Fabrication of durable hydrophobic surfaces through surface texturing

Applied Surface Science 257 (2011) 5688–5693

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Fabrication of durable hydrophobic surfaces through surface texturing Samuel Beckford a , Nicholas Langston a , Min Zou a,∗ , Ronghua Wei b a b

Department of Mechanical Engineering, University of Arkansas, 863 W. Dickson St., Fayetteville, AR 72701, USA Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238, USA

a r t i c l e

i n f o

Article history: Received 5 September 2010 Received in revised form 12 December 2010 Accepted 15 January 2011 Available online 25 January 2011 Keywords: Fluorinated carbon film Durability Hydrophobic Water contact angle Friction

a b s t r a c t Low surface energy polymer thin-films can be applied to surfaces to increase hydrophobicity and reduce friction for a variety of applications. However, wear of these thin films, resulting from repetitive rubbing against another surface, is of great concern. In this study, we show that highly hydrophobic surfaces with persistent abrasion resistance can be fabricated by depositing fluorinated carbon thin films on sandblasted glass surfaces. In our study, fluorinated carbon thin films were deposited on sandblasted and as-received smooth glass using deep reactive ion etching equipment by only activating the passivation step. The surfaces of the samples were then rubbed with FibrMet abrasive papers in a reciprocating motion using an automatic friction abrasion analyzer. During the rubbing, the static and kinetic friction forces were also measured. The surface wetting properties were then characterized using a video-based contact angle measuring system to determine the changes in water contact angle as a result of rubbing. Assessment of the wear properties of the thin films was based on the changes in the water contact angles of the coated surfaces after repetitive rubbing. It was found that, for sandblasted glass coated with fluorinated carbon film, the water contact angle remained constant throughout the entire rubbing process, contrary to the smooth glass coated with fluorinated carbon film which showed a drastic decrease in water contact angle with the increasing number of rubbing cycles. In addition, the static and kinetic friction coefficients of the sandblasted glass were also much lower than those of the smooth glass. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Hydrophobic surfaces, having water contact angles (WCAs) larger than 90◦ , have many applications, such as for biocompatibility of medical implants [1], micro-electro-mechanical system/nano-electro-mechanical system (MEMS/NEMS) stiction solution [2], anti-icing [3], and friction and wear reduction between parts in dynamic contact [4,5]. Because of their wide practical applications, surfaces’ wetting properties have been of significant interest for many years. There have been considerable theoretical and experimental investigations to determine the structural and dynamic interactions between water and solid surfaces. An important factor that affects the interaction is the solid surface molecular structure, specifically its polarity and charge [6,7]. The polarity and charges in atomic interactions have a significant influence on the strength of the bond created in molecular interfaces. High binding energy materials are hydrophilic and these materials are generally ionic, covalent or metallic. On the other hand, low binding energy materials are hydrophobic and include molecular crystals and plastics [8]. The hydrophobicity or hydrophilicity of a surface is determined by measuring the angle

∗ Corresponding author. Tel.: +1 479 575 6671; fax: +1 479 575 6982. E-mail address: [email protected] (M. Zou). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.01.074

a water droplet forms between the liquid/solid and liquid/vapor interface [9]. Hydrophobic surfaces are often generated by applying low surface energy polymer thin-films to a substrate surface [10,11]. In addition to chemical modification, creating micro-scale topography on material surfaces can also change their wetting properties. In 1936, R.N. Wenzel determined that by microscopically increasing the surface roughness of a substrate, it is possible to increase its hydrophobicity or hydrophilicity [12,13]. The Wenzel state has been called the collapsed state [14] and refers to the fact that the water droplet sits below the peaks of the roughened surface. Wenzel related the WCA, , of a roughened surface to the specific contact angle, ϕ, of a smooth surface of the same chemistry and the ratio, r, of the actual area to the projected area of a roughened surface. The relationship of the WCA to these variables is expressed in the equation: cos  = r cos ϕ [15]. In 1944, A.B.D. Cassie and S. Baxter established the suspended state which relates the WCA,  C , of a roughened surface to the ratio of the liquid/solid contact area to the total projected area of the surface, ϕ, and , the specific WCA of a smooth surface of the same chemistry. The relationship of the WCA to these variables is expressed in the equation:  C = ϕ(1 + cos ) − 1 [12,14–17]. In both cases, by increasing the surface roughness, the resulting WCA is enhanced for specific WCAs above 90◦ . For engineering surfaces, rubbing occurs frequently. One practical consideration is that hydrophobic surfaces need to maintain

