Sliding of Water Droplets on the Superhydrophobic Surface with ZnO

Jun 15, 2009 - (26) In the current study, using a high-speed camera system, we evaluated the sliding acceleration during the downfall of water droplet...
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Sliding of Water Droplets on the Superhydrophobic Surface with ZnO Nanorods† Munetoshi Sakai,‡ Hiroki Kono,‡,§ Akira Nakajima,*,‡, Xintong Zhang,‡ Hideki Sakai,§ Masahiko Abe,§ and Akira Fujishima‡

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‡ Kanagawa Academy of Science and Technology, 308 East, Kanagawa Science Park, 3-2-1 Sakado, Takatsu-ku, Kawasaki-shi, Kanagawa 213-0012, Japan, §Department of Pure and Applied Chemistry, Science University of Tokyo, 2641 Yamazaki, Noda-shi, Chiba 278-8510, Japan, and Department of Metallurgy and Ceramics Science, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan.

Received April 23, 2009. Revised Manuscript Received May 30, 2009 In this study, we prepared various superhydrophobic surfaces using ZnO nanorod arrays (ZnO-NR) of different diameters. The contact angle was equivalent to the calculated value if it is assuming that the topmost surface of the rods is a solid-liquid contact area. On the superhydrophobic ZnO-NR surfaces, the 5 μL water droplets slid down by constant acceleration motion. Sliding acceleration was governed by the solid area fraction. The resistance force of the actual measurement was consistent with that calculated using the model.

I. Introduction Coatings with a water contact angle greater than 150° (i.e., superhydrophobic coatings) have attracted great interest. The small contact area between a solid and water on a superhydrophobic coating limits chemical reactions and bond formation through the water. Various phenomena are inhibited on such a coating: snow adhesion, oxidation, and electrical conduction.1 Superhydrophobic coatings have been prepared using several methods to roughen surfaces and lower the surface energy. Wenzel proposed a model describing the contact angle θ0 on a rough surface.2 He modified Young’s equation to the following: cos θ0 ¼

rðγSV - γSL Þ ¼ r cos θ γLV

Therein, γSL, γSV, and γLV, respectively, signify the interfacial free energies per unit area of solid-liquid, solid-gas, and liquidgas interfaces. In fact, r is the roughness factor, defined as the ratio of the actual area of a rough surface to the geometrically projected area. In this equation, r is always larger than unity. Therefore, surface roughness enhances hydrophobicity of hydrophobic surface under these parameters. Cassie proposed an equation that describes contact angle θ0 at a surface composed of solid and air. When a unit area of the surface has a wetted solid surface area fraction f with a water contact angle θ, the contact angle on the surface can be expressed using the following equation, assuming a 180° water contact angle for air:3 cos θ0 ¼ f cos θ þ ð1 - f Þcos 180° ¼ f cos θ þ f - 1 Appropriate roughness is important for processing superhydrophobic coatings. Numerous reports describe methods to † Part of the “Langmuir 25th Year: Wetting and superhydrophobicity” special issue. *Corresponding author. Akira Nakajima. Tel.: þ81-3-5734-2525. Fax: þ81-3-5734-3355. E-mail: [email protected].

(1) Nakajima, A.; Hashimoto, K.; Watanabe, T. Monatsh. Chem. 2001, 132, 31. (2) Wenzel, R. N. J. Phys. Colloid Chem. 1949, 53, 1466. (3) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546.

