesh Template

a b s t r a c t In this study, micro-textured surfaces for a high hydrophobicity were fabricated through evaporative patterning of a hydrophobic polymer, CYTOP using various types of screen meshes as the templates. The screen meshes were placed over a hydrophilic glass substrate followed by casting a polymer solution onto the templates to wet the surface entirely. Slow evaporation of the solution gave various types of micro-textured patterns depending on the polymer concentration, the mesh counts, the evaporation temperature, the type of the mesh, whether or not the mesh is calendared. The structures were characterized by optical and scanning electric microscopy, and contact angle (CA) measurements. The mechanism of the pattern formation is discussed. Finally, addition of graphene to the micro-textured surface gave a superhydrophobic surface (CA: 155◦ ), which is concluded that a hierarchical micro/nano structure could be formed. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Hydrophobic coatings have been extensively studied due to the extremely high expectation for their potential applications such as microfluidic [1], and biomedical devices [2], self-cleaning [3], impermeable textiles [4], anti-fog [5], and anti-corrosion surfaces [6]. Many kinds of techniques have been reported to create hydrophobic surfaces such as textile coating [7,8], electrospinning [9], template [10,11], lithography [12], and many other methods [13–15]. One of the basic ideas to enhance the water repellent property is making bumpy surfaces of hydrophobic materials with roughness of micrometer sizes because the hydrophobic surface areas to contact with water can increase (Wenzel effect) compared to the projected areas, or the air can be trapped in the dented parts when water sits on the uneven surface (Cassie-Baxter effect) so that the contact angle of water increases up to more than 150◦ (superhydrophobic state) because a contact angle of water on the air is as high as 180◦ . There are some reports to fabricate hydrophobic surfaces using screen meshes because they have threads woven alternately in lengthwise and crosswise to form concave and convex structures

∗ Corresponding author. E-mail address: [email protected] (H. Tokuhisa). http://dx.doi.org/10.1016/j.apsusc.2016.11.213 0169-4332/© 2016 Elsevier B.V. All rights reserved.

with tens-micrometer periods. They can be templates and even frameworks to generate a high hydrophobic surface [16–19]. For example, a facile method for manufacturing superhydrophobic surfaces using the stainless steel wire mesh as the template is reported [17]. Polymer surfaces were hot-embossed with the mesh to increase the surface roughness resulting in the contact angle up to 152◦ . For the framework, oil/water separation is demonstrated using the superhydrophobic/superoleophilici epoxy/attapulgite nanocomposite coatings on the stainless steel meshes [19]. The coating was done by a facile spray-coating method. The water contact angle was 160 ± 1◦ to make water run off the mesh, while the oil contact angle was 0◦ to allow oil to permeate through. In this work, we demonstrate a simple, low-cost, energy efficient method to create micro-textured surfaces of a hydrophobic perfluorinated polymer (CYTOP) for a high hydrophobic coating, potentially, up to superhydrophobicity over a hydrophilic surface. It is based on slow evaporative patterning of CYTOP with the help of various types of screen meshes as the templates. A screen mesh as an evaporation template was placed on a hydrophilic glass substrate followed by the deposition of a hydrophobic solution containing CYTOP to fill the void generated between the screen mesh and the substrate basically through a capillary force. The slow evaporation of the solvent left on the substrate uneven patterns of CYTOP reflecting on the screen mesh architectures. Different from the method using meshes mentioned above, our method allows

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Table 1 Distance between the centers of the repeated neighboring patterns. Direction

r1 r2

Fig. 1. Schematic of the experimental setup for evaporative patterning using screen meshes.