S. Beckford et al. / Applied Surface Science 257 (2011) 5688–5693

their wetting properties despite any abrasive contact they may be exposed to. At the moment, there has been very little study carried out to investigate the change of thin polymer films’ wetting properties after rubbing. In an investigation, Zheng et al. studied the changes in the wetting properties after rubbing hydrophilic polyimide thin films on a glass substrate in an effort to understand liquid crystal alignment mechanisms [18]. The rubbing was performed using a custom made rubbing machine composed of a rotational drum covered by a velvet textile. Zheng et al. found that the rubbing of the glass surface produced grooves and slightly increased the surface roughness. The grooves caused the surface wettability to no longer be uniform, but vary azimuthally. In a similar investigation, Paek suggested a reorientation of polyimide molecules determined by an increased hysteresis in their dynamic water contact angle measurement [19]. In this case the samples were unidirectionally rubbed with a 15 cm diameter roller covered by a cotton cloth. The roller speed and the substrate stage speed were set at 200 rpm and 24 mm/s, respectively. Zimmermann et al. studied the effects of rubbing on polymethylsilsesquioxane coating on glass in an effort to compare the results on glass to certain textiles [20]. The samples were rubbed using a textile friction analyzer simulating approximately 150% of the contact pressure that occurs when a person rubs a fabric with their fingers. The results for Zimmermann’s study showed a complete destruction of the surface structure of the silicone nanofilament coating leaving a residual hydrophobic layer producing a WCA of 95◦ [20]. In this paper, a systematic study was performed on glass substrates to investigate the effects of rubbing on the wetting properties of fluorinated carbon film coated surfaces. It will be shown that large surface roughness generated by sandblasting can help maintain the hydrophobicity of a surface. In contrast, a smooth surface easily looses the hydrophobic properties after rubbing. The coefficient of friction between the rubbing surface and the glass substrates during rubbing was also investigated since, for some tribological applications, the friction also needs to be as low as possible in order to minimize frictional energy consumption and reduce wear.

2. Experimental details 2.1. Sample preparation Pre-cleaned plain micro glass slides (VWR, Inc.) were selected as the substrates for this study. Some of the glass slides were sandblasted at 240 kPa for 10 s, while maintaining the sandblasting nozzle perpendicular to the sample surface at a 10 cm distance. Aluminum oxide media (Trinity Tool Company, Fraser MI, Trin-blast 80 with average particle size of 165 ␮m) was used for the sandblasting. The sandblasted samples, together with the un-blasted (asreceived) samples, were cleaned with acetone in an ultrasonic bath for 20 min. They were then soaked in isopropyl alcohol (IPA) in an ultrasonic bath for 5 min and rinsed in deionized water. Subsequently the samples were blow dried using a nitrogen blower. Fluorinated carbon films of about 10 nm in thickness were deposited on the sandblasted glass slides and as-received glass slides using a deep reactive ion etching (DRIE) system (Surface Technology Systems Ltd.) by only activating the passivation step using perfluorocyclobutane (C4 F8 ) gas. During this process, the gas flow rate was kept at 85 sccm, the coil power was 200 W, the platen RF power was 12 W, and the deposition time was 20 s. Two groups of samples, as-received and sandblasted, with two different surface roughnesses, were prepared in this study. Within each group, 3 samples were coated with fluorinated carbon thin films to alter the surface energy and three samples were left uncoated. Numbering sequentially, therefore, samples SM1

