Fabrication of Hydrophobic Surfaces by Coupling of Langmuir

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Fabrication of Hydrophobic Surfaces by Coupling of Langmuir-Blodgett Deposition and a Self-Assembled Monolayer Ping-Szu Tsai, Yu-Min Yang,* and Yuh-Lang Lee Department of Chemical Engineering, National Cheng Kung UniVersity, Tainan 70101 Taiwan ReceiVed NoVember 21, 2005. In Final Form: April 20, 2006 A novel method coupling the Langmuir-Blodgett (LB) deposition of silica particles and the formation of a selfassembled monolayer (SAM) of alkylsilane is proposed for fabricating hydrophobic surfaces. The LB deposition and the SAM are supposed to confer the substrate surface roughness and low surface energy, respectively. By controlling the hydrophobic-hydrophilic balance of the silica particle surface through the adsorption of surfactant molecules, deposition of monolayers consisting of hexagonally close-packed arrays of particles on a glass substrate can then be successfully conducted in a Langmuir trough. LB particulate films with a particle layer number up to 5 were thereby prepared. A sintered and hydrophobically finished particulate film with roughness factor of 1.9 was finally fabricated by sintering and surface silanization. Effects of particle size and particle layer number on the wetting behavior of the particulate films were systematically studied by measuring static and dynamic water contact angles. The experimental results revealed that a static contact angle of about 130° resulted from the particulate films regardless of the particle size and particle layer number. This is consistent with the predictions of both the Wenzel model and the Cassie and Baxter model in that roughness of a hydrophobic surface can increase its hydrophobicity and a switching of the dominant mode from Wenzel’s to Cassie and Baxter’s. In general, an advancing contact angle of about 150°, a receding contact angle of about 110°, and a contact angle hysteresis of about 40° were exhibited by the particulate films fabricated.

Introduction A coating with self-cleaning properties would be interesting and attractive since it could save a lot of time and cost for maintenance.1 Many plants in nature including the lotus leaf possess the unusual wetting characteristic of superhydrophobicity.2,3 A self-cleaning surface results since the rolling rainwater droplets wash off contaminants and dust.4,5 Much effort has been devoted to try to mimic the self-cleaning property of the lotus leaf. Synthetic superhydrophobic surfaces have been fabricated through various approaches, including creating a rough surface covered with low-surface-energy molecules,6-17 roughening the surface of hydrophobic materials,18-20 and generating wellordered microstructured surfaces with a small ratio of the liquid* To whom correspondence should be addressed. Phone: 886-62757575 ext. 62633. Fax: 886-6-2344496. E-mail: [email protected]. (1) Blossey, R. Nat. Mater. 2003, 2, 301. (2) Neinhuis, C.; Barthlott, W. Ann. Bot. 1997, 79, 677. (3) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1. (4) Chen, W.; Fadeev, A. Y.; Hsieh, M. C.; Oner, D.; Youngblood, J.; McCarthy, T. J. Langmuir 1999, 15, 3395. (5) Oner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777. (6) 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. (7) Feng, L.; Li, S.; Li, Y.; Li, H.; Zhang, L.; Zhai, J.; Song, Y.; Liu, B.; Jiang, L.; Zhu, D. AdV. Mater. 2002, 14, 1857. (8) Feng, L.; Song, Y.; Zhai, J.; Liu, B.; Xu, J.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2003, 42, 800. (9) Feng, L.; Li, S.; Li, H.; Zhai, J.; Song, Y.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2002, 41, 1221. (10) Li, H.; Wang, X.; Song, Y.; Liu, Y.; Li, Q.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2001, 40, 1743. (11) Thieme, M.; Frenzel, R.; Schmidt, S.; Simon, F.; Hennig, A.; Worch, H.; Lunkwitz, K.; Scharnweber, D. AdV. Eng. Mater. 2001, 3, 691. (12) Youngblood, J. P.; McCarthy, T. J. Macromolecules 1999, 32, 6800. (13) Tadanaga, K.; Morinaga, J.; Matsuda, A.; Minami, T. Chem. Mater. 2000, 12, 590. (14) Zhai, L.; Cebeci, F. C.; Cohen, R. E., Rubner, M. F. Nano Lett. 2004, 4, 1349. (15) Nan, Z.; Feng, S.; Zhiqiang W.; Xi, Z. Langmuir 2005, 21, 4713. (16) Zhang, X.; Shi, F.; Yu, X.; Liu, H.; Fu, Y.; Wang, Z.; Jiang, L.; Li, X. J. Am. Chem. Soc. 2004, 126, 3064. (17) Feng, S.; Zhiqiang W.; Xi, Z. AdV. Mater. 2005, 17, 1005. (18) Morra, M.; Occhiello, E.; Garbassi, F. Langmuir 1989, 5, 872.