The distance between the repeated patterns [␮m] (a)

(b)

(c)

(d)

67 95

133 95

48 67

48 67

The patterned CYTOP areas were not perfectly uniform depending on the flatness and the warp degree of the meshes. And in some cases, the patterned CYTOP was removed partly with the screen meshes. We evaluated the continuous patterned areas in this study. 2.3. Characterization

us to coat a hydrophilic surface with the hydrophobic material together with the periodic uneven structure formations. We fabricated micro-textured surfaces of CYTOP using different meshes in terms of the mesh count, calendared or not, plain- or twill- woven fabrics, and compared the CA among them. We will also discuss the mechanism of how the patterns are formed as the evaporation proceeds. 2. Materials and methods 2.1. Materials A commercially available fluoropolymer CYTOP (CTX-809A) (Asahi Glass, Japan) was used as the hydrophobic polymer. It is diluted with a perfluorocarbon-based solvent (CT-SOLV180) (Asahi Glass, Japan) to obtain 1 wt% and 10 wt% CYTOP solution. A 2 × 5 cm2 float glass plate (Matsunami Glass Ind., Japan) was used as the substrate. 2 × 2 cm2 plain-woven fabric stainless screens of 325 and 500 meshes with the thread diameter of 28 and 19 ␮m, respectively, calendared meshes which is flattened at the highest point in the weave for plain-woven fabric screens of 325 and 500 meshes with the tread diameter of 28 and 19 ␮m and twilled-woven fabric screens of 500 and 900 meshes made of stainless steel with the thread diameter of 19 and 14 ␮m were used as the templates. All meshes were kindly provided by Asada Mesh Co., Ltd. (Japan). Poly (vinyl alcohol) (PVA) with polymerization degree about 1500 (Wako, Japan) was used for a control experiment as a hydrophilic material. It was mixed with water to obtain 1 wt% aqueous solution. 5 wt% graphene with the median lateral size of 500 nm and mostly with 1–8 layers stacked in N-methylpyrrolidone (Graphene Platform, Japan) was kindly provided by Sonocom Co. Ltd. (Japan), and diluted with CYTOP solution to obtain 10 wt% CYTOP solution with 0.1 wt% graphene. For Cu etching, 22 mmol/l FeCl3 (Sigma-Aldrich, USA) methanol (Wako, Japan) solution was prepared. A CYTOP patterned surface on a sputtered Cu film on a glass was soaked in the solution for 10 min, washed with methanol, and dried with N2 . 2.2. Formation of textured surfaces with the mesh templates Glass substrates and screen meshes were washed with acetone, ethanol, and distilled water under sonication, followed by drying with air flowing. Fabrication of micro-textured surfaces of the fluoropolymer is as follows: the screen-mesh was first placed on the cleaned glass substrate, and then fixed with magnet plates by sandwiched between the top of the mesh and the bottom of the glass substrate (Fig. 1). Then, a certain amount (15–50 ␮L) of the solution containing the fluoropolymer was dropped on the center of the mesh. After the solution was thoroughly spread over the mesh, the sample was dried at room temperature overnight. The mesh was removed almost naturally following the magnets were detached.

A movie for a real time observation of the evaporative pattern formation and optical images of the resulting micro-textured surfaces were taken using a VW-9000/VW-600c digital microscope (KEYENCE, Japan). Scanning electron microscopy (SEM) images were obtained on a JSM-7400F scanning electron microscope (JEOL, Japan) at 10 kV. The samples were coated with a thin platinum layer before observation. The pattern heights were measured using a stylus surface profiler, Dektak XT-(ULVAC, Japan). The highest heights at ten different repeated patterns in each sample were collected and averaged. Surface roughness of all the samples was measured with a laser 3D measurement microscopy VK-8500 (KEYENCE, Japan) and evaluated by two parameters: arithmetic mean roughness, Ra; root-mean-square roughness, Rq. Contact-angle measurements of the textured surfaces were determined using FTA 125 Contact Angle Analyzer (First Ten Ångstroms, USA) and the FTA software. At least three measurements on three samples each were performed after dropping 2 ␮L of water on them. The roll-off angle of the pattern containing graphene was measured by placing the sample on a level platform mounted on a Drop Master DM500 (Kyowa Interface Science, Japan) and inclining the sample. Drops (16 ␮L) of water was placed on the surface, the stage was tilted and the angle of the stage was recorded when each drop rolled off. 3. Results and discussion 3.1. Hydrophobic pattern formation using plain woven fabrics 1 wt% and 10 wt% CYTOP (a fluorinated polymer) solution was cast on screen meshes over glasses. The overnight drying at room temperature followed by the removal of the meshes left unique patterns composed of the polymer on the glasses (Fig. 2). The 10 wt% solution deposited over a 325 screen mesh gave mesh-like patterns while the 1 wt% solution showed a dot or small ring shape pattern, roughly. In the case of the 500 mesh the patterns also showed the same trend for the concentration differences of the polymer. The distance between the neighboring pattern centers (r1 and r2 ) is almost the same as the pitch of the screen mesh except for the 1 wt% polymer pattern using the 325 mesh that had twice the r1 distance compared to the template (Table 1). This indicates that the patterns were formed based on the mesh structures, and the ring pattern for the 1wt%–325 mesh seems to be formed along one of the woven thread, weft or warp. 3.2. Slow evaporation process A movie, which is given as a typical progression of drying in the supplementary material hints how the patterns are growing. It was taken from the backside of a 325 mesh over a glass slide