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through SM3 are as-received smooth glass slides coated with fluorinated carbon film; SM4 through SM6 are as-received smooth glass slides left uncoated; SB1 through SB3 are sandblasted glass slides coated with fluorinated carbon film and SB4 through SB6 are sandblasted glass slides left uncoated. These four types of samples were then subjected to rubbing and friction tests. It should be noted that glass substrates were selected because there is a large difference in WCAs for glass substrates coated with and without the fluorinated carbon thin film, which allows the use of WCA as an indicator to detect the wear of the fluorinated carbon film after rubbing. 2.2. Sample characterization 2.2.1. Topography characterization Scanning electron microscopy (SEM, Philips/FEI XL30 ESEM) was used to characterize the surface topographies of all samples. SEM micrographs at different magnifications over several locations across each sample were taken to examine the micro-scale uniformity of the sample surfaces. To better show the surface topography, 45◦ oblique angle views were used for all measurements. A stylus surface profiler (Dektak 150, Veeco Instruments, Inc.) was used to measure surface roughness of the samples. The measurement was taken using a 12.5 ␮m radius stylus with a 9.8 ␮N contact force at a 100 ␮m/s scan speed for a scan length of 3 mm. Three measurements were taken at different locations on each sample. The average roughness and RMS roughness were recorded on each type of samples. 2.2.2. Surface chemical composition characterization The fluorinated carbon films were investigated using X-ray photoelectron spectroscopy (XPS, ULVAC-PHI/PHI 5000 Versaprobe). The XPS analyses were performed with a monochromatic Al K␣ (1486.6 eV) source. Both XPS survey spectrum and high-energy resolution (Hi-Res) scans of the F 1s regions were acquired. 2.2.3. Wettability characterization A video-based contact angle measurement system (OCA 15, DataPhysics Instruments GmbH, Germany) was used to measure the WCAs of the samples. The static WCAs of each sample were measured at least 3 times across the sample surface using the sessile drop method by dispensing 0.5 ␮L drops of deionized water on the sample surfaces. All WCA measurements were taken under ambient laboratory conditions with temperatures of about 25 ◦ C and 45% relative humidity. 2.2.4. Rubbing and friction test An automatic friction abrasion analyzer (Triboster, Kyowa Interface Science Co., Ltd.) was used to perform the rubbing test and measure the static and dynamic coefficients of friction (COF) during the rubbing test. This apparatus provides a linear reciprocating motion and uses an internal load cell to translate the horizontal load into a voltage reading, which is then converted back to a load value. This value is then divided by the amount of weight placed on the weight pan of the balance arm to obtain a COF. The static COF is determined by the highest friction coefficient registered within the first 0.5 s. The dynamic COF is determined by taking the average of all of the coefficients registered between 10 and 90% of the total stroke length. The stroke length is defined as the total distance over which the COF is to be measured, and it is user-defined specifically for each set of measurements. The number of cycles can vary between 1 and 1000 based on the requirements for the experiment. The rubbing test was performed by repetitively rubbing the test sample with a fine polishing paper (FiberMet Abrasive Disk from Buehler containing 0.05 ␮m alumina particles) that was mounted to a flat facet stainless steel piece attached to the Triboster. The

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Fig. 1. SEM micrographs (45◦ oblique angle views) of (a) an as-received glass sample, (b) a fluorinated carbon film coated as-received glass sample, (c) a sandblasted glass sample, and (d) a fluorinated carbon film coated sandblasted glass sample.

3.1. Surface topography Fig. 1 shows representative SEM micrographs (45◦ oblique angle views) of as-received and sandblasted samples coated with and without fluorinated carbon thin films. Fig. 1(a) and (b), taken from the as-received samples before and after fluorinated carbon film deposition, show that the surfaces are very smooth. Comparing Fig. 1(b) with Fig. 1(a), one can see that fluorinated carbon film deposition did not change the surface topography notably. Fig. 1(c) and (d), taken from the sandblasted samples before and after fluorinated carbon film deposition, show that the surfaces are very rough, having large peaks and valleys. The distances between peaks are tens of micrometers. Comparing Fig. 1(d) with Fig. 1(c), one can see again that fluorinated carbon film deposition did not notably change the topography of the rough surface. It should be pointed out that SEM images in Fig. 1(a) and (b) were not taken from the same sample, neither were Fig. 1(c) and (d). Table 1 Sample surface roughness. Group #

Sample description

Ra (␮m)

Rq (␮m)

1 2

As-received glass Fluorinated carbon thin film coated as-received glass Sandblasted glass Fluorinated carbon thin film coated sandblasted glass

0.016 0.020

0.030 0.039

3 4

3.2. Surface chemical composition The presence of the fluorinated carbon film and its chemical state on glass substrate is depicted in Fig. 3. The XPS C1s spectrum gives the binding energy of the fluorocarbon films resulting from

As-received glass

Sandblasted glass 50

1 As-received glass: Ra = 0.016 µm Rq = 0.030 µm Skew = 4.37

0.5

Sandblasted glass: Ra = 7.083 µm Rq = 8.669 µm Skew = -0.06 left axis

0

0

-0.5

-25

-1

7.083 7.48

8.669 8.797

25

right axis

Height (µm)