solid contact area.4,5,21,22 Most of the methods disclosed to date, however, are either expensive, substrate limited, require the use of harsh chemical treatments, or cannot be easily scaled-up to create large-area uniform coatings.14 Although it cannot be easily scaled-up to create large-area coatings, the Langmuir-Blodgett (LB) deposition is well known to be capable of preparing highly ordered monomolecular films with a densely packed structure and precisely controlled thickness.23 Surface characterization of the monolayer, influence of transfer promoters, and LB films of a phthalocyanine for gas-sensing purposes have also been studied by the authors.24-27 On the other hand, self-assembled monolayers (SAMs) formed on solid substrates by spontaneous assembly of amphiphilic molecules from dilute solutions are often considered as solidstate analogues to LB films prepared on a liquid subphase and transferred to a solid support.23 Despite the striking similarities between these two classes of highly organized, supramolecular systems, regarding the type of film molecules and their uniform, densely packed assembly on the substrate surface, SAM films are generally strongly chemisorbed and often show pronounced, substrate-dependent properties unknown for LB films but rather typical for epitaxial overlayers. The surface chemistry of many materials, such as gold, alumina, mica, and oxidized silicon can (19) Morra, M.; Occhiello, E.; Garbassi, F. J. Colloid Inerface Sci. 1989, 132, 504. (20) Erbil, H. Y.; Demirel, A. L.; Avci, Y.; Mert, O. Science 2003, 299, 1377. (21) Shiu, J.-Y.; Kuo, C.-W.; Chen, P.; Mou, C.-Y. Chem. Mater. 2004, 16, 561. (22) Yoshimitsu, Z.; Nakajima, A.; Watanabe, T.; Hashimoto, K. Langmuir 2002, 18, 5818. (23) Ulman, A. An Introduction to Ultrathin Organic Films from LangmuirBlodgett to Self-Assembly; Academic Press: San Diego, 1991. (24) Lee, Y.-L.; Chen, Y.-C.; Chang, C.-H.; Yang, Y.-M.; Maa, J.-R. Thin Solid Films 2000, 370, 278. (25) Ku, I.-H.; Lee, Y.-L.; Chang, C.-H.; Yang, Y.-M.; Maa, J.-R. Colloids Surf., A 2001, 191, 223. (26) Sheu, C.-W.; Lin, K.-M.; Ku, I.-H.; Chang, C.-H.; Lee, Y.-L.; Yang, Y.-M.; Maa, J.-R. Colloids Surf., A 2002, 207, 81. (27) Sheu, C.-W.; Chang, C.-H.; Lee, Y.-L.; Yang, Y.-M,; Maa, J.-R. J. Chin. Inst. Chem. Engrs. 2002, 23, 573.

10.1021/la053152m CCC: $33.50 © 2006 American Chemical Society Published on Web 05/24/2006

