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Langmuir 2008, 24, 796-801
A Topography/Chemical Composition Gradient Polystyrene Surface: Toward the Investigation of the Relationship between Surface Wettability and Surface Structure and Chemical Composition Jilin Zhang and Yanchun Han* State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Graduate UniVersity of the Chinese Academy of Sciences, Changchun 130022, P. R. China ReceiVed August 20, 2007. In Final Form: October 14, 2007 In this paper, we have prepared of a topography/chemical composition gradient polystyrene (PS) surface, i.e., an orthogonal gradient surface, to investigate the relationship between surface wettability and surface structure and chemical composition. The prepared surface shows a one-dimensional gradient in wettability in the x, y, and diagonal directions, including hydrophobic to hydrophilic, superhydrophobic to hydrophobic, superhydrophobic to superhydrophilic gradients, and so forth. These one-dimensional gradients have different gradient values, gradient range, and contact angle hysteresis, which lie on both the surface roughness and the surface compositions. From the trend of variation of contact angle hysteresis, it can be concluded that the transition from the Cassie’s model to the Wenzel’s model occurs both by decreasing surface roughness and by increasing surface hydrophilic compositions. Moreover, the transition is more effective via changing surface chemical composition than changing surface roughness herein.
1. Introduction The wettability of solid surfaces, governed by both the surface chemical composition and the geometrical microstructure, is a very important property.1-3 Recently, the control of surface wettability, such as superhydrophobicity (with water contact angle (CA) greater than 150°), superhydrophilicity (CA less than 5°), and reversibly switchable wettability, and so forth, has aroused great interest because of its wide variety of applications.4 In general, there are two ways to fabricate functional wettability surfaces, controlling the surface structure or chemically modifying the surfaces. For example, artificial superhydrophobic surfaces, also named “lotus-effect” surfaces, are commonly achieved by creating micro/nanostructures on hydrophobic substrates or chemically modifying a micro/nanostructured surface with low surface energy materials.4a-d On the contrary, artificial superhydrophilic surfaces, which can be applied in preparation of antifogging glass, self-cleaning construction materials, are commonly prepared by creating micro/nanostructures on hydrophilic substrates or chemically modifying a micro/nanostructured surface with high surface energy materials.4e Although the surface wettability can be investigated point-by-point by these functional wettability surfaces,5-7 gradient surfaces in wettability are usually employed to systematically investigate the relationship between the surface wettability and surface structure and chemical compositions.8-18 * To whom correspondence should be addressed. E-mail: ychan@ ciac.jl.cn. (1) [a] Abbott, N. L.; Folkers, J. P.; Whitesides, G. M. Science 1992, 257, 1380. [b] Bico, J.; Tordeux, C.; Que´re´, D. Europhys. Lett. 2001, 55, 214. (2) Shull, K. R.; Karis, T. E. Langmuir 1994, 10, 334. (3) [a] Onda, T.; Shibuichi, S.; Satoh, N.; Tsujii, K. Langmuir 1996, 12, 2125. [b] O ¨ ner, D.; McCarthy, J. Langmuir 2000, 16, 7777. (4) [a] Nakajima, A.; Hashimoto, K.; Watanabe, T. Monatsh. Chem. 2001, 132, 31. [b] Sun, T.; Feng, L.; Gao, X.; Jiang, L. Acc. Chem. Res. 2005, 38, 644. [c] Callies, M.; Que´re´, D. Soft Mater. 2005, 1, 55. [d] Que´re´, D. Rep. Prog. Phys. 2005, 68, 2495. [e] Feng, X.; Jiang, L. AdV. Mater. 2006, 18, 3063. (5) Erbil, H. Y.; Demirel, A. L.; Avci, Y.; Mert, O. Science 2003, 299, 1377. (6) Yabu, H.; Takebayashi, M.; Tannaka, M.; Shimomura, M. Langmuir 2005, 21, 3235. (7) Han, J. T.; Xu, X.; Cho, K. Langmuir 2005, 21, 6662. (8) Chaudhury, M. K.; Whiteside, G. M. Science 1992, 256, 1539.