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produce such roughness.1,4-9 Cassie’s model is more important than Wenzel’s for superhydrophobicity with a high water-shedding property.10 For assessing hydrophobic surfaces’ water-shedding properties, static hydrophobicity such as the sliding angle (the critical angle at which a droplet starts sliding down an inclined surface by gradual tilting) and contact-angle hysteresis (the difference between the receding and advancing contact angles of a sliding droplet on an inclined surface) are often evaluated as criteria. However, these criteria are insufficient to represent water-shedding kinetics such as sliding acceleration and velocity, and are not always useful criteria for material design. Recently, recognition of the importance of dynamic hydrophobicity such as a droplet’s sliding acceleration or velocity on a tilted surface has been growing gradually. Various related studies have been reported.11-17 Water droplets are known to slide down a hydrophobic solid surface by a caterpillar-like rolling motion with or without slippage at the solid-liquid boundary;15-19 a direct observation method for internal fluidity of sliding water (4) Onda, T.; Shibuichi, S.; Satoh, N.; Tsujii, K. Langmuir 1996, 12, 2125. (5) Hozumi, A.; Takai, O. Thin Solid Films 1997, 303, 222. (6) Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watanabe, T. Adv. Mater. 1999, 16, 1365. (7) Youngblood, J. P.; McCarthy, T. J. Macromolecules 1999, 32, 6800. (8) Lau, K. K. S.; Bico, J.; Teo, K. B. K.; Chhowalla, M.; Amaratunga, G. A. J.; Milne, W. I.; McKinley, G. H.; Gleason, K. K. Nano Lett. 2003, 3, 1701. (9) Sun, T.; Wang, G.; Feng, L.; Liu, B.; Ma, Y.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2004, 43, 357. (10) Miwa, M.; Fujishima, A.; Nakajima, A.; Hashimoto, K.; Watanabe, T. Langmuir 2000, 16, 5754. (11) Rio, E.; Daerr, A.; Andreotti, B.; Limat, L. Phys. Rev. Lett. 2005, 94, 024503. (12) Richard, D.; Quere, D. Europhys. Lett. 1999, 48, 286. (13) Kim, H.-Y.; Lee, H. J.; Kang, B. H. J. Colloid Interface Sci. 2002, 247, 372. (14) Suzuki, S.; Nakajima, A.; Sakai, M.; Sakurada, Y.; Yoshida, N.; Hashimoto, A.; Kameshima, Y.; Okada, K. Chem. Lett. 2008, 37, 58. (15) Quere, D. Rep. Prog. Phys. 2005, 68, 2495. (16) Gogte, S.; Vorobieff, P.; Truesdell, R.; Mammoli, A.; van Swol, F; Shah, P.; Brinker, C. J. Phys. Fluids 2005, 17, 51701. (17) Sakai, M.; Song, J.-H.; Yoshida, N.; Suzuki, S.; Kameshima, Y.; Nakajima, A. Langmuir 2006, 22, 4906. (18) Hodges, S. R.; Jensen, O. E.; Rallison, J. M. J. Fluid Mech. 2004, 512, 95. (19) Mahadevan, L.; Pomeau, Y. Phys. Fluids 1999, 11, 2449.

Published on Web 06/15/2009

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droplets was established recently using particle image velocimetry (PIV).17,20 For a superhydrophobic surface, water droplets commonly slide down with a large sliding acceleration that is attributable to the large contribution of slipping motion.17 Various studies of the relation between the sliding angle and surface structure or chemical composition have been conducted. Nevertheless, understanding of the relation between the microstructure of a solid surface and dynamic hydrophobicity remains limited, especially for surface states near superhydrophobicity. Recently, thin solid films of ZnO nanorod arrays (hereinafter denoted as ZnO-NR) were reported.21-25 These ZnO-NR films become superhydrophobic when properly processed. Moreover, the ZnO-NR nanorod sizes are controllable according to the concentration of the precursor solution used in the hydrothermal synthesis method.26 In the current study, using a high-speed camera system, we evaluated the sliding acceleration during the downfall of water droplets on various superhydrophobic ZnONR films with different microstructures.