3. Results and discussion

Fig. 2 shows the representative surface profiles measured by Dektak 150 of an as-received and a sandblasted sample. It can be seen that the sandblasted surface is much rougher than the asreceived surface. The as-received surface has a large skew of 4.37 with quite a few sharp peaks, while the sandblasted surface has both peaks and valleys more or less evenly distributed as indicated by a very small skew of −0.06. The average roughness of the sandblasted sample is 7.083 ␮m, which is about 440 times that of the as-received sample of 0.016 ␮m. The surface profiles of all samples were measured by Dektak, which reveals that deposition of fluorinated carbon films does not affect the surface topography significantly (Table 1).

Height (µm)

test sample was mounted on the Triboster stage with a removable double-stick tape such that the polishing paper rested in the desired testing region of the sample. Each sample was divided into 6 regions which were rubbed for 10, 20, 50, 100, 200 and 500 cycles, respectively, under an applied load of 100 g and sliding at 5 mm/s for a stroke length of 15 mm.

0

500

1000

1500

2000

2500

-50 3000

Length (µm) Fig. 2. Surface profiles of an as-received glass sample and a sandblasted glass sample.

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Sandblasted glass with PTFE As-received glass with PTFE

Sandblasted glass without PTFE As-received glass without PTFE

Water Contact Angle (Deg)

160

120

80

40

0 0

100

200

300

400

500

600

Rubbing Cycles Fig. 5. Changes in water contact angle with rubbing cycles.

Fig. 3. Hi-res XPS spectrum of fluorinated carbon film coated on glass substrate.

the DRIE passivation process. The C1s peak is conformed of 5 components: CF3 at 293.28 eV, CF2 at 291.20 eV, CF at 289.09 eV, C O at 286.82 eV, and C–C, C–H at 284.5 eV. These peaks and their binding energies are close to that of similar fluorinated carbon films, such as Teflon film [21]. 3.3. Surface wetting property The surface wetting properties of all samples were characterized by a video-based contact angle measurement system, and the results are shown in Fig. 4. It can be seen that an as-received glass surface has a WCA of 20◦ and a sandblasted sample has a WCA of

0◦ . After coating the surfaces with fluorinated carbon film, the WCA of the as-received sample and the sandblasted sample increased to 109◦ and 136◦ , respectively. The significantly increased WCA on the sandblasted samples after fluorinated carbon film is due to the combined effects of surface topography and chemistry. The WCA of the fluorinated carbon film coated sandblasted sample (136◦ ) is much higher than that of the fluorinated carbon film coated as-received sample (109◦ ) showing the importance of surface roughness on improving the surface hydrophobicity. When a water droplet is dispensed onto the sandblasted surface, air is likely to be trapped between the peaks of the topography and the water droplet, which reduces the probability of the water droplet wetting the surface between the peaks of the topography. Fig. 5 shows the average and standard deviations of the WCAs obtained from all samples measured after different number of rubbing cycles. It can be seen that, even though coating fluorinated carbon film on an as-received glass surface increased the WCA from 20◦ to 109◦ , a drastic decrease in the WCA to 50◦ with increasing number of rubbing cycles is also observed. On the other hand, coat-

Fig. 4. Water contact angle optical images of (a) an as-received glass sample, (b) a fluorinated carbon film coated as-received glass sample, (c) a sandblasted glass sample, and (d) a fluorinated carbon film coated sandblasted glass sample.

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Fig. 6. Water contact angles of (a) a fluorinated carbon film coated as-received sample and (b) a fluorinated carbon film coated sandblasted sample after 500 cycles of rubbing.

