pubs.acs.org/Langmuir © 2009 American Chemical Society
Application of the Restricting Flow of Solid Edges in Fabricating Superhydrophobic Surfaces Xianliang Sheng,† Jihua Zhang,*,‡ and Lei Jiang § †
College of Science, Inner Mongolia Agriculture University, Hohhot 010018, PR China, ‡Aerospace Research Institute of Material and Processing Technology, Beijing 100076, PR China, and §Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, PR China Received March 26, 2009. Revised Manuscript Received May 12, 2009
In this article, a simple method of pressing a conical frustum into liquid was adopted to explore the ability to restrict flow around their edges. On the basis of experiments and theoretical analyses, the restricting force Δf and the pressing work ΔEw were used to characterize the ability to restrict flow around the edge for water or formamide, which were found to be closely related to the geometric morphologies of edges and the liquid and material characteristics. The ability to restrict flow around the edge may be enhanced by increasing the rise angle ω and the size of edge circles and using a high-surface-energy liquid. Inspired by this, the superhydrophobicity of the materials with lower hydrophobicity has been successfully obtained by constructing close microedges on their flat surfaces. We believe that these findings would help to widen several novel applications to high-adhesion superhydrophobic surfaces.
1. Introduction Surface wettability is an important characteristic of solid materials and is involved in molecular, microscopic surface structures and macroscopic geometrical morphology.1-3 Researchers have paid a great deal of attention to altering surface wettability by chemical composition and microstructures.4-14 However, studies about the effect of edges, a special part of a solid surface, on surface wettability remain few although it is well known that wetting hysteresis phenomena widely occur when a *E-mail:
[email protected]. (1) Adamson, A. M. Physical Chemistry of Surfaces, 6th ed.; John Wiley & Sons: Toronto, 1997; Chapter 1. (2) Chappuis, J. In Multiphase Science and Technology; Hewitt, G. F., Delhaye, J. M., Zuber, N., Eds.; Hemisphere Pub. Corp.: Washington, 1985; Vol. 1, p 387. (3) de Gennes, P. G. Rev. Mod. Phys. 1985, 57, 827. (4) McHale, G.; Shirtcliffe, N. J.; Aqil, S.; Perry, C. C.; Newton, M. I. Phys. Rev. Lett. 2004, 93, 036102. (5) Onda, T.; Shibuichi, S.; Satoh, N.; Tsujii, K. Langmuir 1996, 12, 2125. (6) Shibuchi, S.; Onda, T.; Satoh, N.; Tsujii, K. J. Phys. Chem. 1996, 100, 19512. (7) McCarthy, T. J.; Oner, D. Langmuir 2000, 16, 7777. (8) Yoshimitsu, Z.; Nakajima, A.; Watanabe, T.; Hashimoto, K. Langmuir 2002, 18, 5818. (9) Miwa, M.; Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watanabe, T. Langmuir 2000, 16, 5754. (10) Lam, P.; Wynne, K. J.; Wnek, G. E. Langmuir 2002, 18, 948. (11) Xie, Q.; Xu, J.; Feng, L.; Jiang, L.; Tang, W.; Luo, X.; Han, C. C. Adv. Mater. 2004, 16, 302. (12) Shi, F.; Wang, Z.; Zhang, X. Adv. Mater. 2005, 17, 1005. (13) Gau, H.; Herminghaus, S.; Lenz, P.; Lipowsky, R. Science 1999, 283, 46. (14) (a) Feng, L.; Li, S.; Li, H.; Zhai, J.; Song, Y.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2002, 41, 1221. (b) Gao, X.; Jiang, L. Nature 2004, 432, 36. (c) 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. (d) Feng, L.; Song, Y.; Zhai, J.; Liu, B.; Xu, J.