Gradient wettability surfaces with a gradually changing surface energy along their length are of great interest for numerous practical applications, such as biomolecular interaction investigations, cell-motility studies, diagnostics, nanotribology, liquid self-transportation, and microfluidics.8-18 In the past years, there have been many reports concerning the gradient wettability surfaces from hydrophobic to hydrophilic. For example, Ganzer et al. demonstrated that one-dimensional planar molecular gradients with tunable wettabilities could be fabricated by combining the asymmetric vapor deposition method.10 Recently, we fabricated a one-dimensional gradient wettability surface from superhydrophobicity to hydrophobicity by changing polystyrene (PS) microsphere topography.14 Zhang’s group obtained a onedimensional gradient wettability surface from superhydrophobicity to superhydrophilicity by self-assembling two kinds of thiol molecules on the rough gold surface.15 Until now, there have been many reports concerning the preparation methods of one-dimensional gradient wettability surface, which can be used to investigate the relationship between surface wettability and surface structure or surface wettability and surface chemical compositions. However, to systematically investigate the relationship both between surface wettability and surface structure and between surface wettability and surface chemical compositions, orthogonal gradient surfaces should be considered. Furthermore, in a recent study, people found that the processes of cell adhesion, spreading, migration, and so forth are very (9) Ruardy, T. G.; Schakenraad, J. M.; van der Mei, H. C.; Busscher, H. J. Surf. Sci. Rep. 1997, 29, 3. (10) Efimenko, K.; Genzer, J. AdV. Mater. 2001, 13, 1560. (11) Morgenthaler, S.; Lee, S.; Zu¨rcher, S.; Spencer, N. D. Langmuir 2003, 19, 10459. (12) Choi, S. H.; Zhang, Newby, B. M. Langmuir 2003, 19, 7427. (13) Ionov, L.; Houbenov, N.; Sidorenko, A.; Stamm, M.; Luzinov, I.; Minko, S. Langmuir 2004, 20, 9916. (14) Zhang, J.; Xue, L.; Han, Y. Langmuir 2005, 21, 5. (15) Yu, X.; Wang, Z.; Jiang, Y.; Zhang, X. Langmuir 2006, 22, 4483. (16) Liu, H.; Xu, J.; Li, Y.; Li, B.; Ma, J.; Zhang, X. Macromol. Rapid Commun. 2006, 27, 1603. (17) Ballav, N.; Shaporenko, A.; Terfort, A.; Zharnikov, M. AdV. Mater. 2007, 19, 998. (18) Ito, Y.; Heydari, M.; Hashimoto, A.; Konno, T.; Hirasawa, A.; Hori, S.; Kurita, K.; Nakajima, A. Langmuir 2007, 23, 1845.
10.1021/la702567w CCC: $40.75 © 2008 American Chemical Society Published on Web 12/22/2007
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Figure 1. Schematic illustration of the preparation process of a orthogonal gradient polystyrene surface. (a-c) Preparation of rectangular PS pieces (size: 30.0 ( 2.0 mm (length) (the x direction) × 22.0 ( 2.0 mm (width) (the y direction) × 3.0 ( 1.0 (thickness)). (d,e) Templating by porous anodic aluminum oxide membrane, microrod-structured PS surface can be obtained. (f) Preparation of textured gradient PS surface in a temperature gradient field. (g) Dripping 50 °C 98% H2SO4 solution into the container gradually, chemical composition gradient PS surface can be obtained. (h) An orthogonal gradient PS surface.