Fabrication of Hydrophobic Surfaces

be modified using SAMs. A hydrophilic surface such as glass can be made hydrophobic through the self-assembly of molecules with hydrophobic tail groups such as alkylsilanes. Similarly, SAMs with chemically reactive tail groups can be formed and further chemically modified, thus allowing for the formation of multilayers.23,28 In contrast, incomplete sub-monolayers can be formed by quenching the self-assembly process before it reaches completion.29 The LB technique is frequently used to make monolayers of particles at air/water or air/oil interfaces.30-32 The main interest is the investigation of interaction forces between the particles33-37 and coagulation in two dimensions.38-42 Solid films of particles transferred from the water surface onto supports have been prepared from spread particle layers43 and by using the technique of adsorbing nanoparticles from the aqueous subphase onto the floating charged molecular monolayers.44,45 Recently, the possibility of preparing two-dimensional colloidal crystals from silica spheres by the LB method and succeeded in depositing monolayers consisting of hexagonally close-packed (hcp) arrays of silica on various substrates was studied.46-50 Their results suggest that a successful synthesis of ordered monolayers of monodisperse silica with the LB technique critically depends on the hydrophilic/hydrophobic balance. This balance was investigated by modification of the surface of the silica particles through the adsorption of surfactants.47,48 The surface-modified solid particle then behaves like an amphiphile at the air/water interface due to hydrophilic-hydrophobic balance, and the fabrication of particulate LB films is therefore made possible. Furthermore, the monolayer behavior of the particle films at air/water interface has been well studied.48 In this work, a novel method for fabricating hydrophobic surfaces is proposed by coupling the LB deposition of silica particles on a glass substrate and the formation of a SAM of alkylsilane on the outer surface of the particulate films. The (28) Mitzi, D. B. Chem. Mater. 2001, 13, 3283. (29) Schwartz, D. K.; Steinberg, S.; Israelachvili, J.; Zasadzinski, J. A. N. Phys. ReV. Lett. 1992, 69, 3354. (30) Pieranski, P. Phys. ReV. Lett. 1980, 45, 569. (31) Collier, C. P.; Saykally, R. J.; Shiang, J. J.; Henrich, S. E.; Heath, J. R. Science 1997, 277, 1978. (32) Kurth, D. G.; Lechmann, P.; Lesser, C. Chem. Commun. 2000, 949. (33) Armstrong, A. J.; Mockler, R. C.; O’Sullivan, W. J. J. Phys. Condens. Matter 1989, 1, 1707. (34) Robinson, D. J.; Earnshaw, J. C. Langmuir 1993, 9, 1436. (35) Aveyard, R.; Binks, B. P.; Clint, J. H.; Fletcher, P. D. I.; Neumann, B.; Paunov, V. N.; Annesley, J.; Nees, D.; Parker, A. W.; Ward, A. D.; Burgess, A. N. Central Laser Facility Annual Report 2000/2001; CLRC Rutherford Appleton Laboratory: Didcot, UK, 2001. (36) Sun, J.; Stirner, T. Langmuir 2001, 17, 3103. (37) Martı´nez-Lo´pez, F.; Cabrerizo-Vı´lchez, M. A.; Hidalgo-AÄ lvarez, R. J. Colloid Interface Sci. 2000, 232, 303. (38) Tolnai, Gy.; Csempesz, F.; Kabai-Faix, M.; Ka´lma´n, E.; Keresztes, Zs.; Kova´cs, A. L.; Ramsden, J. J.; Ho´rvo¨lgyi, Z. Langmuir 2001, 17, 2683. (39) Ghezzi, F.; Earnshaw, J. C.; Finnis, M.; McCluney, M. J. Colloid Interface Sci. 2001, 238, 433. (40) (a) Robinson, D. J.; Earnshaw, J. C. Phys. ReV. A 1992, 46, 2045. (b) Robinson, D. J.; Earnshaw, J. C. Phys. ReV. A 1992, 46, 2055. (c) Robinson, D. J.; Earnshaw, J. C. Phys. ReV. A 1992, 2065. (41) Hurd, A. J.; Schaeffer, D. W. Phys. ReV. Lett. 1985, 54, 1043. (42) Onoda, G. Y. Phys. ReV. Lett. 1985, 55, 226. (43) Cao, L.; Wan, H.; Huo, L.; Xi, S. J. Colloid Interface Sci. 2001, 244, 97. (44) Muramatsu, K.; Takahashi, M.; Tajima, K.; Kobayashi, K. J. Colloid Interface Sci. 2001, 242, 127. (45) Iakovenko, S. A.; Trifonov, A. S.; Mamedov, A.; Nagesha, D. K.; Hanin, V. V.; Soldatov, E. C.; Kotov, N. A. AdV. Mater. 1999, 11, 388. (46) Van Duffel, B.; Ras, R. H. A.; De Schryver, F. C.; Schoonheydt, R. A. J. Mater. Chem. 2001, 11, 3333. (47) Szekeres, M.; Kamalin, O.; Schoonheydt, R. A.; Wostyn, K.; Clays, K.; Persoons, A.; Dekany, I. J. Mater. Chem. 2002, 12, 3268. (48) Reculusa, S.; Ravaine, S. Chem. Mater. 2003, 15, 598. (49) Bardosˇova´, M.; Hodge, P.; Sˇ matko, V.; Whitehead, D. Acta Phys. SloVaca 2004, 54, 409. (50) Lee, Y.-L.; Du, Z.-C.; Lin, W.-X.; Yang, Y.-M. J. Colloid Interface Sci. 2006, 296, 233.