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Fig. 2. Optical microscopic images of the resulting polymer patterns after solvent evaporation using (a) 10 wt% and (b) 1 wt% CYTOP solution in the case of the 325 mesh, and (c) 10 wt% and (d) 1 wt% in the case of the 500 mesh.

Table 2 Averaged contact angles of the resulting CYTOP patterns on glass substrates. Mesh count

325

CYTOP concentration [wt%] Height [␮m] Surface roughness Ra, Rq [␮m] Averaged contact angle (max/min contact angles) [degree]

1 0.72 ± 26% 0.30, 0.40

10 4.5 ± 17% 1.4, 2.1

1 0.21 ± 24% 0.08, 0.09

10 2.9 ± 18% 0.51, 0.69

113 (117/112)

114 (118/108)

107 (113/101)

112 (118/103)

after dropping 10 wt% of the polymer solution, and focused on the position where the screen mesh contacted with the front glass side. Fig. 3 shows the cut images of the movie. It can be seen that open areas of the screen started drying 90 min after the solution spreading between the screen and the substrate to cover the entire hydrophilic glass substrate (CA <20◦ ) (Fig. 3(a) and (b)). The solution wetting the mesh formed a grid structure to fill the gap between the mesh screen grid and the substrate, which might be due to a capillary force generated between them (Fig. 3(c)). As the open area dried to the certain level the liquid grid started breaking so as to store the liquid at each weave points (or knuckles) of the screen mesh that contact with the glass substrate (designated by arrows in Fig. 3(d)). In the case of the 325 mesh the liquid moves further from the lengthwise to the crosswise thread. The liquid around the knuckles of the crosswise threads (designated by the dotted arrows in Fig. 3(d)) attracted the one of the lengthwise threads (designated by the solid arrow in Fig. 3(d)). Although the plain-woven fabric screen should be symmetry, there seemed to be some slight differences in the height between the lengthwise and crosswise threads at the junctions. The capillary force might increase as the gap between the asperity of the knuckles and the substrate decreases as the reported simulation [20]. Therefore, the thread with the closer knuckles to the substrate might draw the liquid finally. Interestingly, the liquid movement after the open area started drying almost completed within ten minutes. So, the drying time of overnight was mostly occupied by the solvent evaporation after the pattern formation.

500

3.3. Plausible mechanism of the pattern formation The resulting patterns of the fluorinated polymer shown in Fig. 2 told the drying history because the polymers should be solidified when the solvent evaporation caused the viscosity to increase over time to the point where the polymer could not have enough time to diffuse into the inside of the moving liquid along the screen architecture. In the case of 1 wt% solution the polymer was precipitated out at the final stage of the drying where the liquid transferred to the closest knuckles to the substrate. The pattern using the 325 mesh had an oval ring shape array under the knuckles of one side thread, and the 500 one gave a dot array under all the junctions. The shape difference can come from the difference in the distance between the knuckles and the substrate. A pendular ring of the polymer solution [21–24], which is often called a small amount of liquid at the point of contact between the solid surfaces, can be formed around the contact knuckle when the 325 mesh was used. As the evaporation fully proceeded, the footprints of the knuckles were appeared as the ring shape of the polymer. In the case of the 500 mesh the mesh was a little away from the substrate, the solution tend to form a liquid bridge between the asperity of the knuckle and the underneath substrate [25]. It might turn out to be a dot-like pillar structure under the knuckle. Unfortunately, our method could not control the distance so well that some areas are the ring shape and others are dot-shape array even on the same substrate. It is because the contact relies on the capillary force generated between