ing fluorinated carbon film on a sandblasted glass surface increased the WCA from 0◦ to 136◦ , but the WCA, after an initial decrease of a few degrees, remains constant at approximately 131◦ even after 500 rubbing cycles. It is also seen from Fig. 5 that after 500 cycles of rubbing, the WCA of the fluorinated carbon film coated as-received glass approaches the WCA of the plain as-received glass, indicating the fluorinated carbon film is completely worn off. Fig. 6 shows the WCA of a fluorinated carbon film coated asreceived sample and a fluorinated carbon film coated sandblasted sample after 500 cycles of rubbing. Comparing Figs. 6(a) and 4(b), one can see the drastic change in WCA after rubbing for the asreceived sample, in contrast to the small change observed for the sandblasted sample by comparing Fig. 6(b) with Fig. 4(d). Fig. 7 shows representative SEM micrographs (45◦ oblique angle views) of the four types of samples investigated after 500 cycles of rubbing. The uncoated as-received sample (Fig. 7(a)) shows rubbing of isolated asperities, while the coated as-received sample (Fig. 7(b)) shows unidirectional grooves on the surface after rubbing, presumably due to fluorinated carbon film molecules aligning with the rubbing direction. Both sandblasted samples (Fig. 7(c) and (d)) show debris generated among sandblasted peaks, with much less debris generated on the fluorinated carbon film coated sand-

blasted sample (Fig. 7(d)). It should be noted here that since glass is brittle, it is easy to break the sharp asperities and generate debris. If a ductile material is used, such as a metal, there will be far fewer debris generated. From Figs. 6 and 7, it can be concluded that sandblasting to roughen the surface will help maintain the surface hydrophobicity. This is because, for the fluorinated carbon film coated sandblasted samples, when rubbing against a polishing paper, there are only very few asperities in contact with the polishing paper due to the large surface roughness of the sandblasted sample. The fluorinated carbon film in the valleys remains essentially untouched and protected by the asperity peaks. Even though, some debris are generated due to rubbing, they fall in the valleys of the asperities, thus do not affect the surface wetting property significantly; and the fluorinated carbon film coated sandblasted sample is thus able to maintain its hydrophobic properties. 3.4. Friction analysis Fig. 8 shows the static and kinetic COF of the four types of samples investigated. As shown in Fig. 8(a) and (b), after initial running in period, the static and kinetic COF of the sandblasted

Fig. 7. SEM micrographs (45◦ oblique angle views) of (a) an uncoated as-received glass surface, (b) a fluorinated carbon film coated as-received glass surface, (c) an uncoated sandblasted glass surface, and (d) a fluorinated carbon film coated sandblasted glass surface, all after 500 cycles of rubbing.

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a

Coefficient of Static Friction

Sandblasted glass with PTFE As-received glass with PTFE

Sandblasted glass without PTFE As-received glass without PTFE

1 0.8

0.4 0.2

0

100

200

300

400

500

600

Rubbing Cycles Sandblasted glass with PTFE As-received glass with PTFE

Coefficient of Kinetic Friction

b

Sandblasted glass without PTFE As-received glass without PTFE

0.8 0.6 0.4 0.2 0

the as-received samples. The sandblasted samples also showed a decreasing trend in kinetic COF, in contrast to the increasing trend of the as-received samples, making it the preferable surface for creating durable hydrophobic surfaces. Acknowledgments

0.6

0

5693

0

100

200

300

400

500

600

Rubbing Cycles Fig. 8. Coefficients of friction change with rubbing cycles. (a) Static COF and (b) kinetic COF.

samples are much lower than those of the as-received samples. Additionally, it is observed that there is no significant difference in the COF between the fluorinated carbon film coated sample and uncoated sample. This is because, for fluorinated carbon film coated as-received glass, rubbing with a polishing paper quickly removed the fluorinated carbon thin film as indicated by water contact angle measurement. For fluorinated carbon film coated sandblasted glass, however, rubbing only removed the fluorinated carbon film at the asperity peaks of the surface where the polishing paper is in contact with the glass surface. Therefore, a similar contact pair was formed to the contact pair between a polishing paper and a surface without fluorinated carbon film resulting in similar COF. Moreover, it is also observed that the static and kinetic COF on the as-received samples showed increasing trends with increasing rubbing cycles, suggesting higher possibility of failure upon further rubbing. 4. Conclusions The surface wetting properties observed after continuous rubbing by a polishing paper suggest that the roughened, fluorinated carbon film coated sample retains its hydrophobic properties for a much longer period of time than the fluorinated carbon film coated, as-received sample. In addition, the sandblasted samples also showed much lower static and kinetic COF when compared to