II. Experimental Section II-1. Sample Preparation. Monoethanolamine 0.75 M (MEA, NH2CH2CH2OH; Wako Pure Chemical Industries Ltd., Tokyo, Japan) and zinc acetate dihydrate 0.75 M (Zn(CH3COO)2H2O; Wako Pure Chemical Industries, Tokyo, Japan) were dissolved into 2-methoxyethanol (CH3OCH2CH2OH; Wako Pure Chemical Industries, Tokyo, Japan). After reflux of the solution at 60 °C for 30 min, a ZnO sol solution was prepared. The sol solution was spin-coated onto a Si wafer (n-type Si (100); Aki Corp., Japan) at 3000 rpm and annealed at 300 °C for 10 min. After repeating this coating-annealing process three times, final heat treatment was conducted at 420 °C for 1 h. Zinc nitrate hexahydrate (Zn(NO3)2 3 6H2O; Wako Pure Chemical Industries, Tokyo, Japan) and hexamethyltetramine (HMT, (CH2)6N4; Wako Pure Chemical Industries, Tokyo, Japan) were dissolved into distilled water (molar ratio, 1:1). The solution was diluted with distilled water, and the concentration of zinc nitrate hexahydrate was changed to 0.00625, 0.0125, 0.025, 0.05, 0.075, or 0.1 M. Then, the obtained ZnO films were soaked in the solutions (film size, 15 mm  40 mm; solution volume, 200 mL), and hydrothermal treatment was conducted at 90 °C for 3 h. Finally, to modify the film surface to the superhydrophobic, octadecyltrimethoxysilane (ODS, CH3(CH2)17Si(OCH3)3; Gelest Inc.) was coated onto the surface using chemical vapor deposition (CVD). The Si plate was heated with 0.2 mL of ODS in a sealed flask by filling N2 at 150 °C for 3 h; then, the surface was rinsed and dried. A flow diagram of film processing is presented in Figure 1. II-2. Evaluation. The surface structure was observed using field emission scanning electron microscopy (FE-SEM, S-4800; Hitachi High-Technologies Corp., Tokyo, Japan). The surface roughness Ra and roughness factor r were evaluated in a 2-μmsquare area using atomic force microscopy (AFM, Nano Scope IV; Veeco Instruments, USA) with a Si cantilever (spring constant 21-78 N/m, resonance frequency 260-410 kHz, NCH-10T; Nanoworld AG, Switzerland,) using tapping mode with a scan (20) Sakai, M.; Hashimoto, A.; Yoshida, N.; Suzuki, S.; Kameshima, Y.; Nakajima, A. Rev. Sci. Instrum. 2007, 78, 045103. (21) Kwok, W. M.; Djurisic, A. B.; Leung, Y. H.; Li, D.; Tam, H.; Phillip, D. L.; Chan, W. K. Appl. Phys. Lett. 2006, 89, 183112. (22) Guo, M.; Diao, P.; Cai, S. J. Solid State Chem. 2005, 178, 1864. (23) Ma, T.; Guo, M.; Zhang, M.; Zhang, Y.; Wang, X. Nanotechnology 2007, 18, 035605. (24) Feng, X. J.; Feng, L.; Jin, M. H.; Zhai, Z.; Jiang, L.; Zhu, D. B. J. Am. Chem. Soc. 2004, 126, 62. (25) Pan, Q.; Cheng, Y.-X. Appl. Surf. Sci. 2009, 255, 3904. (26) Vayssieres, L. Adv. Mater. 2003, 15(5), 464.