dependent on not only the surface composition19 but also the surface roughness.20 To systematically study the mechanism of these processes, orthogonal gradient surfaces should also be considered. In this paper, we first prepared a PS topography/chemical composition gradient surface, i.e., an orthogonal gradient surface, to investigate the relationship between surface wettability and surface structure and surface chemical composition. Herein, the topography gradient was fabricated in a temperature gradient field and the chemical composition gradient was prepared via controlling the surface sulfonation degree. The prepared surface shows a one-dimensional gradient in wettability in the x, y, and diagonal directions with different gradient values, gradient range, and contact angle hysteresis, which lies on both the surface roughness and the surface compositions. The transition from the Cassie’s model to the Wenzel’s model occurs by both decreasing surface roughness and increasing surface hydrophilic compositions, and is more sensitive to surface compositions. The prepared orthogonal gradient surfaces may be applied to systematically study the mechanism of cell activity, such as cell adhesion, spreading, migration, and so forth, which, in a recent study, is very dependent on not only the surface composition19 but also surface roughness.20 2. Experimental Section 2.1. Materials. The commercial PS particles (made in Panjin Petrochemical of China, Mw ) 210K, PDI ) 2.84) were sandwiched between the two cleaned glass slides (washed by 1 wt % NaOH solution, rinsed with deionized water, and dried by N2 at room temperature), and heated at 160 °C with a pressure of ∼2000 Pa for 3 h. (Figure 1a,b) Then, the PS piece and glass slides were put flatly into 10-20 °C water, and the glass slides were cleaved from the PS (19) Gallant, N. D.; Lavery, K. A.; Amis, E. J.; Becker, M. L. AdV. Mater. 2007, 19, 965. (20) Simon, K. A.; Burton, E. A.; Han, Y.; Li, J.; Huang, A.; Luk, Y. J. Am. Chem. Soc. 2007, 129, 4892.
piece. Subsequently, the PS piece was cut into rectangle pieces, 30.0 ( 2.0 mm (length) (the x direction) × 22.0 ( 2.0 mm (width) (the y direction) × 3.0 ( 1.0 (thickness). These rectangle pieces were thoroughly rinsed with deionized water and dried by N2 (Figure 1c). 2.2. Preparation of the Textured Gradient Surface. A smooth PS rectangle piece was put on a cleaned glass side and covered by a commercial anodic aluminum oxide membrane with a pore diameter of about 0.2 µm (Figure 1d), then heated at 160 °C with a pressure of ∼500 Pa for 3 h, and cooled to room temperature, and put into 10 wt % NaOH solution for 24 h to etch the anodic aluminum oxide membrane. After that, the structured piece was thoroughly rinsed with deionized water and dried by N2. The gradient topography piece was fabricated in a gradient temperature field, which we have reported before14 (Figure 1e,f). 2.3. Preparation of the Chemical Gradient Surface. A smooth PS rectangle piece was put vertically in a special PTFE container (50.0 mm (length) (the x direction), 5.0 mm (width), and 25.0 mm (depth) (the y direction)) with its back standing against the wall of the well, and heated at 50 °C for 3 h in an oven. Then, 50 °C 98% H2SO4 was dripped from a drop funnel (10 mL) into the PTFE container along its wall. The dripping velocities are 5, 10, and 15 s/drop. When the PS piece was taken out immediately, it was almost immersed by H2SO4, and thoroughly rinsed with deionized water. 2.4. Preparation of the Orthogonal Gradient Surface. The textured gradient piece was put vertically in the PTFE container and heated at 50 °C for 3 h in an oven. Then, 50 °C 98% H2SO4 was dripped from a 10 mL drop funnel into the PTFE container along its wall (Figure 1g). The dripping velocity is around 10 s/drop. When the PS piece was taken out immediately, it was almost immersed by H2SO4, and thoroughly rinsed with deionized water (Figure 1h). 2.5. Characterization. The surface chemical compositions were measured by X-ray photoelectron spectroscopy (XPS). The XPS experiments with varying title angles were measured with VG ESCALAB MK IIat room temperature by using an Al KR monochrom (hν ) 1486.6 eV) at 14 KV and 20 mA. The micrographs of the sample were investigated by a Micro FEI PHILIPS XL-30-ESEMFEG field emission scanning electronic microscope (FESEM)
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operating at 20 kV. The samples for scanning electron microscopy were coated with a 20-30 Å layer of Au to make them conductive. The CAs were measured by a drop shape analysis system G10/ DSA10 (Kruess, Germany) at ambient. The volume of solvent droplets as the probe fluid was about 0.5-2 µL. The volume of the probing liquid does not affect the data.4 The CA hysteresis is calculated by θa - θr, where θa is the advancing contact angle and θr is the receding contact angle. The advancing and receding angle measurements are to tilt the sample until the drop just begins to roll downhill.21 The photograph of the overview wettability of the orthogonal gradient sample was caught by a Sony digital camera (T5 made in Japan).
