Langmuir 2005, 21, 911-918
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Macroscopic-Wetting Anisotropy on the Line-Patterned Surface of Fluoroalkylsilane Monolayers Masamichi Morita,†,‡ Tomoyuki Koga,† Hideyuki Otsuka,†,§ and Atsushi Takahara*,†,§ Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan, Fundamental Research Department, Chemical Division, Daikin Industries, Ltd.,1-1 Nishi Hitotsuya, Settsu-shi, Osaka 566-8585, Japan, and Institute for Materials Chemistry and Engineering, Kyushu University, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan Received June 15, 2004. In Final Form: October 28, 2004 Micropatterned fluoroalkylsilane monolayer surfaces with liquidphilic/liquidphobic area (line width 1-20 µm) were prepared with few defects by vacuum ultraviolet (VUV) photolithography. The anisotropic wetting of a macroscopic droplet with a 0.5-5 mm diameter on the micropatterned surfaces was investigated. The strong anisotropy of the contact angle and the sliding angle and droplet distortion for fluoroalkylsilane/ silanol patterned surfaces was attributed to the difference in the energy barrier of wetting between parallel and orthogonal lines. The wetting anisotropy decreased with decreases in the liquidphilic area. Fluoroalkylsilane/alkylsilane patterned surfaces with small differences in the surface free energies of the components showed anisotropic wetting only for the low-surface-tension liquids.
1. Introduction In recent years, well-defined micropatterned surfaces have been actively studied for practical applications such as electronic, optical, and medical devices.1,2 Several investigations3-15 in these research fields have focused on wetting on micropatterned surfaces, and the results of these investigations have received remarkable attention from the perspective of both fundamental research and practical applications. It has been revealed that a droplet on patterned surfaces with different surface free energies shows anomalous wetting behavior,5-7,9,12,16-18 unlike droplets on homoge* Corresponding author. E-mail:
[email protected]. † Graduate School of Engineering, Kyushu University. ‡ Daikin Industries, Ltd. § Institute for Materials Chemistry and Engineering, Kyushu University. (1) Wilbur, J. L.; Whitesides, G. M. Self-Assembly and Self-Assembled Monolayers in Micro- and Nanofabrication in Nanotechnology; Springer: New York, 1999; Chapter 8. (2) Xia, Y. Adv. Mater. 2004, 16, 1245-1246. (3) Dulcey, C. S.; Georger, J. H.; Krauthamer, J. V.; Stenger, D. A.; Fare, T. L.; Calvert, J. M. Science 1991, 252, 551-554. (4) Kumar, A.; Biebuyk, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498-1511. (5) Naidich, Y. V.; Vorrovich, R. P.; Zabuga, V. V. J. Colloid Interface Sci. 1995, 174, 104-111. (6) Drelich, J.; Miller, J. D.; Whitesides, G. M. Langmuir 1996, 12, 1913-1922. (7) Kawai, A.; Okada, A. J. Adhes. Soc. Jpn. 1998, 34, 22-25. (8) Gau, H.; Herminghaus, S.; Lenz, P.; Lipowsky, R. Science 1999, 283, 46-49. (9) Oner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777-7782. (10) Zhao, B.; Moore, J. S.; Beebe, D. J. Science 2001, 291, 10231026. (11) Kagan, C. R.; Breen, T. L.; Kosbar, L. L. Appl. Phys. Lett. 2001, 79, 3536-3538. (12) Yoshimitsu, Z.; Nakajima, A.; Watanabe, T.; Hashimoto, K. Langmuir 2002, 18, 5818-5822. (13) Lam, P.; Wynne, K. J.; Wnek, G. E. Langmuir 2002, 18, 948951. (14) Koga, T.; Otsuka, H.; Takahara, A. Chem. Lett. 2002, 12, 11961197. (15) Wang, J. Z.; Zheng, Z. H.; Li, H. W.; Huck, W. T. S.; Sirringhaus, H. Nat. Mater. 2004, 3, 171-176. (16) Neumann, A. W.; Good, R. J. J. Colloid Interface Sci. 1972, 38, 341-358.