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Fig. 3. Cut images of the movie for the progression of solvent drying when the 10 wt% CYTOP solution was dropped over the 325 screen mesh after (a) 60, (b) 90, (c) 95, (d) 98, (e) 100 min.

3.4. Drying temperature dependence

Fig. 4. An optical microscopic image of the resulting polymer pattern after solvent evaporation of the 1 wt% polyvinyl alcohol solution.

the flexible mesh and the substrate after the solution was spread over the surface of the substrate. For 10 wt% solution, the solidification of the polymer seemed to happen at the earlier stage. Basically, there are larger footprints of the knuckles compared to 1 wt% case for the both mesh templates. This indicates that the solidification might start when the solution drop was still large soon after it moved to under the knuckles. In the case of the 325 mesh, there are some tails with the oval ring footprints. They can be appeared as the solution is shrinking along the mesh grid. A closer inspection allows us to find a dot between the footprints in the both cases. That may be caused by a satellite droplet that was generated when a liquid bridge between knuckles was broken [26]. The formation of the droplet can be driven by relatively high interface free energy between the hydrophobic solution and the hydrophilic surface so that the liquid bridge tries to minimize the contact area with making a high curvature at the breaking point. In contrast, using a hydrophilic PVA aqueous solution on the hydrophilic substrate gave a continuous line between the footprints of knuckles as shown in Fig. 4. This case might reveal that the liquid bridges between the pendular rings can be stabilized by low interface free energies between the solution and the substrate [23].

Drying temperature dependence of the deposit patterns was investigated and the resulting patterns were shown in Fig. 5. Drying at 50 ◦ C for 1 wt% CYTOP solution deposition using the 325 mesh gave the larger footprints of lengthwise knuckles together with the smaller crosswise ones while the slow evaporation at room temperature showed only the one direction deposition at the junction as mentioned above. Further increase in the drying temperature up to 100 ◦ C caused more spread pattern along the grid to form the screen-mesh-like pattern together with the broadening of the footprints patterns from 25 ␮m at room temperature to 35 ␮m. This means that at higher temperature the polymer was solidified prior to or without the liquid movement. We assume that the liquid movement and liquid bridge breaking between the knuckles rely on a subtle difference in capillary forces generated between knuckles and the substrate. In general, higher temperature leads to lower capillary force [25]. Therefore, the droplet movement was thought to be negligible at higher temperature.

3.5. Contact angles of the micro-textured surfaces The patterned heights, the surface roughness, and the contact angles of the resulting patterned surfaces were measured and summarized in Table 2. As we expected, all in all, the CAs became larger compared to the spin-coated, flat, film (CA: 104◦ ). A SEM-EDX analysis reveals that more than 70% of the underlying glass surface is uncovered with the fluorinated polymer. Given that the CA of the glass surface is less than 20◦ , the hydrophobic protruded regions should efficiently enhance the CA. If the whole surface is in the Cassie-Baxter state where the footprints can suspend the water over all the hydrophilic surfaces, a simple calculation gives over 140◦ of the CA [27]. From the results, this is a little extreme case. Therefore, the wetting should be a mixed state of hydrophilic and hydrophobic solid-water contacts (Wenzel regimes), and the Cassie-Baxter regimes. The 1 wt% CYTOP solution using the 500 mesh showed that the CA is about the same as the spin-coated one, although the exposed glass area occupied more than 90% of the surface. This also suggests that the water droplet should be lifted up around the dot pattern to trap the air although the height of the footprints was only about 0.21 ␮m (Table 2).