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Figure 1. Liquid drop on idealized hydrophobic surfaces of various roughnesses. (a) Flat surface, (b) noncomposite surface of concentric grooves, and (c) composite surface of concentric grooves.

former and the latter are supposed to confer the substrate surface roughness and low surface energy, respectively.

Background It is well known that increasing the roughness of a hydrophobic surface can increase its hydrophobicity dramatically. Two distinct wetting behaviors, however, have been observed depending upon the nature and extent of the surface roughness.51,52 As shown in Figure 1a, a contact angle, θ, of a liquid droplet on a flat solid surface is given by the classical Young’s equation, γLV cos θ ) γSV - γSL, where γSL, γSV, and γLV are the interfacial free energies per unit area of the solid-liquid, solid-vapor, and liquid-vapor interfaces, respectively. Wenzel proposed a theoretical model describing the contact angle θ′ at a rough surface.51 He modified Young’s equation as follows.

cos θ′ ) r cos θ

(1)

where r is a roughness factor, defined as the ratio of the actual area of a rough surface to the geometric projected area. This factor is always larger than unity. As shown in Figure 1b, a surface of concentric grooves exemplifies the roughness.53 In the regime of Wenzel’s equation (this roughness regime will be referred as “Wenzel’s regime”), the surface free energy of the solid part of a rough surface is r times higher than that of a flat surface and the hydrophobicity of a rough hydrophobic surface is augmented by the increase of the solid-liquid contact area. In contrast, Cassie and Baxter proposed an equation describing the contact angle θ′ at a heterogeneous (composite) surface composed of two different materials.52 When a unit area of the surface has a surface area fraction f1 with a contact angle θ1 and an area fraction f2 with a contact angle θ2, the contact angle on the surface can be expressed as cos θ′ ) f1 cos θ1 + f2 cos θ2. They applied this equation to a rough hydrophobic surface trapping air in the hollows of the rough surface or with the existence of liquid-air interface by assuming that the surface is composed of solid and air. When f2 represents the area fraction of trapped air or other liquid-air interface, as shown in Figure 1c for example, the equation can be modified as follows.

cos θ′ ) rf1 cos θ1 + f2 cos 180° ) rf1 cos θ1 - f2 (2) where f1 is the wetted fraction of the projected sample area, (51) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988. (52) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546. (53) Oliver, J. P.; Huh, C.; Mason, S. G. Colloids Surf. 1980, 1, 79.

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increase while the hysteresis starts decreasing. This decrease in hysteresis occurs as a consequence of the switching of the dominant hydrophobicity mode from Wenzel’s to Cassie and Baxter’s due to the increase of the air fraction at the interface between solid and water. The Wenzel roughness factor of the hexagonally close-packed particulate monolayer is calculated as shown in Figure 3. On the basis of the hemispherical model,54 the surface area, S, and a Wenzel roughness factor, r, are calculated as follows.

S)6×

(

)

x3 2 π 2 1 πD2 + D - D 12 2 4 r)

S

x3 2 D 2

= 1.9

(3) (4)

It is noteworthy that the Wenzel roughness factor is 1.9 and is independent of particle diameters for the hcp structure considered. Experimental Section

Figure 2. Schematic of contact angle hysteresis on a model rough surface for θ ) 120°. θ ) intrinsic contact angle; θmax, θmin ) maximum and minimum possible contact angles; θW, θC&B ) conact angles calculated from Wenzel’s and Cassie and Baxter’s equations, respectively; θa, θr ) advancing and receding contact angles, respectively. (After Johnson and Dettre55)

Figure 3. Unit cell of the hexagonally close-packed (hcp) monolayer by particles with diameter D and the calculated Wenzel roughness factor, r, based on the hemispherical close-packed model.