neous surfaces. The authors call such unusual wettability “anisotropic wetting”, as opposed to the two-dimensional isotropic wetting against homogeneous surfaces. The anisotropic wetting of a pure liquid on a patterned surface has been studied so far. The wetting behavior on the patterned surface has been analyzed by the Cassie equation,19 with which the contact angle of the heterogeneous surface can be theoretically predicted, because it has well-defined patterned areas.5-7,9,12 As a result, the relationships between the patterned structure and the anisotropic wetting have been discussed on the basis of the comparison of the theoretical values obtained from the Cassie equation with the experimental ones. The effect of the pattern shape on the anisotropy of the sliding angle12 and the droplet distortion5,7 also has been studied. However, most of the above investigations have been done for surfaces with height differences of 20-150 µm between the patterned surface areas; as a result, the combined effects of surface roughness and surface composition will appear in the wetting behavior. On the other hand, the anisotropic wetting of macroscopic droplets on the ideal flat patterned surface has also been studied from a theoretical perspective,16-18 and verification of the theory is expected because of the development of precise control of patterned surface structure. Recently, fabrication technology for patterned surfaces using self-assembled monolayers (SAMs) has been developed. The most typical methods are the use of SAMs composed of organosilane molecules adsorbed on a Si/ SiO2 substrate20 or alkanethiol molecules adsorbed on a Au substrate.21 SAMs have been developed as useful materials to control wettability because of their ease of (17) Boruvka, L.; Neumann, A. W. J. Colloid Interface Sci. 1978, 65, 315-330. (18) Schwartz, L. W.; Garoff, S. Langmuir 1985, 1, 219-230. (19) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546551. (20) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92-98. (21) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 1990-1995.
10.1021/la0485172 CCC: $30.25 © 2005 American Chemical Society Published on Web 12/23/2004
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fabrication and their physicochemical characteristics such as their very few defects and their mechanical and chemical robustness. Organosilane monolayers are patterned by photolithography.3,14,22 In contrast, thiol monolayers are patterned on a Au substrate with stamps prepared by photolithography, that is, a microcontact printing method.4 The above-mentioned methods are a combination of a bottom-up self-assembly technique and a top-down photolithography technique. Therefore, these methods enable reproducible preparation of a micron-scale patterned surfaces with low fabrication costs. Furthermore, the use of a patterned surface prepared with SAMs allows us to ignore the effects of roughness on wetting, as the height difference between the liquidphilic and liquidphobic areas is Rh(C10) > Rf Figure 1. Schematic representation of the fabrication of twocomponent line-patterned organosilane monolayers. tion of VUV light of 172 nm through a photomask with a line and space width of 1-20 µm. The same VUV excimer lamp used for substrate cleaning served as a light source for this procedure. Unless otherwise specified, the line/space ratio was 1/1 for this study. Photomasks with a line/space ratio of 3/1 were used for this portion of the study. The photomask consisted of a 2 mm thick quartz glass plate with a 100 nm thick chromium pattern; a cylindrical stainless steel weight of 340 g was placed on the photomask to ensure contact between the mask and the sample surface. Photoirradiation of VUV light leads to the dissociation of covalent bonds such as C-C, C-H, and Si-C bonds. The decomposed monolayer is further oxidized with active oxygen species generated by the photoexcitation of atmospheric oxygen molecules.30 Finally, the monolayer was removed as a volatile species, and Si-OH residues were formed. The VUV irradiation was carried out under 100 Pa for 30 min, and the water contact angles of the homogeneous monolayers [Rf, Rh(C10), and Rh(C18)] approached 90 64 >90 73 50 44 54 40
parallel direction of line (y-axis)
a
F ) γl(cos θr - cos θa). b w: droplet width. c Rcalcd ) sin-1[wγl(cos θr - cos θa)/mg]. Table 7. Static and Dynamic Contact Angles and Sliding Angles for Water (20 µL) and n-Hexadecane (5 mL) on Rf/ Rh(C18) Patterned Surfaces sliding direction
liquid
θs/deg
θa/deg
θr/deg
Fa/mN
wb/mm
Rexptl/deg
Rcalcdc/deg
orthogonal direction of line (x-axis)
water n-hexadecane water n-hexadecane
100 47 100 35
106 62 107 37
96 29 95 27
12 29 15 7
5.1 6.1 5.1 4.3
15 66 17 11
19 64 23 8
parallel direction of line (y-axis) a
F ) γl(cos θr - cos θa). b w: droplet width. c Rcalcd ) sin-1[wγl(cos θr - cos θa)/mg].
because the line width is considerably smaller than the droplet size. The results with our substrates with a line width of