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Figure 1. Flow diagram of film proceeding. rate of 0.500 Hz. The crystal phase of ZnO-NR was evaluated using an X-ray diffractometer (XRD, RINT 1500; Rigaku Corp., Tokyo, Japan). The chemical composition of the ZnO-NR surface was confirmed using X-ray photoelectron spectroscopy (XPS, S-probe; Seiko Instruments Inc., Chiba, Japan). Sessile drop method using a contact angle meter (Dropmaster 500; Kyowa Interface Science Co. Ltd., Saitama, Japan) was used to measure contact angles. For measurement of the contact angle, the droplet size should be less than the capillary length of the liquid to render the effect of gravity negligible. For water, that length is 2.7 mm. In the present study, the droplet volume for the contact angle measurement was 3 μL (1.8 mm droplet diameter). Contact angles were measured at five different points for each coating. The surface was blown with ionized air (Winstat BF-Z; Shishido Electrostatic Ltd., Tokyo, Japan) before each measurement to eliminate static electricity on the surface. A sliding angle measurement system (SA-20; Kyowa Interface Science) recorded the sliding angles of a 5 μL water droplet. Although we generally employ 30 μL droplets for sliding angle measurement,14,17 smaller droplets were used for this study to evaluate the surface effect more precisely. The water droplet’s sliding acceleration was measured using the “Image Analysis System for Evaluating Sliding Behavior of a Liquid Droplet”.20 The sample was tilted at 15°. A 5 μL water droplet was then placed gently on the inclined sample surface. Sequential photographs of the sliding action of the water droplets on the surface were taken every 1 ms using a high-speed digital camera system (512 PCI; Photron Ltd., Tokyo, Japan). The sliding acceleration was estimated by measuring the sliding distance of the front or rear edge of the contact line between the droplet and sample surface from the initial starting point using commercial software (Dipp Macro-KAST; Ditect Co. Ltd., Tokyo, Japan). The internal fluidity of the droplet was measured using PIV.20 The water droplet contained 0.06 wt % fluorescent particles (diameter, 3 μm; density, 1.05 cm3/g; excitation wavelength, 542 nm; emission wavelength, 612 nm; R0300, Duke Scientific Corp., CA, USA). The droplet’s central section was imaged using a sheet-like Ar ion laser (intensity, 1000 mW; sheet width, 200 μm; wavelengths, 488 and 514 nm; Seika Corp., Tokyo). The droplet was then made to slide. Sequential images depicting the central section of the sliding droplet were obtained DOI: 10.1021/la901461k

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Figure 4. XRD spectra of the thin solid surface on the Si wafer after hydrothermal treatment.

Figure 2. FE-SEM images of the ZnO-NR on the Si wafer after hydrothermal treatment. The precursor concentrations were (a) 0.1 M, (b) 0.075 M, (c) 0.05 M, (d) 0.025 M, (e) 0.0125 M, and (f ) 0.00625 M.

Figure 5. Relation between the precursor solution concentration and the surface microstructure. The vertical axis of the left side shows the surface roughness Ra. The vertical axis of the right side shows the roughness factor r.

Figure 3. SEM images of ZnO-NR for diameter measurement. using a high-speed camera (1024 PCI; Photron Ltd., Tokyo, Japan). An image correlation method was used in this system (Dipp Flow; Ditect, Tokyo, Japan). An outline and figures of this measurement are presented in an earlier study.27

III. Results and Discussion III-1. Structure and Properties of Samples. Figure 2 presents SEM images of ZnO-NR on the Si wafer after hydrothermal treatment. The ZnO-NR with a hexagonal prism shape had 20-150 nm diameter. The averaged diameter of ZnO-NR depended on the concentration of precursor solutions. The average (27) Furuta, T.; Nakajima, A.; Sakai, M.; Isobe, T.; Kameshima, Y.; Okada, K. Langmuir 2009, 25, 5417.

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diameter d (Figure 3) and its standard deviation σ increased concomitantly with increasing concentrations of precursor solutions. Their respective practical values for solutions of (a) 0.1 M, (b) 0.075 M, (c) 0.05 M, (d) 0.025 M, (e) 0.0125 M, and (f ) 0.00625 were (a) 123.1 nm (d ) and 35.2 nm (σ), (b) 93.8 and 28.9 nm, (c) 69.2 and 21.8 nm, (d) 37.9 and 9.8 nm, (e) 29.3 and 7.5 nm, and (f ) 19.8 and 3.6 nm. The surface composition was almost stoichiometric ZnO; no impurity was identified by XPS except carbon on the surface. The XRD patterns revealed that the films were wurtzite-type ZnO in the hexagonal system (Figure 4). The strong intensity of the reflection from the (002) plane reflects the strong orientation to the c-axis direction. This trend was consistent with that described in an earlier report.24 Figure 5 presents the relation between the concentration of the precursor solution and the surface structure. The surface structure becomes rough with decreasing precursor solution concentration. Therefore, a high contact angle was obtained from the ZnO-NR film treated in a thin solution (Figure 6). The sample surface became superhydrophobic when the concentration of the precursor solution was less than 0.075 M. Miwa et al. assumed a uniform needle-like structure on the rough surface and obtained the following equation by combining Langmuir 2009, 25(24), 14182–14186