f1 + f2 ) 1 and r is the roughness factor of the wetted area. In the regime of Cassie and Baxter’s equation (this roughness regime will be referred as “Cassie and Baxter’s regime”), the hydrophobicity of a rough surface is emphasized by the decrease of the solid-liquid contact area. Johnson and Dettre simulated the variation of the water contact angle for hydrophobic surfaces with various roughnesses by assuming idealized sinusoidal surfaces.55 Their results are summarized in Figure 2 for a hydrophobic surface with θ ) 120°. During the roughness regime where Wenzel’s mode is dominant, they showed that the contact angle and its hysteresis (the difference between advancing and receding contact angles) on hydrophobic rough surfaces increase as the roughness factor increases. They also demonstrated that when the roughness factor exceeds a certain level (∼1.7), the contact angle continues to (54) Nakae, H.; Inui, R.; Hirata, Y.; Saito, H. Acta Mater. 1998, 46, 2313. (55) (a) Johnson, R. E., Jr.; Dettre, R. H. AdV. Chem. Ser. 1963, 43, 112. (b) Johnson, R. E., Jr.; Dettre, R. H. Surface and Colloid Science; Matijevic, E., Ed.; Wiley: New York, 1969; Vol. 2, pp 85-153.

Materials. Silica powders of 0.5, 1.0, and 1.5 µm in diameter were purchased from Lancaster Synthesis, Inc. These powders were free from organic contamination and can be easily dispersed in aqueous medium. Cationic surfactant hexadecyltrimethylammonium bromide (HTMAB, 99% pure, Sigma) was used as the physical modification agent for silica particle surfaces. Chloroform (99.9% pure), methanol (99.8% pure), hydrochloric acid (36.5-38.0%), and 2-propanol (99.8% pure) were supplied by Baker, Mallinckrodt, Backer, and Fluka, respectivley. Dodecyltrichlorosilane (C12H25Cl3Si) was purchased from Fluka and used as a SAM component without further purification. All experiments were conducted with pure water that was passed through a Milli-Q plus purification system (Millipore) with a resistivity of 18.2 MΩ‚cm. Surface Modification of Silica Particles. Bare silica particles were dispersed in 2-propanol by sonication for 24 h and followed by sonication in the presence of the surfactant for 1 h. All solutions were allowed to stand to reach equilibrium for another 24 h. The solvent was then removed by a vacuum system. Finally, these surfacemodified silica particles were redispersed in chloroform by sonication. The surfactant concentration and particle concentration were 1000 ppm and 20 mg‚mL-1, respectively, in all experiments. As mentioned above, surfactants were added to the suspension to control the hydrophilic-hydrophobic balance of the particle surfaces. The effect of the type, concentration, and chain length of the surfactant and the composition of the dispersion medium on the quality of particle ordering have been studied in details elsewhere.44,45 The dispersion of the silica particles in such a solution is stable as revealed by the variation of particle size with time by using a computerized particle size analyzer (model Zetasizer 3000 HS, Malvern). Particulate Monolayer Preparation and LB Film Deposition. The silica suspensions were agitated in an ultrasonic bath prior to use. The monolayer experiments and the film deposition were conducted in a Langmuir trough (KSV mini-trough, KSV Instruments Ltd.) with a working area of 32 cm × 7.5 cm on a vibration isolation table. The film pressure at the air/water interface were measured by the Wilhelmy plate attached to a microbalance. The water subphase temperature was always controlled at 25 °C by a thermostat. In a typical experiment, an appropriate amount of the silica suspension was spread on a pure water subphase by using a microsyringe (1705N, Hamilton Co.), and a waiting period of 20 min was allowed for solvent evaporation. The monolayer at the air/water interface was then continuously compressed at a barrier speed of 5 mm‚min-1 to yield a π-A isotherm. For film deposition, a 24 mm × 50 mm × 0.1 mm glass microscope slide was thoroughly cleaned with rinsing liquids as indicated by a measured contact angle less than 5° with water. The particulate film was deposited in the upstroke direction at a speed of 1 mm‚min-1, at a selected surface pressure of 10 mN‚m-1, which was kept constant

Fabrication of Hydrophobic Surfaces

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Figure 4. π-A isotherm of silica particulate monolayer. The particle surface was hydrophobically modified by physical adsorption of cationic surfactant.

