pubs.acs.org/Langmuir © 2010 American Chemical Society
Tip Geometry Controls Adhesive States of Superhydrophobic Surfaces Zhongjun Cheng, Jun Gao, and Lei Jiang* Center of Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, 100190 Beijing, China Received December 6, 2009. Revised Manuscript Received January 25, 2010 Inspired by biological attachment systems, aligned polystyrene (PS) nanopillars terminating in flat, concave tips and nanotubes were fabricated by a simple and reproducible method. All the obtained surfaces show both the contact angles larger than 150° (superhydrophobicity) and high adhesion of water to it. The tip geometry plays an important role in determining the adhesive property. Surface with the concave tips has the highest adhesion, and then the surface with flat tips, whereas aligned nanotube surface has a relatively lower adhesion. Besides different van der Waals forces between the PS surfaces and water, another important factor, i.e., different negative pressures produced by the different volumes of sealed air, may be the crucial factor for their different adhesions. These findings provide the experimental evidence of the influence of the tip geometry on the adhesion of structured superhydrophobic surfaces, which is helpful for us to further understand the biological attachment systems and to optimum design of artificial analogues.
Introduction Geckos and some insects are exceptional in their ability to climb rabidly up smooth vertical surfaces.1,2 This remarkable ability is explained by the unique micro- and nanostructures on their attachment pads that provide sufficient adhesions with solid surfaces.3 Taking from the inspiration of these findings, many surfaces with novel adhesion have been prepared.4,5 In addition to the application in dry adhesion, the animals’ remarkable ability can also be applied in other fields, for example, superhydrophobic surfaces. Traditionally, superhydrophobic surfaces all have a large contact angle and a small sliding angle. Such surfaces have attracted great interest for many potential applications and can be effectively fabricated by combining appropriate surface roughness with *To whom correspondence should be addressed: e-mail jianglei@ iccas.ac.cn; Tel þ861082621396; Fax þ861082627566.
(1) Autumn, K.; Liang, Y. A.; Hsieh, S. T.; Zesch, W.; Chan, W. P.; Kenny, T. W.; Fearing, R.; Full, R. J. Nature 2000, 405, 681–684. (2) Lee, H.; Lee, B. P.; Messersmith, P. B. Nature 2007, 448, 338–341. (3) Tian, Y.; Pesika, N.; Zeng, H.; Rosenberg, K.; Zhao, B.; McGuiggan, P.; Autumn, K.; Israelachvili, J. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 19320–19325. (4) Qu, L.; Dai, L.; Stone, M.; Xia, Z.; Wang, Z. L. Science 2008, 322, 238–242. (5) Geim, A. K.; Dubonos, S. V.; Grigorieva, I. V.; Novoselov, K. S.; Zhukov, A. A.; Shapoval, S. Y. Nat. Mater. 2003, 2, 461–463. (6) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1–8. (7) Blossey, R. Nat. Mater. 2003, 2, 301–306. (8) Lafuma, A.; Quere, D. Nat. Mater. 2003, 2, 457–460. (9) Feng, L.; Li, S.; Li, Y.; Li, H.; Zhang, L.; Zhai, J.; Song, Y.; Liu, B.; Jiang, L.; Zhu, D. Adv. Mater. 2002, 14, 1857–1860. (10) Ma, M.; Gupta, M.; Li, Z.; Zhai, L.; Gleason, K. K.; Cohen, R. E.; Rubner, M. F.; Rutledge, G. C. Adv. Mater. 2007, 19, 255–259. (11) Ming, W.; Wu, D.; van Benthem, R.; de With, G. Nano Lett. 2005, 5, 2298– 2301. (12) Lee, Y.; Park, S.-H.; Kim, K.-B.; Lee, J.-K. Adv. Mater. 2007, 19, 2330– 2335. (13) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Chabrol, G.; Perry, C. C. Adv. Mater. 2004, 16, 1929–1932. (14) Zimmermann, J.; Reifler, F. A.; Fortunato, G.; Gerhardt, L.; Seeger, S. Adv. Funct. Mater. 2008, 18, 3662–3669. (15) Xiu, Y.; Zhu, L.; Hess, D. W.; Wong, C. P. Nano Lett. 2007, 7, 3388–3393. (16) Gao, L.; McCarthy, T. J. Langmuir 2008, 24, 362–364. (17) Tsujii, K.; Yamamoto, T.; Onda, T.; Shibuichi, S. Angew. Chem., Int. Ed. 1997, 36, 1011–1012. (18) Zhang, X.; Shi, F.; Yu, X.; Liu, H.; Fu, Y.; Wang, Z.; Jiang, L.; Li, X. J. Am. Chem. Soc. 2004, 126, 3064–3065. (19) Zou, J.; Chen, H.; Chunder, A.; Yu, Y.; Huo, Q.; Zhai, L. Adv. Mater. 2008, 20, 3337–3341. (20) Zorba, V.; Stratakis, E.; Barberoglou, M.; Spanakis, E.; Tzanetakis, P.; Anastasiadis, S. H.; Fotakis, C. Adv. Mater. 2008, 20, 4049–4054.