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2003, 42, 800. (e) Feng, X.; Feng, L.; Jin, M.; Zhai, J.; Jiang, L.; Zhu, D. J. Am. Chem. Soc. 2004, 126, 62. (f) Wang, S.; Feng, X.; Yao, J.; Jiang, L. Angew. Chem., Int. Ed. 2006, 45, 1264. (g) Wang, S.; Song, Y.; Jiang, L. J. Photochem. Photobiol. C 2007, 8, 18. (h) Hong, X.; Gao, X.; Jiang, L. J. Am. Chem. Soc. 2007, 129, 1478. (i) Zheng, Y; Gao, X.; Jiang, L. Soft Matter 2007, 3, 178. (j) Guo, C.; Feng, L.; Zhai, J.; Wang, G.; Song, Y.; Jiang, L.; Zhu, D. ChemPhysChem 2004, 5, 750. (15) Gibbs, J. W. The Collected Works of J. Willard Gibbs; Yale University Press: New Haven, CT, 1961; Vol. 1, p 326. (16) Oliver, J. F.; Huh, C.; Mason, S. G. J. Colloid Interface Sci. 1977, 59, 568. (17) Oliver, J. P.; Huh, C.; Mason, S. G. Colloids Surf. 1980, 1, 79. (18) Huh, C.; Mason, S. G. J. Colloid Interface Sci. 1977, 60, 11. (19) Oliver, J. F.; Mason, S. G. Colloid Interface Sci. 1977, 60, 480. (20) Zhang, J.; Gao, X.; Jiang, L. Langmuir 2007, 23, 3230.
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liquid (drop or fluid) spreads to the corner of the edges.15-22 Recently, we found that lotus rims could avoid water flow over their surfaces, thus remaining self-cleaning.21 Inspired by this, a new strategy to construct superhydrophobic edges was used to solve the outflowing problems of tubes’ or rods’ edges.20 Notably, theories on the wettability of solid edges suggested by Gibbs (1870s) have been applied to explain some novel wetting phenomena, such as the suspension of drops on superhydrophobic surfaces.22-26 Therefore, we believe that it is very important to explore the wetting effects and applications of solid edges. Superhydrophobic surfaces with a contact angle (CA) of >150° have attracted researchers’ attention recently because of their importance in fundamental research and potential industrial applications of self-cleaning.4-14 Until now, various methods were used by material scientists to synthesize superhydrophobic surfaces, such as a solution method, sol-gel method, chemical vapor deposition (CVD) method, electrospinning method, and others.5-12 In previous work by our group, some interesting phenomena on superhydrophobic surface have been reported.14 For example, a wetting transition occurs between superhydrophobicity and superhydrophilicity induced by ultraviolet radiation.14e-14g A highly adhesive or anisotropic superhydrophobic surface was discovered by investigating natural rice leaves and the wings of butterflies.14a,14c,14d,14h,14i However, according to classic Cassie and Wenzel theories, superhydrophobic surfaces are mostly prepared by inducing surface microstructures and using low-surface-energy materials (hydrophobic materials).27,28 Moreover, expensive and/or noxious low-surface-energy compounds, such as fluorine and fluorine-containing compounds, and complex processing on microstructures have become distinct (21) Zhang, J.; Wang, J.; Zhao, Y.; Xu, L.; Gao, X.; Zheng, Y.; Jiang, L. Soft Matter 2008, 4, 2232. (22) Zhang, J.; Sheng, X.; Jiang, L. Langmuir 2009, 25, 1371. (23) Extrand, C. W. Langmuir 2002, 18, 7991. (24) Extrand, C. W. Langmuir 2004, 20, 5013. (25) Extrand, C. W. Langmuir 2006, 22, 1711. (26) Extrand, C. W. Langmuir 2005, 21, 10370. (27) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988. (28) Cassie, A. B. D; Baxter, S. Trans. Faraday Soc. 1944, 40, 546.