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Fig. 5. Optical microscopic images of the resulting polymer patterns after solvent evaporation of the 1 wt% CYTOP solution in the case of the 325 mesh at different drying temperatures (a) room temperature, (b) 50, and (c) 100 ◦ C.

Fig. 6. Optical microscopic images of the resulting polymer patterns after solvent evaporation of the 10 wt% CYTOP solution in the case of the calendared meshes of different types of woven fabric with different mesh counts: plain-woven fabric with 325 (a) and 500 meshes (b); twill-woven fabric with 500 (c) and 900 meshes (d).

The CAs of 10 wt% solution increased compared to the corresponding one of the 1 wt%. This is not surprising, considering that they have more hydrophobic areas to cover the glass and more surface roughness, and the distance between the nearest repeated polymer patterns are closer because of the widened footprints. These factors might increase the area of Cassie-Baxter domains as well as the surface areas to be contacted with water. In order to validate this trend, calendared screen meshes where the knuckles of the wires were flattened were used for the evaporation mask, expecting that the footprints fabricated through the pendular rings at the contact area of threads with the substrate become larger, and thus the distance between the nearest rims of the neighboring patterns become shorter to form more favorable structures to support water droplets. 3.6. Patterning using calendared screen mesh templates Calendared meshes of plain-woven and twill-woven fabrics with different mesh counts (325–900) were used as the templates. The room temperature drying of the 10 wt% polymer solution provided quite different patterns in the optical images as shown in Fig. 6. This time a large area of the surfaces including the part of the opening area was covered with the polymer. The flattened knuckles gave the footprints made of the polymer with diamond

or gourd shapes that linked each other with lines at the vertexes. The open area of the mesh and the inside of the footprints had colored patterns. The SEM image of the typical pattern formed by the calendared mesh reveals that the dark lines with the diamond shape in the optical image are thick polymers that would have been adhered to the side of the knuckle before the removal of the mesh while the colored areas are a thin polymer film (Fig. 7). The colors in the optical images might come from a structural color due to a thin film with thickness of sub micrometers [28]. We think that the mechanism of the pattern formation is basically the same as the not-calendared ones. After the solution was cast on the mesh, it spread over the entire surface, followed by the formation of pendular rings around the flattened knuckles together with a liquid bridge formation between the nearest vertexes of the knuckles. Since the contact area of the flattened knuckles with the substrate become larger (>20% in the case of the 325 plain-woven fabric mesh) compared to the not-calendared mesh (about 4%), even at the final stage of the drying where the liquid level is getting close to the interface between the top of the knuckles and the surface of the substrate, the solution might still spread over the open area of the mesh accompanying with the solidification of the polymer. We found that the inside of the circles in the colored open area is the uncoated glass surface. This was confirmed by a FeCl3 -etching of a sputtered Cu film with the CYTOP pattern as a resist. After etching, the etched

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Table 3 Averaged contact angles of the resulting CYTOP patterns when calendered meshes were used. Types of weave

Plain weave

Mesh count Height [␮m] Surface roughness Ra, Rq [␮m] Averaged contact angle (max/min contact angles) [degree]

325 3.9 ± 13% 1.2, 2.0

500 1.8 ± 16% 2.0, 3.7

500 2.3 ± 28% 1.9, 3.7

900 2.5 ± 25% 2.2, 4.2

122(127/117)

131(140/123)

132(136/115)

141(143/135)

Fig. 7. A SEM image of the polymer pattern after solvent evaporation of the 10 wt% CYTOP solution using the 500 calendared mesh of a plain woven fabric.