Figure 5. Transfer ratios for the deposition of a five-layer silica particulate film. Silica particles of 0.5 µm diameter were used. Surface pressure ) 10 mN‚m-1.

during the deposition by automatic adjusting of the barriers. Multilayers were deposited successively on a microscope glass slide in the same way as described for the monolayers. A 10-min waiting period at the end of each upstroke deposition, however, was allowed to dry the film, and the dipping rate of the slide was controlled at 100 mm‚min-1. It should be noted that the upstroke and downstroke speeds chosen in this work ensured each upstroke deposition a transfer ratio close to 1. Sintering of Particulate Films. The LB deposited particulate films were heated in a furnace under air atomsphere for 30 min at 450 °C to obtain sintered and organics-free SiO2 films. Silanization of Particulate Films. The clean glass and particulate films were finished by a SAM of dodecyltrichlorosilane. Typically, the clean glass and particle deposited glass substrates were dipped into a 0.25 wt % solution of dodecyltrichlorosilane in chloroform at room temperature for 30 min. They were then removed from the silane solution, washed with chloroform twice, and dried with nitrogen gas. Characterization of Particulate Films. Surface morphology and cross-sections of the particle deposited films were examined with a scanning electron microscope (SEM, Hitachi S4100). The static contact angle and dynamic contact angles were measured with water by using a contact angle meter (GBX, PX610) and a dynamic contact angle analyzer (Thermo Cahn, WinDCA 300), respectively.

was observed. It is noteworthy that the Wenzel roughness factor of such a surface is 1.9 and independent of particle diameters, as calculated by eq 4. However, in all cases, defects and/or aggregates of silica particles can also be seen. Figure 7 shows the SEM micrograph in cross-section view of a three-layer particulate film with particles of 1.5 µm diameter. The surface morphology of this particulate film also revealed hcp ordering and defects of the silica particles. The formation of defects in particulate films is due mainly to the nonuniformity in size and shape of the particles and the shrinkage of particles by sintering. A five-layer particulate film is shown schematically in Figure 8a. Actually, this can be realized experimentally by successive LB deposition of particles. Figure 8b shows the SEM micrography in cross-section view of such a five-layer particulate film by using SiO2 particles with 0.5 µm diameter. For fabricating a hydrophobic surface by coupling of LB deposition and a SAM, we are now in a position to endow the surface of the particulate film with a hydrophobic property. As shown schematically in Figure 8c, a hydrophilic surface of LB-deposited SiO2 particulate film is made hydrophobic through the self-assembly of alkylsilane molecules with hydrophobic tail groups. Images of water droplets on various treated surfaces are shown in Figure 9. They are (a) silanized glass substrate (r = 1) and silanized one-layer particulate films (r = 1.9) with particles of (b) 0.5, (c) 1.0, and (d) 1.5 µm diameters. The experimental results confirmed the hydrophobicity introduced by surface silanization. Moreover, an ∼20° increase in water contact angle may have resulted from one-layer particle deposition for all three particle diameter values. Significant hydrophobicity increase is achieved by increasing the roughness of a hydrophobic surface. The particle size, however, has insignificant effect on contact angle increase. This is consistent with the predictions of both the Wenzel model and the Cassie and Baxter model in that roughness of hydrophobic surface can increase its hydrophobicity and a switching of the dominant mode from Wenzel’s to Cassie and Baxter’s. It is noteworthy that the contact angles between a water droplet and a five-layer particulate film with particles of 0.5 µm diameter were also measured before and after sintering. The initial contact angle was measured to be 40° for the particulate film before sintering. However, water completely penetrated into the particulate film in 12 min. On the other hand, water penetrated immediately into the sintered particulate film. This indicated that surfactant molecules (HTMAB) had been removed by sintering. Figure 10 shows, for examples, the immersion-emmersion cycles during dynamic contact angle (DCA) measurements of

Results and Discussion A typical π-A isotherm of the particulate monolayer is shown in Figure 4, where the surface pressure is plotted against the area/particle. Three regimes can be distinguished. There is a sharp transition from gaseous phase (regime Ι) to the solid condensed phase (regime ΙΙ), where the surface pressure increase is very steep and linear. In regime ΙΙΙ, the surface pressure increase in a nonlinear fashion. The surface pressure at the transition from regime ΙΙ to regime ΙΙΙ is regarded as the collapse pressure and is about 25 mN‚m-1 for the demonstrated case. Furthermore, it is noteworthy that the limiting area (A0 in Figure 4) is about 0.215 µm2‚particle-1, which is very close to the area of the hcp unit cell. Figure 5 shows the transfer ratios for the transfer of silica particles with 0.5 µm diameter at a constant surface pressure of 10 mN‚m-1 onto a glass substrate by the LB deposition technique. A transfer ratio close to 1 was always found for each upstroke deposition, implying nearly perfect transfer of silica particulate monolayer. In this work, particulate films with a particle layer number up to 5 were fabricated. The SEM micrographs of one-layer silica particles on a glass substrate by LB deposition are shown in Figure 6 for particles with various sizes. In all cases, hcp ordering of the silica particles

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Figure 7. SEM micrograph in cross-section view of a three-layer particulate film with particles of 1.5 µm diameter.