Published on Web 06/05/2009
DOI: 10.1021/la901058y
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shortcomings in making superhydrophobic surfaces.14j,29,30 Therefore, there may be an urgent need to provide some simple, affordable methods of preparing more superhydrophobic surfaces using materials with either high or low surface energy. In this article, various smooth conical frusta were made, and then a simple method of pressing conical frusta immersed in water and formamide was adapted to study the restricting flow of the edges. The ability of the edges to restrict flow was measured by dynamic contact angle measurements and observed by a highspeed CCD camera. Then, we analyzed in detail the effects of edge size, rise angle, and surface energy of liquids on enhancing the ability to restrict flow over the edge. Finally, an application of constructing microgooves or microconvexes on a flat surface was proposed to fabricate the superhydrophobic surfaces by using a highly wetting restriction of the edge effect. Therefore, superhydrophobic surfaces can be fabricated by materials with lower hydrophobicity by such a method. We believe that these findings would widen many potential applications of edge effects, such as in liquid transportation or coagulating drops to reserve liquid.
2. Experimental Section Sample Preparation. Samples of various shapes and sizes were machined from Teflon rods (Keyi factory, Beijing, China.). These rods were machined into small conical frusta (the accuracy of the finish was (0.01 mm) with edge angles ω of 30, 45, 60, and 90° (a right angle, cylinder). All of these tops were carefully ground with various types of sand paper and then polished with a 100-grit diamond rag wheel. The roughness of the top and edges was 150°) by constructing some edges on the flat surfaces with a less hydrophobic material. It is clear that the edge structure with a high ability to restrict flow is needed in such surfaces. To elucidate this better, we did the following experiments. A circle-shaped polycarbonate (PC) microgroove with a large rise angle of 90° was first prepared with a laser cutter. Meanwhile, the other edge on a flat surface, a polydimethylsilicone (PDMS) microconvex surface, was manufactured by using the microgroove as a negative template (Figure 1). A liquid of high surface tension (72.56 mN/m) (i.e., water) was chosen in the experiments to further enhance the restriction of edges on the movements of the contact line. Figure 5a shows a 5 μL water drop on the close microconvex surface with an outer diameter d of ∼0.9 mm. The CA value of the drop on the PDMS microconvex surface was measured to be 153.4°. Apparently, the PDMS (31) Extrand, C. W.; Moon, S. I. Langmuir 2009, 25, 992.
9906 DOI: 10.1021/la901058y
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Figure 4. Dependence of restricting work ΔEw on various sizes of edges in the liquid: (a) water and (b) formamide.
surfaces were superhydrophobic; however, the instinct water CA of PDMS was only 109.1° (Figure S2). Figure 5b shows the profile of a 3 μL drop on the PC surface with the microgroove. Noticeably, the CA value of the PC microgroove was 145.3° when a 3 μL drop was placed on the PC microgroove with an outer diameter d of ∼0.9 mm, but the smooth PC was less hydrophobic with an instinct CA of 79.3° (Figure S3). The geometrical relations between the instinct and apparent CA on the surfaces with microedges, such as microconvexes or microgrooves, can be estimated by the Gibbs equation (eq 1). When a drop is deposited in the area of microconvexes or microgrooves, a spherical cap is then formed around these microstructures. By using the geometrical calculation for the volume of a spherical cap, the diameter d of the microgrooves or microconvexes can be expressed as a simple function of the apparent CA, θ, and the volume of drop V32 " #1=3 24 Vsin3 θ ð4Þ d ¼ π ð1 -cos θÞ2 ð2 þ cos θÞ (32) Assuming the shape of a drop on a circle edge to be a spherical cap on the basis of ref 20, the volume of the drop, V, can be expressed as a function of the apparent CA, θ, and the diameter of the edge circle, d0:V = π/24 d3[(1 - cos θ)2(2 + cos θ)/sin3 θ]. Thus, a deduction about the diameter of the microedge circles, microgrooves, or microconvexes can be obtained in the form of eq 4. Notably, eq 4 is valid only when the drops are sufficiently small because gravity does not distort them. If some drops are large enough, then they would be almost certainly flattened to some extent, and the hypothesis of a spherical cap for drops around the edge does not come into existence.