This work was supported in part by the National Science Foundation under grants CMMI-0645040 and DMR-0520550, and in part by the Arkansas Biosciences Institute and Southwest Research Institute under its internal grant IR8114. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. The authors also wish to thank Mr. Edward Langa of Southwest Research Institute for the Dektak measurement. References [1] C. Eriksson, H. Nygren, K. Ohlson, Implantation of hydrophilic and hydrophobic titanium discs in rat tibia: cellular reactions on the surfaces during the first 3 weeks in bone, Biomaterials 25 (2004) 4759–4766. [2] Y. Zhao, Stiction and anti-stiction in MEMS and NEMS, Acta Mech. Sinica (English Series) 19 (2003) 1–10. [3] L. Cao, A.K. Jones, V.K. Sikka, Anti-icing superhydrophobic coatings, Langmuir 25 (2009) 12444–12448. [4] Z. Tao, B. Bhushan, Surface modification of AFM silicon probes for adhesion and wear reduction, Tribol. Lett. 21 (2006) 1–16. [5] Y. Song, R. Premachandran Nair, M. Zou, Y.A. Wang, Adhesion and friction properties of micro/nano-engineered superhydrophobic/hydrophobic surfaces, Thin Solid Films 518 (2010) 3801–3807. [6] S.H. Lee, P.J. Rossky, A comparison of the structure and dynamics of liquid water at hydrophobic and hydrophilic surfaces – a molecular dynamics simulation study, J. Chem. Phys. 100 (1994) 3334–3335. [7] N. Giovambattista, P.G. Debenedetti, P.J. Rossky, Effect of surface polarity on water contact angle and interfacial hydration structure, J. Phys. Chem. B 111 (2007) 9581–9587. [8] P.D. Gennes, F. Brochard-Wyart, D. Quere, Capillarity and Wetting Phenomena: Drops, Bubbles, Pearls, Waves, Springer, New York, NY, 2004, pp. 291. [9] D.Y. Kwok, A.W. Neumann, Contact angle measurement and contact angle interpretation, Adv. Colloid Interface 81 (1999) 167–249. [10] N. Yoshida, Y. Abe, H. Shigeta, Preparation and water droplet sliding properties of transparent hydrophobic polymer coating by molecular design for self-organization, J. Sol–Gel Sci. Technol. 31 (2004) 195–199. [11] Y. Sakai, Y. Sadaoka, M. Matsuguchi, Humidity sensors based on polymer thin films, Sens. Actuators B: Chem. B35 (1996) 85–90. [12] R.N. Wenzel, Resistance of solid surfaces to wetting by water, Ind. Eng. Chem. 28 (1936) 988–994. [13] I. Tsuji, Y. Ohkubo, K. Ogawa, Study on super-hydrophobic and oleophobic surfaces prepared by chemical adsorption technique, Jpn. J. Appl. Phys. 48 (2009) 40205–40207. [14] R.J. Vrancken, H. Kusumaatmaja, K. Hermans, Fully reversible transition from Wenzel to Cassie–Baxter States on corrugated superhydrophobic surfaces, Langmuir 26 (2010) 3335–3341. [15] H. Yildirim Erbil, C. Elif Cansoy, Range of applicability of the Wenzel and Cassie–Baxter equations for superhydrophobic surfaces, Langmuir 25 (2009) 14135–14145. [16] E. Bormashenko, Y. Bormashenko, G. Whyman, Micrometrically scaled textured metallic hydrophobic interfaces validate the Cassie–Baxter wetting hypothesis, J. Colloid Interface Sci. 302 (2006) 308–311. [17] A.B.D. Cassie, S. Baxter, Wettability of porous surfaces, Trans. Faraday Soc. 40 (1944) 546–551. [18] W. Zheng, C.-H. Lu, Y.-C. Ye, Effects of mechanical rubbing on surface tension of polyimide thin films, Jpn. J. Appl. Phys. 47 (2008) 1651–1656, Part 1 (Regular Papers Short Notes & Review Papers). [19] S. Paek, Comparative study of effects of rubbing parameters on polyimide alignment layers and liquid crystal alignment, J. Ind. Eng. Chem. 7 (2001) 316–325. [20] J. Zimmermann, F.A. Reifler, G. Fortunato, A Simple, one-step approach to durable and robust superhydrophobic textiles, Adv. Funct. Mater. 18 (2008) 3662–3669. [21] D.K. Sarkar, M. Farzaneh, R.W. Paynter, Superhydrophobic properties of ultrathin rf-sputtered Teflon films coated etched aluminum surfaces, Mater. Lett. 62 (2008) 1226–1229.