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Article Table 1. Sliding Accelerations of a Water Droplet during Downfall concentration of precursor (M) 0.00625 0.0125 0.025 0.05

solid area fraction

sliding acceleration (mm/s2)

0.07 0.17 0.23 0.33

2.229 2.154 2.124 2.118

Figure 6. Relation between concentration of the precursor solution and the water contact angles of the ZnO films.

the Cassie mode with the Wenzel mode.10 cos θ0 ¼ rf cos θs þ f - 1 In that equation, f is the area fraction of the solid, r signifies the roughness factor, θ0 is the expected contact angle, and θs is the contact angle of the solid without roughness. Assuming that f is the area fraction of the nanorod head (top plane) and that θs is the contact angle on the flat surface by ODS coating (100°),28 the expected contact angle values of ZnO-NR films are calculable. The area of the ZnO-NR head Shead was calculated from the average diameter d as shown below.  Shead ¼ 6 

 d d  2 sin 60 2

Therefore, when ZnO-NR per unit area is multiplied to Shead, the solid area fraction f is calculable as pffiffiffi D 3  d2  f ¼ Sf 2 where D represents the ZnO-NR numbers in the SEM images and Sf denotes the frame area of the SEM image. The respective heights and widths of SEM images were 600 and 850 nm. The calculated contact angles based on this assumption for the surface superhydrophobicity were almost equivalent to the actual values (Figure 6). This result suggests that the practical contact area between solid and water is mainly the top plane of ZnO-NR when the surface is superhydrophobic. It is noteworthy that precise measurement of surface roughness using AFM is difficult because of the limitation of intruding tip’s top to the surface structure. The obtained values might have some margin of error. However, the calculated contact angle values using Miwa’s equation are almost identical to the practical ones, which implies that its margin of error is not large. On the other hand, the sliding angle of the 5 μL water droplet was less than 11° when the ZnO-NR was synthesized in precursor solutions whose concentrations were 0.00625-0.05 M. These samples were used for measurements of dynamic hydrophobicity. (28) Hashimoto, A.; Sakai, M.; Yoshida, N.; Suzuki, S.; Kameshima, Y.; Nakajima, A. J. Surf. Finishing Soc. Jpn. 2008, 59/12, 907.

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Figure 7. Images for the distribution of the velocity vector of the internal fluidity in the sliding droplet. The solid area fraction on the superhydrophobic surface was 0.17. The tilt angle was 15°.

III-2. Relation between Dynamic Hydrophobicity and Solid Structure in Superhydrophobic Region. Actual obtained sliding accelerations are presented in Table 1. Figure 7 displays a movement distance in the elapsed time and a velocity distribution in a droplet on the superhydrophobic surface with the solid area fraction 0.17. The sliding behavior of the water droplet exhibited almost constant accelerated motion (a: 2.26 m/s2). Moreover, the velocity distributions from PIV were uniform for the slope direction, suggesting that the water droplet fluidity on the superhydrophobic coating is almost entirely a slipping motion. This analysis revealed that a 5 μL water droplet slides down on all these superhydrophobic ZnO-NR films almost entirely by a slipping motion with little deformation. The sliding behavior of the water droplet exhibited almost constant accelerated motion in the measurement range. For that reason, the droplet motion is treated in this study as a mass point system. The equation of motion is expressed as ma ¼ mg sin R - R where m represents the droplet mass, a denotes the sliding acceleration, and R signifies the resistance force against the sliding direction of the droplet. The air resistance was ignored in relation to the sliding behavior. For the case of the mass point system, the resistance coefficient between the solid surface and the bottom of the liquid is assumed. The velocity dependence of this resistance is negligible. Therefore, the former equation can be changed to produce ma ¼ mg sin R - μN DOI: 10.1021/la901461k