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Figure 5. Optical images of a 5 μL drop deposited on a circular PDMS microconvex surface and a 3 μL drop on a PC microgroove. It is obvious that the two surfaces are superhydrophobic.
Figure 6. (a) Plots of CAs of various volume drops (V = 1, 3, 5,
and 10 μL) versus the contact diameter d of microedges by eq 4 (d = 0.9 mm and 0.35 mm). (b) Plots of experimental CAs of drops versus their volumes V on the fixed contact diameter of microedges where the real and dashed lines were calculated by eq 4.
where the drops are sufficiently small when the weight of a drop cannot distort them. Figure 6a shows the theorical plots of the CA, θ versus the diameter of d on the condition of various drop volumes (1, 3, 5, and 10 μL, respectively) as predicated by eq 4. Note that for a certain drop volume the value of θ increased with
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the decrease in the microedge diameter d. Moreover, for a certain diameter d, the value of θ increased with the increase of V of a drop. For examples, for a 5 μL drop, when it dropped onto the PDMS microconvexes with a diameter of 0.9 mm, a CA value of 155° was predicated by eq 4. The measured CA (153.4°) in Figure 5a agreed well with the theoretical CA value. Figure 6b shows the calculated plots and the experimental data of the value of θ versus drop volume V on the PDMS microconvexes and PC microgrooves with diameters of 0.9 and 0.35 mm, respectively. We found a sharp increase in the apparent CA when the volume of the drop gradually increased. For example, the CA value of a 0.1 μL drop placed on a PC microgroove with a diameter of 0.9 mm was 79.3°, but for a 1 μL drop, the CA value was measured to be 130.6°. Although they were not superhydrophobic, they indeed increased the CA value by improving the volume of the drop. It was implied that the superhydrophobic state of a drop on our surfaces with microedges may be limited, depending on the sizes of the microedges (similarly, those superhydrophobic states determined by the Wenzel or Cassie theory were also limited below the sizes of the rough microstructures, such as the evaporation of water.33) Nevertheless, the measured results agreed well with the theorical results obtained via eq 4. It was addressed that the close microedges constructed by microconvex surfaces or microgrooves restricted the spreading of the drops and made superhydrophobic surfaces. Because of the highly restricting abilities of the microedges for liquid, we believe that some application of superhydrophobic surfaces may be realized, such as the transport of biologic or medical aqueous drops.34
4. Conclusions We adopted a simple method of pressing a conical frustum into liquid to explore the ability to restricting flow around their edges. On the basis of the experiments and theoretical analyses, the restricting force Δf and the pressing work ΔEw were used to characterize the ability of water or formamide to restrict flow around the edge. It was found that the geometric morphologies, the liquid, and the material characteristic played an important role in restricting flow at the edges. Increasing the rise angle ω, and the size of edge circles and using a high-surface-energy liquid may enhance the ability to restrict flow around the edges. Inspired by this, a strategy has been successfully developed to obtain superhydrophobicity by constructing close microedges on flat surfaces, such as microconvexes and microgrooves, to restrict the spreading of water drops. More superhydrophobic surfaces may be fabricated by materials with lower hydrophobicity. We believe that these findings would help to widen several novel applications of edge effects around high-adhesion superhydrophobic surfaces. Supporting Information Available: The experimental setup, results of the advancing angle around the edges measured by CCD images, critical forces f0, fc, and Δf, and critical heights h0, hc, and Δh. This material is available free of charge via the Internet at http://pubs.acs.org. (33) Dorrer, C.; R€uhe, J. Langmuir 2007, 23, 3820. (34) Jin, M.; Feng, X.; Feng, L.; Sun, T.; Zhai, J.; Li, T.; Jiang, L. Adv. Mater. 2005, 17, 1977.
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