circles in the Cu film were observed, which are corresponding to the circle patterns. The uncovered circle indicates that the solution in the open area was drawn into the knuckles as the evaporation proceeded. In terms of surface roughness, the parameters (Ra and Rq) were larger compared to the not-calendared ones (Table 3). This might suggest that the calendared patterns are more intricate. With regard to the uniformity of the pattern, as can be seen from a large standard deviation (>10%) (Table 3) it still needs to be improved. The ununiformity can be caused by warp of flexible screen meshes during drying. The templates were thought to be detached from the substrate before the solvent fully evaporated. As for the water repellency, the CAs increased significantly (122–141◦ ) compared to the not-calendared case (107–113◦ ) (Table 3). This suggests that the increase in the footprint area might enhance the surface roughness and/or increase the area to trap the air under a water droplet. It can also be seen that as the mesh count increases, the CA of the resulting surface increases. Especially, masking with the 900 mesh screen gave a high CA of 141◦ (Table 3). Different from other meshes, the occupied area of the open areas of the mesh is comparable or less than that of footprints. Considering that the furthest distance between the rims of footprints is within 15 ␮m, which is almost the same distance between protrusions of the surface microstructure of a natural lotus leaf (10–20 ␮m) [29,30], the Cassie-Baxter regime can be dominant. 3.7. Addition of graphene to the micro-textured surface Since there are many reports on addition of nano-scale structure onto micro-textured surface to achieve superhydrophobicity (CA >150◦ ) [30–32], we also attempted to modify the micro-textured surface of CYTOP with a nanomaterial. We chose graphene sheet as the nanomaterial because it can form aggregates to expose a bundle of nanosheets on the surface. Indeed, a SEM image of the aggregates used in this study shows there are many sheet-like protrusion with the size of tens to hundreds of nanometers on the surfaces (Fig. S2).

Twill weave

Fig. 8. An optical image of the polymer pattern of the 10 wt% CYTOP solution containing 0.1 wt% graphene using the 900 calendared mesh of a twill woven fabric.

Thus, a mixed solution of CYTOP (10 wt%) and graphene (0.1 wt%) was prepared. Since the graphene could not dissolve well in the fluorinated solvent, after sonication it was used as a suspension. The mixture was cast on a glass substrate covered with the 900 calendared mesh that showed the highest CA in this study. Fig. 8 showed the optical image of the resulting pattern. Basically, the same textured surface of the polymer as the case without graphene was reproduced. The graphene sheets were mainly observed around the rim of the footprints as aggregates, and most of them were protruded from the polymer surface, which was confirmed by SEM-EDX analysis (Fig. S3). We think that the graphene aggregates moved along the solution flow during drying, and mixed with the polymer to stick out of the microstructure mainly due to the aggregates size (one to several tens ␮m) compared to the polymer height (2.5 ␮m) As a result, the CA of 155◦ and the roll-off angle of 2◦ was obtained, which is superhydrophobicity. A control experiment confirmed no significant change in the CA of spin-coated CYTOP film before and after addition of graphene. Therefore, it can be concluded that the superhydrophobicity is attributable to the structural change in the micro-textured surface, that is, formation of micro/nano hierarchy structures of the fluoropolymer/graphene hybrid. 4. Conclusions We have reported a new method to fabricate micro-textured surfaces with a high water repellency using screen meshes as a template based on evaporative pattern formation of solutions containing a hydrophobic polymer, CYTOP. Various types of the meshes were applied, and the resulting polymer patterns provided hydrophobic textured surfaces on a hydrophilic glass substrate, depending on the mesh geometries, the polymer concentration and the evaporation temperature. Subtle differences in the capillary forces that were generated between the asperity of the knuckles in the mesh and the flat substrate determined the resulting polymer patterns on the surface. The pendular ring or liquid bridge formation of the solution at the contact point of the knuckles with