3. Results and Discussion 3.1. The Textured Gradient Surface. The surfaces composed of different structures have a big difference in wettabilty. The CA hysteresis of microtube films is much higher than that of microrod ones.22 When a cleaned PS rectangle piece was put on a cleaned glass slide, covered by a commercial anodic aluminum oxide membrane with a pore diameter of about 0.2 µm (Figure 1d), and heated PS at 160 °C for 3 h as shown in Figure 1d,e, either microtubes or microrods can be obtained. These structures are dependent on two factors. One is the temperature, and the other is polymer molecular weight.23 Zhang et al. studied that microrods tended to form by using high polymer molecular weight PS or at low heating temperature condition.23 In order to obtain PS microrod surfaces, a cleaned PS rectangle piece with high molecular weight (Mw ) 210K) was chosen. During the process, the chosen temperature (160 °C) was higher than the glass transition temperature (Tg) of PS (∼100 °C). So, the PS chains could move and wet the cylindrical micropores of the alumina member to form PS microrods. After etching the alumina member in 10% NaOH aquatic solution for 24 h, the surface showed a CA of 156 ( 2° and the CA hysteresis around 7°. The textured gradient PS surface was prepared in a gradient temperature field, which has been studied in detail in our previous report.14 The microstructure of the film at different locations was investigated by FESEM (Figure 2). On the heated side (160 °C), the temperature was much higher than Tg. Thus, the PS chains could move at a high speed freely, which made the rods melt to adhere to the adjacent ones and fall over rapidly. After heating the film for 3 h, the rod microstructure disappeared completely, and a relatively flat surface formed (Figure 2b). Along the length from the heated side to the unheated side, i.e., the x direction in Figure 1f, the temperature decreased obviously. At the location where the temperature was a little bit higher than Tg of PS, the polymer chains in the PS microrods could move at a low velocity, which resulted in the PS microrods beginning to adhere to the adjacent rods little by little. With decreasing temperature, the adjacent degree decreased too. Therefore, the microrod diameter size decreased from 18 µm to 200 nm gradually (Figure 2c-i). On the unheated side, the temperature was much less than the Tg of the PS. At this time, the polymer chains were frozen and could not move freely. Thus, the microstructure did not change at all (Figure 2a,h,i), and the microrod diameter size was around 200 nm. Thus, we were successful in preparation of the textured gradient PS surface. Since surface wettability can be governed by surface roughness, we calculated the ratio of the height and the width of the microrods as surface roughness (r). Here, from the heated side to the unheated one, r increased from 0 to 20.5 gradually. Correspondingly, the surface hydrophobicity increased from 93.7° to 155.8° gradually along the x direction (Figure 3). (21) Johnson, R. E.; Dettre R. H. AdV. Chem. Ser. 1964, 43, 112. (22) Jin, M.; Feng, X.; Feng, L.; Sun, T.; Zhai, J.; Li, T.; Jiang, L. AdV. Mater. 2005, 17, 1977. (23) Zhang, M.; Dobriyal, P.; Chen, J.; Russell, T. P. Nano Lett. 2006, 6, 1075.
Figure 2. Typical FESEM images of polystyrene (PS) surface. (a) The imprinted PS surface. (b,d,f,h) The overview of the PS gradient surface along the length after processing in a temperature-gradient field. (c,e,g,i) The magnification of PS gradient surface along the length after processing in a temperature-gradient field.