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materials of low surface energy.6-30 Recently, inspired by the high adhesive property of the gecko’s feet, superhydrophobic surfaces with high adhesive force, i.e., a large sliding angle, have also aroused much attention.31-39 A good example is the superhydrophobic polystyrene (PS) nanotube surface reported by us previously,31 which has a high adhesive force to water and has been successfully used as a “mechanical hand” for transportation of microdroplets.32 A similar phenomenon can also be observed on the surface of hairy h-PDMS film.33 Besides these polymeric superhydrophobic surfaces, some inorganic superhydrophobic surfaces, such as ZnO film,34 aluminum alloy,35 and patterned hydrophobic silicon substrate,36 are also interesting for their high adhesions.37-39 These works all believe that the geometrical structures on these surfaces are crucial for their special adhesions, but a clear picture of how they contribute to the final adhesive performance is still missing. Fortunately, nature gives us the inspiration; for different animals, the (21) Li, X.; Reinhoudt, D.; Crego-Calama, M. Chem. Soc. Rev. 2007, 36, 1350– 1368. (22) 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, 1701–1705. (23) Min, W.; Jiang, B.; Jiang, P. Adv. Mater. 2008, 20, 3914–3918. (24) Ji, J.; Fu, J.; Shen, J. Adv. Mater. 2006, 18, 1441–1444. (25) Srinivasan, S.; Praveen, V. K.; Philip, R.; Ajayaghosh, A. Angew. Chem., Int. Ed. 2008, 47, 5750–5754. (26) Kako, T.; Nakajima, A.; Irie, H.; Kato, Z.; Uematsu, K.; Watanabe, T.; Hashimoto, K. J. Mater. Sci. 2004, 39, 547–555. (27) Han, J. T.; Kim, S. Y.; Woo, J. S.; Lee, G. Adv. Mater. 2008, 20, 3724–3727. (28) Hong, J.; Bae, W. K.; Lee, H.; Oh, S.; Char, K.; Caruso, F.; Cho, J. Adv. Mater. 2007, 19, 4364–4369. (29) Garcı´ a, N.; Benito, E.; Guzman, J.; Tiemblo, P. J. Am. Chem. Soc. 2007, 129, 5052–5060. (30) Luo, Z.; Zhang, Z.; Hu, L.; Liu, W.; Guo, Z.; Zhang, H.; Wang, W. Adv. Mater. 2008, 20, 970–974. (31) Jin, M.; Feng, X.; Feng, L.; Sun, T.; Zhai, J.; Li, T.; Jiang, L. Adv. Mater. 2005, 17, 1977–1981. (32) Hong, X.; Gao, X.; Jiang, L. J. Am. Chem. Soc. 2007, 129, 1478–1479. (33) Cho, W. K.; Choi, I. S. Adv. Funct. Mater. 2008, 18, 1089–1096. (34) Li, Y.; Zheng, M.; Ma, L.; Zhong, M.; Shen, W. Inorg. Chem. 2008, 47, 3140–3143. (35) Guo, Z.; Liu, W. Appl. Phys. Lett. 2007, 90, 223111. (36) Winkleman, A.; Gotesman, G.; Yoffe, A.; Naaman, R. Nano Lett. 2008, 8, 1241–1245. (37) Boscher, N. D.; Carmalt, C. J.; Parkin, I. P. J. Mater. Chem. 2006, 16, 122– 127. (38) Liao, C.; Wang, C.; Lin, H.; Chou, H.; Chang, F. Langmuir 2009, 25, 3359– 3362. (39) Zhao, N.; Xie, Q.; Kuang, X.; Wang, S.; Li, Y.; Lu, X.; Tan, S.; Shen, J.; Zhang, X.; Zhang, Y.; Xu, J.; Han, C. C. Adv. Funct. Mater. 2007, 17, 2739–2745.