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the substrate was the key process for the evaporative patterning. The contact angles of the textured surfaces were higher than that of just spin-coated one, although the polymer did not cover all the hydrophilic surfaces. The patterning using the 900 mesh of the twill-woven fabric with calendared knuckles gave the highest CA in this study, which might be due to the high roughness surface and the highly efficient structure to trap the air under the water droplet. Finally, addition of graphene nanosheets to the micro-textured surfaces resulted in a superhydrophobic surface (CA: 155◦ ), implying the micro/nano hierarchy structure formation by CYTOP/graphene hybrid. This method can extend to hydrophobic coating with shape degrees of freedom like a curved surface because of the template flexibility. It can be applied for printed, flexible, and wearable devices. Acknowledgement A part of this work was conducted at Tsukuba West and the Nano Processing Facility (NPF) of AIST. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2016.11. 213. References [1] M.C. Draper, C.R. Crick, V. Orlickaite, V.A. Turek, I.P. Parkin, J.B. Edel, Superhydrophobic surfaces as an on-chip microfluidic toolkit for total droplet control, Anal. Chem. 85 (2013) 5405–5410. [2] Y. Lu, Y. Wu, J. Liang, M.R. Libera, S.A. Sukhishvili, Self-defensive antibacterial layer-by-layer hydrogel coatings with pH-triggered hydrophobicity, Biomaterials 45 (2015) 64–71. [3] B. Bhushan, Y.C. Jung, K. Koch, Self-cleaning efficiency of artificial superhydrophobic surfaces, Langmuir 25 (2009) 3240–3248. [4] B. García, J. Saiz-Poseu, R. Gras-Charles, J. Hernando, R. Alibés, F. Novio, J. Sedó, F. Busqué, D. Ruiz-Molina, Mussel-inspired hydrophobic coatings for water-repellent textiles and oil removal, ACS Appl. Mater. Interfaces 6 (2014) 17616–17625. [5] Y. Lai, Y. Tang, J. Gong, D. Gong, L. Chi, C. Lin, Z. Chen, Transparent superhydrophobic/superhydrophilic TiO2-based coatings for self-cleaning and anti-fogging, J. Mater. Chem. 22 (2012) 7420–7426. [6] F. Zhang, L. Zhao, H. Chen, S. Xu, D.G. Evans, X. Duan, Corrosion resistance of superhydrophobic layered double hydroxide films on aluminum, Angew. Chem. Int. Ed. 47 (2008) 2466–2469. [7] S. Ghiassian, H. Ismaili, B.D.W. Lubbock, J.W. Dube, P.J. Ragogna, M.S. Workentin, Photoinduced carbene generation from diazirine modified task specific phosphonium salts to prepare robust hydrophobic coatings, Langmuir 28 (2012) 12326–12333. [8] Y. Liu, J. Tang, R. Wang, H. Lu, L. Li, Y. Kong, K. Qi, J.H. Xin, Artificial lotus leaf structures from assembling carbon nanotubes and their applications in hydrophobic textiles, J. Mater. Chem. 17 (2007) 1071–1078. [9] M. Ma, M. Gupta, Z. Li, L. Zhai, K.K. Gleason, R.E. Cohen, M.F. Rubner, G.C. Rutledge, Decorated electrospun fibers exhibiting superhydrophobicity, Adv. Mater. 19 (2007) 255–259.