Figure 3. Water contact angle and the corresponding shapes of sessile water droplets (2 µL) along the x direction of the sample. When the surface roughness values are 0, 2.6, 7.4, and 20.5, the water contact angles are 93.7°, 114.1°, 133.6°, and 155.8°, respectively.
3.2. The Chemical Composition Gradient Surface. When a PS piece is placed into 50 °C 98% H2SO4 solution, the PS
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Figure 5. Water contact angle and the corresponding shapes of sessile water droplets (2 µL) along the y direction of the sample. When sulfonation degrees are 0%, 11.2%, 28.2%, and 50%, the water contact angles of the different locations of the film are 46.2°, 52.8°, 77.4°, and 89.2°, respectively.
Figure 4. (a) X-ray photoelectron spectroscopy (XPS) spectra of S 2p region of the -HSO3 group gradient as a function of position among the y direction of the sample (2.5 mm steps) (dripping velocity: 10 s/drop). (b) At different dripping velocities, the sulfonation degree as a function of position among the y direction of the sample.
surface will be sulfonated (eq 1). The sulfonation degree depends on the reaction time. Herein,
we put the as-prepared smooth PS surface in a PTFE well with its back standing against the wall of the well. By adding 50 °C 98% H2SO4 solution in the well gradually, the positions along the latitude of the PS surface correspond directly to a continuous immersion time, i.e., the reaction time decreases gradually from the lower part to the upper part along the surface, which means that the different position of the surface possesses different sulfonation degree.17 Figure 4a is the S 2p XPS spectra of the surface different positions (∼168.5 eV). With the distance increasing from the lower part to the upper one of the surface, the area of the S 2p peak decreases gradually, which means the
sulfur content of the surface decreases along the y direction gradually. To extract the value of sulfonation degree, the sulfurto-carbon ratios were calculated. Herein, the sulfonation degree was calculated by the ratio of the S 2p peak area/0.54 and C 1s peak area/(8 × 0.25). Figure 4b gives the sulfonation degree of the different surface points at different dripping velocities. In Figure 4b, because the PS piece was taken out immediately after it was almost immersed by H2SO4, and then thoroughly rinsed with deionized water. The sulfonation degree of the initial place (y ≈ 20 mm) is around 0 (the acid initial reaction). When the dripping velocity is as high as 10.0 µL/s, the gradient is gentle, and the sulfonation degree of the surface only decreases from 26% to 0% along the y direction. On the contrary, when the dripping velocity is low (3.3 µL/s), the surface sulfonation degree decreases from 61% to 7%. When the dripping velocity is around 5.0 µL/s, the sulfonation degree of the surface decreases from 50% to 0%. With the enhancement of surface sulfonation degree, the surface composition of the hydrophilic group (-SO3H) increases gradually. Since the wettability of the solid lies on the surface compositions, a gradient surface with a continuous change of CA can be obtained because of the gradual variation of the surface compositions. Figure 5 shows a good gradient in wettability with CA increasing from 46.2°, to 52.8°, 77.4°, and 89.2° along the y direction (the dripping velocity: 5.0 µL/s). 3.3. The Orthogonal Gradient Surface. An orthogonal gradient surface is necessary to investigate the surface wettability because of the wettability governed by both the chemical composition and the geometrical microstructure of the surface. Herein, after putting the textured gradient PS piece instead of the flat one into the PTFE container and dripping 50 °C 98% H2SO4 solution into it at a velocity of 5.0 µL/s, the orthogonal gradient surface (textured gradient in the x direction and chemical composition gradient in the y direction) was obtained. Then, the surface wettability of different positions was investigated by dripping the 0.5-2.0 µL drops of 2 ppm KMnO4 solution onto the orthogonal gradient surface (Figure 6a). The surface shows a good orthogonal gradient in wettability. Not only at the x and y direction and but also at the diagonal direction, each line is an individual one-dimensional gradient in wettability. Gradient X1 is a hydrophobic to hydrophilic gradient. Gradients X2 and X3 are gradients from hydrophobic to superhydrophilic stage. Gradient X4 is a superhydrophobic to superhydrophilic gradient. Gradients Y1, Y2, and Y3 are hydrophilic to superhydrophilic gradients. Gradient Y4 is a gradient from superhydrophobic to hydrophobic stage. It should be noted that Figures 3 and 5 are included in Figure 6a. In Figure 6a, when the sulfonation degree is 0, the gradient (gradient Y4) is the same to the one of Figure 3. When the surface roughness is 0, the gradient (gradient X1) is the same to the one of Figure 5.