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Figure 1. Schematic illustration of the preparation process of PS nanopillars with flat and concave tips.
tips of the fine hairs in biological systems show different geometries (spherical, conical, flat, concave shapes, and so on), which endow these animals with different adhesions,40 and recent works already point out the importance of the tip geometry in determining the dry adhesion.41 However, to the best of our knowledge, this factor has been considered limited in the artificial superhydrophobic systems up to now.42 It is well-known that PS is a common material used to fabricate superhydrophobic surfaces.43-45 Herein, superhydrophobic aligned PS nanopillars terminating in flat, concave tips and nanotubes, resembling those found in the attachment pads of different animals, were fabricated by a simple and reproducible method. The surface with concave tips has the largest adhesive force to water, and then the surface with flat tips, whereas the aligned nanotube surface has a relative lower adhesion, indicating that the tip geometry plays an important role in determining the adhesive property. Besides the different van der Waals forces between these PS surfaces and the water, another important factor, i.e., different negative pressures produced by the different volumes of sealed air, may be the crucial factor for their different adhesions. Furthermore, the adhesion of superhydrophobic surfaces can be controlled by simply tuning the volume of the sealed air. These findings provide the useful experimental evidence of the influence of the tip geometry on the adhesion of superhydrophobic surfaces and pave the road to a better understanding of biological attachment systems and to optimum designs of artificial analogues.
Experimental Section Samples Preparation. The method used to fabricate the PS
nanopillars with different tip geometries is simple (Figure 1).46-48 The precursor solution was prepared by dissolve PS (3 g, Mw = 230 000, Aldrich) in dimethylbenzene (7 g) to form a 30 wt % solution. Then, the PS smooth films with a thickness of about 50 μm were prepared via solution casting on clean glass slides. After drying at room temperature, a porous template of commercial alumina membrane, whose porosity consists of an array of (40) Gao, H.; Yao, H. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 7851–7856. (41) del Campo, A.; Greiner, C.; Alvarez, I.; Arzt, E. Adv. Mater. 2007, 19, 1973–1977. (42) Bok, H.; Kim, S.; Yoo, S.; Kim, S. K.; Park, S. Langmuir 2008, 24, 4168– 4173. (43) Lupitskyy, R.; Roiter, Y.; Tsitsilianis, C.; Minko, S. Langmuir 2005, 21, 8591–8593. (44) Ionov, L.; Houbenov, N.; Sidorenko, A.; Stamm, M.; Luzinov, I.; Minko, S. Langmuir 2004, 20, 9916–9919. (45) Motornov, M.; Minko, S.; Eichhorn, K. J.; Nitschke, M.; Simon, F.; Stamm, M. Langmuir 2003, 19, 8077–8085. (46) Zhang, M.; Dobriyal, P.; Chen, J.-T.; Russell, T. P.; Olmo, J.; Merry, A. Nano Lett. 2006, 6, 1075–1079. (47) Zhang, J.; Han, Y. Langmuir 2008, 24, 796–801. (48) Lee, W.; Jin, M.-K.; Yoo, W.-C.; Lee, J.-K. Langmuir 2004, 20, 7665–7669.