[10] E. Bormashenko, T. Stein, G. Whyman, Y. Bormashenko, R. Pogreb, Wetting properties of the multiscaled nanostructured polymer and metallic superhydrophobic surfaces, Langmuir 22 (2006) 9982–9985. [11] L. Zhang, Z. Zhou, B. Cheng, J.M. DeSimone, E.T. Samulski, Superhydrophobic behavior of a perfluoropolyether lotus-leaf-like topography, Langmuir 22 (2006) 8576–8580. [12] D. Öner, T.J. McCarthy, Ultrahydrophobic surfaces. effects of topography length scales on wettability, Langmuir 16 (2000) 7777–7782. [13] S. Huang, L. Ren, J. Guo, H. Zhu, C. Zhang, T. Liu, The preparation of graphene hybrid films decorated with poly[2-methoxy-5-(2’-ethyl-hexyloxy)-1,4-phenylene vinylene] particles prepared by non-solvent induced precipitation, Carbon 50 (2012) 216–224. [14] S. Peng, D. Tian, X. Yang, W. Deng, Highly efficient and large-scale fabrication of superhydrophobic alumina surface with strong stability based on self-congregated alumina nanowires, Appl. Mater. Interfaces 6 (2014) 4831–4841. [15] L. Zhang, D.-a. Zha, T. Du, S. Mei, Z. Shi, Z. Jin, Formation of superhydrophobic microspheres of poly(vinylidene-fluoride-hexafluoropropylene)/graphene composite via gelation, Langmuir 27 (2011) 8943–8949. [16] L. Feng, Z. Zhang, Z. Mai, Y. Ma, B. Liu, L. Jiang, D. Zhu, A super-hydrophobic and super-oleophilic coating mesh film for the separation of oil and water, Angew. Chem. Int. Ed. 43 (2004) 2012–2014. [17] T. Darmanin, J. Tarrade, E. Celia, F. Guittard, Superoleophobic meshes with high adhesion by electrodeposition of conducting polymer containing short perfluorobutyl chains, J. Phys. Chem. C 118 (2014) 2052–2057. [18] J. Zhuang, M.M. Du, H.Z. Cai, Y.J. Zhang, D.M. Wu, Preparation of super hydrophobic surfaces by the stainless steel wire mesh as templates, Adv. Mater. Res. 562–564 (2012) 56–59. [19] J. Yanga, Y. Tanga, J. Xub, B. Chena, H. Tanga, C. Lia, Durable superhydrophobic/superoleophilic epoxy/attapulgite nanocomposite coatings for oil/water separation, Surf. Coat. Technol. 272 (2015) 285–290. [20] L. Yang, Y. Tu, H. Fang, Modeling the rupture of a capillary liquid bridge between a sphere and plane, Soft Matter 6 (2010) 6178–6182. ¨ op, ¨ A. s. Dea´ık, [21] N. Nagy, D. n. Za´ımbo´ı, S. r. Pothorszky, E. Gergely-Ful Identification of dewetting stages and preparation of single chain gold nanoparticle rings by colloidal lithography, Langmuir 32 (2016) 963–971. [22] I.U. Vakarelski, D.Y.C. Chan, T. Nonoguchi, H. Shinto, K. Higashitani, Assembly of gold nanoparticles into microwire networks induced by drying liquid bridges, Phys. Rev. Lett. 102 (2009) (058303-058301-058304). [23] I.U. Vakarelski, J.O. Marston, S.T. Thoroddsen, Foam-film-stabilized liquid bridge networks in evaporative lithography and wet granular matter, Langmuir 29 (2013) 4966–4973. [24] I.U. Vakarelski, J.W. Kwek, X. Tang, S.J. O’shea, D.Y.C. Chan, Particulate templates and ordered liquid bridge networks in evaporative lithography, Langmuir 25 (2009) 13311–13314. [25] Z. Wei, M.F. He, W.B. Zhao, Y. Li, Thermodynamic analysis of liquid bridge for fixed volume in atomic force microscope, Sci. China—Phys. Mech. Astron. 56 (2013) 1962–1969. [26] D. Moon, D.J. Im, T. Uhm, I.S. Kang, Pico liter dispensing using liquid bridge breakup in immiscible fluid, Int. J. Chem. Eng. Appl. 3 (2012) 36–39. [27] M. Ramiasa-MacGregor, A. Mierczynska, R. Sedev, K. Vasilev, Tuning and predicting the wetting of nanoenigineered material surface, Nanoscale 8 (2016) 4635–4642. [28] S. Kinoshita, S. Yoshioka, J. Miyazaki, Physics of structural colors, Rep. Prog. Phys. 71 (2008) 076401. [29] Y. Wang, Y. Mo, M. Zhu, M. Bai, Wettability of metal coatings with biomimic micro textures, Surf. Coat. Technol. 203 (2008) 137–141. [30] E. Celia, T. Darmanin, E.T. d. Givenchy, S. Amigoni, F. Guittard, Recent advances in desingning superhydrophobic surfaces, J. Colloid Interface Sci. 402 (2013) 1–18. [31] J. Dong, Z. Yao, T. Yang, L. Jiang, C. Shen, Control of superhydrophilic and superhydrophobic graphene interface, Sci. Rep. 3 (2013) 1733. [32] L. Zhang, D.-a. Zha, T. Du, S. Mei, Z. Shi, Z. Jin, Formation of superhydrophobic microspheres of poly(vinylidene fluoride-hexafluoropropylene)/graphene composite via gelation, Langmuir 27 (2011) 8943–8949.