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relationship (eq 2). The CAs of different surface positions were listed in Figure 6b. From Figure 6b, the surface positions with
CA ) f(r, s)
Figure 6. (a) Water droplets set on the different locations of the orthogonal gradient surface (the exact roughness X1 ) 0; X2 ) 2.6; X3 ) 7.4; X4 ) 20.5; and the sulfonation degrees Y1 ) 50.0; Y2 ) 28.2; Y3 ) 11.2; Y4 ) 0). (b) Water contact angles of the different locations of the orthogonal gradient surface (X1 ) 0; X2 ) 2.6; X3 ) 7.4; X4 ) 20.5; Y1 ) 50.0; Y2 ) 28.2; Y3 ) 11.2; Y4 ) 0), contact angle errors: e (7°. (c) Contact angle hysteresis of the different locations of the orthogonal gradient surface. Contact angle hysteresis errors: e (4°.
Here, due to surface wettability governed by surface roughness (r) and sulfonation degree (s), we create a function to study their
(2)
s ≈ 0 and r ) 20.5 and with s ) 50% and r > 7.5 are the most hydrophobic and the most hydrophilic areas of the surface with CA around 156.7° and 0°, respectively. Furthermore, there are three interesting areas in the Figure 6b: area (I) (r < 7.4), area (II) (r > 7.4 and s > 26%), and area (III) (r > 7.4 and s < 26%). These areas have different gradient values (∂CA/∂r, ∂CA/∂s) and gradient ranges (∆CA ) Max(CA) - Min(CA)). Area (I) is a common gradient area with wettability changing from hydrophobicity to hydrophilicity in a gentle gradient value. Area (II) is quite smooth, which means the gradient value of this area is near 0. Therefore, strictly speaking, this area is not a gradient. Area (III) is the sharpest gradient among the three areas with the highest gradient value. The wettability changes from superhydrophobicity to superhydrophilicity rapidly. Moreover, we should consider the gradient value (∂CA/∂r, ∂CA/∂s) and the gradient range of each gradient (∆CA ) Max(CA) - Min(CA)). Looking at gradient Y1 - Y4, the gradient values of line Y1 - Y3 are negative to that of line Y4 (∂CA/∂r|s)0 > 0), that means, when the strong hydrophilic group (-HSO3) is more than ∼15%, the increase of r would make the surface more hydrophilic (∂CA/∂r|s>15% < 0). From gradient Y1 to Y4, s decreases from 50% to 0%. Correspondingly, ∆CA increases from 45° to 53°, 62°, and 64° gradually, and |∂CA/∂r| decreases from 9.0 to 7.1, 3.0, and 3.0 gradually, too. On the other hand, looking at gradient X1 - X4, the gradient values (∂CA/∂s) and the gradient range (∆CA) have a regular variation. From gradient X1 to X4, r increases from 0 to 20.5, and ∆CA increases from 48° to 106°, 135°, and 157° gradually, and |∂CA/∂s| increases from 1.0 to 2.1, 4.5, and 10.1 gradually, too. Among these onedimensional gradients, gradient X1 is the smoothest one, i.e., the lowest gradient value. Gradients X4 and Y1 are special gradients with high surface roughness and high sulfonation degree, respectively. They have the greatest gradient steepness at the first gradient path. Then, the gradient steepness decreases to 0 because of the high surface roughness and high sulfonation degree. Strictly speaking, except the first path of gradients X4 and Y1, other paths are not wettability gradients because gradient steepness is 0. Accordingly, gradient X4 and Y1 are the shortest among these one-dimensional gradients. Gradient X4 is sharpest with the greatest ∆CA due to the highest surface roughness. So, the increase of surface roughness can improve not only surface wettability but also the gradient value. On the other hand, the CA hysteresis is another important value to surface wettability. Figure 6c shows the CA hysteresis of the different locations. On the fresh textured surface (Figure 2f,g)) (r ) 20.5, s ) 0), the CA hysteresis is as low as 7°. With r decreasing, the CA hysteresis first increases to 62°, and then decreases to 27° finally. Moreover, with s increasing, the CA hysteresis also first increases to 102°, and then decreases. From Figure 6c, most one-dimensional gradients have the same variation trend of CA hysteresis. It is worth noting here that when s and r are larger than ∼15% and ∼7.5, respectively, the CA hysteresis is not measurable due to the strong hydrophilicity of the surface. To understand these phenomena, theoretical consideration is necessary. Here, we employ two models. One is the Wenzel’s model (eq 3),24 where θ and θw are contact angles on the smooth and rough surfaces, respectively. rw is surface roughness. The (24) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988.