Figure 6. SEM micrographs of one-layer particulate film on glass substrate by LB deposition. Particle diameter: (a) 0.5, (b) 1, and (c) 1.5 µm.

the solid surfaces. Very small and nearly equal advancing and receding contact angles were measured for the clean glass. Highly wetting and low-contact-angle hysteresis, therefore, are exhibited by the clean glass. In contrast with the clean glass, the silanized glass exhibits much larger contact angles and noticeable contact angle hysteresis. Obviously, both surface hydrophobicity and surface heterogeneity are introduced through surface silanization. It is evident that even larger contact angles and more significant contact angle hysteresis are exhibited by the silanized particulate film. Although not shown in this figure, the nearly identical

Figure 8. (a) Schematic of SiO2 particle deposition by the LB technique. (b) SEM micrograph in cross-section view of a five-layer particulate film with particles of 0.5 µm diameter. (c) Surface silanization of LB-deposited SiO2 particulate film.

repeated loops of DCA analysis for each surface indicate that stable particulate films can be fabricated by the present method.

Fabrication of Hydrophobic Surfaces

Figure 9. Water droplet on (a) silanized glass substrate, and silanized one-layer particulate film with particles of (b) 0.5, (c) 1, and (d) 1.5 µm diameters.

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wetting property of the particulate films. The experimental results also revealed that particle size has little effect on advancing and receding contact angles. It should be noted that the contact angle hysteresis is the difference between advancing and receding contact angles. Contact angle hysteresis of about 40° was found for particulate films with particles of 0.5, 1.0, and 1.5 µm diameters. In general, an advancing contact angle about 150°, a receding contact angle about 110°, and a contact angle hysteresis about 40° were exhibited by the particulate films fabricated. It is well known that a self-cleaning surface should exhibit both high contact angle and low sliding angle, which can also be expressed as the contact angle hysteresis. The “one-sized” particulate films fabricated in this work were, therefore, not well qualified for the self-cleaning surfaces. However, it is worth noting that a method of fabricating self-cleaning “two-sized” particulate films is being studied by the authors. The particulate films were fabricated by LB depositing silica multilayered particle coatings on glass substrate and followed by subsequent depositing another layer of smaller silica nanoparticles. Superhydrophobic surfaces that exihit both high contact angel and low hysteresis are expected. Detailed results will be reported soon.

Conclusions Figure 10. Dynamic contact angle measurements of solid surfaces. (a) Clean glass, (b) silanized glass, and (c) silanized three-layer particulate film of particles with 0.5 µm diameter.

Figure 11. Effects of particle layer number and particle size on contact angles of silanized particulate surfaces.

Figure 11 shows the effects of the particle layer number and particle size on static contact angl, advancing contact angle, and receding contact angle. Obviously, the particle layer number has no significant effect on static contact angle and neither has the particle size. It is the outermost surface that determines the static

Bare silica particles of 0.5, 1.0, and 1.5 µm diameters were surface-modified and used for fabricating hydrophobic surfaces by coupling the LB deposition of these particles on glass substrate and the formation of a SAM of trichlorododecylsilane on the outer surface of the particulate films. Effects of particle size and particle layer number on the wetting behavior of the particulate films were systematically studied. Some conclusions can be drawn from the study. First, a novel method for fabricating hydrophobic surfaces consisting of hexagonally close-packed arrays of silica particles with a constant roughness factor (r = 1.9), which is independent of particle size, becomes available. Second, for the hydrophobic surface with a constant roughness factor of 1.9, a static water contact angle about 130° resulted from the particulate films regardless of the particle size and particle layer number. This is consistent with the predictions of both the Wenzel model and the Cassie and Baxter model in that roughness of hydrophobic surface can increase its hydrophobicity and a switching of the dominant mode from Wenzel’s to Cassie and Baxter’s. Finally, an advancing contact angle about 150°, a receding contact angle about 110°, and a contact angle hysteresis about 40° were exhibited by the particulate films fabricated. Acknowledgment. This work was partially supported by the National Science Council of the Republic of China (Taiwan) through Grant No. NSC 93-2120-M-006-007. Zhung Ching Du is acknowledged for his assistance with the Langmuir trough. LA053152M