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Figure 8. Schematic diagram showing the function of the air layer when the water droplet slid on the superhydrophobic surface.

where μ signifies the coefficient of the dynamic friction, and N is the normal force. The practical values of μ are 2.0  10-2 to 5.0 10-2 in this study. To elucidate the sliding behavior on the superhydrophobic surface with the controlled nanostructure, we specifically examined the differences in the coefficient of dynamic friction μ1 and the solid area fraction f (Figure 8). Specifically, when the droplet was placed on the surface of such a sample, the normal force N was characterized as follows: N ¼ mg cos R ¼ fN þ ð1 - f ÞN When Nsolid and Nair are the normal forces of solid and air layer, the equations are obtained as presented below. Nsolid ¼ fN ¼ fmg cos R Nair ¼ ð1 - f ÞN ¼ ð1 - f Þmg cos R When it was assumed that the air under the water droplet bottom was supported on the superhydrophobic surface, the normal force per unit was constant, as N=Sall ¼ Nair =Sair ¼ Nsolid =Ssolid where Sall represents the droplet bottom area, Sair is (1- f )  Sall and Ssolid is f  Sall. This is justified by increased Laplace pressure for the air layer in the microstructure on the superhydrophobic surface (Figure 8). The droplet bottom was supported by both the air and the solid because the action-reaction force was acting at the interaction between air and liquid. The resistance force μN consisted of two elements of the air layer and solid under the bottom of the droplet. Therefore, it was formulated as follows: μN ¼ μair Nair þ μsolid Nsolid Therein, μairN signifies the resistance force of the air layer, and μsolidN represents the resistance force of the solid. Because the friction force between the air layer and the liquid is applicable to zero, the following holds: μN ¼ μsolid Nsolid

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Figure 9. Relation between the solid area fraction f to the actual resistance force and the calculated resistance force.

Therefore, the following are obtained: μsolid Nsolid ¼ mg sin R - ma μsolid fmg cos R ¼ mg sin R - ma In those equations, the left side and right side are calculated, respectively, from actual measurements and the logical model. For calculation of the left side, the coefficient of dynamic friction μsolid in the sliding of the droplet on the smooth ODS coating is used. Hashimoto et al. reported that the sliding acceleration was 4.2 m/s2 in the sliding behavior of the water droplet on the coating (i.e., μsolid; 1.77  10-1).27 Figure 9 portrays the resistance force of the actual measurement and the logical model. The resistance force was almost equivalent by order. It is inferred that the sliding acceleration on a superhydrophobic surface such as ZnO-NR was governed by the solid area fraction f. This model is applicable when a droplet slides down by constant acceleration, mainly by slipping with little deformation. When the contribution of the rolling mechanism becomes large, then a different model must be used. Detailed analyses for the contribution of surface roughness, surface shape, chemical composition, droplet mass, tilt angle in the wide range are to be addressed in future work.

IV. Conclusion In this study, we prepared various superhydrophobic surfaces with different surface structures. The surface structures comprised ZnO-NR of different diameters. The contact angle was equivalent to the calculated value by assuming the topmost surface of rods as the solid-liquid contact area. The 5 μL water droplets slid down by constant acceleration motion on the superhydrophobic surface of ZnO-NR. Sliding acceleration was governed by the solid area fraction f. The resistance force of the actual measurement was consistent with that calculated using our model. Acknowledgment. We are grateful to staff at Material Characterization Center at the Kanagawa Academy of Science and Technology for SEM, AFM, XRD, and XPS analyses.

Langmuir 2009, 25(24), 14182–14186