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cos θw ) rw cos θ
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other is the Cassie’s model (eq 4),25 where θ and θc are contact angles on the smooth and rough surfaces, respectively. f is the fractional area of air trapped on the rough surface.
cos θc ) (1 - f) cos θ - f
(4)
On the fresh textured surface (Figure 2f,g), the CA is around 156 ( 2° and the CA hysteresis is around 7°. The CA hysteresis is so low that it indicates there is a big fraction of air beneath the water drop. This wetting phenomenon is according to the Cassie’s model. By calculation, the fraction of the air trapped among the hydrophobic rods is as high as 0.92, which, in our opinion, is essential to such low CA hysteresis. With surface roughness decreasing, the fraction of air decreases obviously, which results in the CAs decreasing. Interestingly, the CA hysteresis has two variation stages: increasing stage and decreasing stage. With r decreasing, the sudden increase of the CA hysteresis indicates that the wetting phenomenon changes from the Cassie’s model to the Wenzel’s model.4d,17,19 After this transition, the CA hysteresis then decreases with r decreasing continuously. Although the wetting phenomenon can change from the Cassie’s model to the Wenzel’s model both via decreasing r and increasing s, the transition via increasing s is more sensitive than that via decreasing r at the high roughness condition. In other words, the transition from the Cassie’s model to the Wenzel’s model is more effective via changing surface chemical composition than via changing surface roughness. (25) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546.
4. Conclusion
(3)
In summary, we successfully prepared for the first time a topography/chemical composition gradient surface, i.e., an orthogonal gradient surface, to investigate the relationship between surface wettability and surface structure and surface chemical composition. The prepared surface shows a good orthogonal gradient in wettability, which contains many special one-dimensional gradients, including hydrophobic to hydrophilic, superhydrophobic to hydrophobic, superhydrophobic to superhydrophilic gradients, and so forth. These one-dimensional gradients have different gradient values and gradient ranges, which lie on both the surface roughness and the surface compositions. When surface roughness is around 0, the gradient (gradient X1) is the smoothest. On the contrary, when surface roughness is the biggest (r ) 20.5), the gradient (gradient X4) is the sharpest with the greatest ∆CA due to the highest surface roughness. So, the increase of surface roughness can improve not only surface wettability but also the gradient value. Moreover, the CA hysteresis is different too. Looking at the variation trend of CA hysteresis, the transition from the Cassie’s model to the Wenzel’s model can be observed. This transition is more effective via changing surface chemical composition than via changing surface roughness herein. The orthogonal gradient surfaces may be applied to systematically study the mechanism of cell adhesion, spreading, migration, and so forth due to these processes are very dependent on not only the surface composition but also surface roughness. Acknowledgment. This work is subsidized by the National Natural Science Foundation of China (20334010, 20621401, 50573077). LA702567W