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parallel and straight channels (see Supporting Information Figure S1) with the diameter of 13 mm and the thickness of 60 μm (Whatman Int. Ltd., England), was placed on top of the smooth PS film. The sample was sandwiched between two glass slides, and then the assembly was fixed by clips to ensure a good contact between the polymer film and the alumina membrane. The assembly was placed in an oven (80 °C for surfaces with flat tips, 160 °C for surfaces with concave tips) and then annealed under vacuum. After 2 h the assembly was taken out and cooled to room temperature. Dissolving the alumina membranes with 3 M NaOH aqueous and then rinsed with pure water, and dried with N2, finally, the aligned PS nanopillars with flat and concave tips were obtained. As to aligned PS nanotubes, the method is the same as we reported previously.31 The same precursor solution was cast onto a clean glass slide and covered by the same alumina membrane. After 24 h, the template was dissolved away similarly as mentioned above; finally, the aligned PS nanotubes film was obtained. Characterization. Field-emission scanning electron microscopy (SEM) (JSM-6700F, Japan) was used to obtain SEM images of the PS nanostructured surfaces. The atomic force microscopy (AFM) images were recorded by SPI3800N/ SPA400 (SII Seiko Instruments Inc.) in dynamic force mode. The water contact angle was measured using an OCA20 (optical contact angle measurement) at ambient temperature. The adhesive force was measured using a high-sensitivity microelectromechanical balance system with a resolution of 10 μg (DataPhysics DCAT 11, Germany). A 3 μL water droplet was suspended with a metal ring first, and the PS surface was placed on the balance table. The surface was moved upward at a constant speed of 0.02 mm s-1 until the substrate contacted the water droplet. Then, the PS surface began to move downward. The water droplet was stretched from the spherical to the elliptical, and the force increased gradually up to maximum. When the PS surface broke away from the droplet, the droplet changed back to spherical and the force rapidly reduced down to zero.
Results and Discussion Figure 2 shows the SEM images of the as-prepared surfaces. Arrays of nanopillars with flat tips were obtained after treating at 80 °C, the density of these nanopillars is ∼6.81 106 nanopillars mm-2 (Figure 2a), and the length is ∼12.4 μm (see Supporting Information Figure S2a). In a larger magnified view of Figure 2a, round aligned nanopillars with flat tips are observed clearly with an average diameter of ∼303.3 nm (Figure 2b). When the heating temperature is increased to 160 °C, one can obtain fine nanopillars with the concave tips. Different from the nanopillars with flat tips, the center of the nanopillars end appears darker than the edge, suggesting the existence of the meniscus at the tips (Figure 2c). The magnified image indicates that the nanopillars are round with an average outer diameter of ∼302.8 nm and an inner diameter of ∼187.3 nm (Figure 2d). Meanwhile, the density of these nanopillars is ∼6.77 106 nanopillars mm-2, and the length is ∼13.3 μm (see Supporting Information Figure S2b). As to aligned PS nanotube surface, which is prepared similarly as we reported previously.31 Figure 2e shows the top view SEM image of the PS nanotube surface, showing good alignment of PS nanotubes with open end-caps. Because of the same template used in the experiment, the density of the nanotubes is similar to the two foregoing surfaces, ∼6.79 106 nanotubes mm-2. In a magnified image (Figure 2f), one can observe that the nanotubes are round with an average out diameter of ∼301.9 nm and a wall thickness of ∼64.5 nm. Different from the surfaces with flat and concave tips, aligned nanotubes have a larger length, ∼53.8 μm (see Supporting Information Figure S2c). In order to have a better understanding of the surfaces with concave tips, the atomic force microscopy was used to investigate the structures. Figure 3a shows the top image of the surface, which Langmuir 2010, 26(11), 8233–8238
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Figure 2. SEM images of the as-prepared aligned PS nanopillars with different tip geometries: (a, c, e) top view of the PS nanopillars with flat, concave shapes, and nanotubes, respectively; (b, d, f) the magnified images corresponding to (a), (c), and (e), respectively.
Figure 5. Schematic illustration of a water droplet on the aligned PS nanopillars with flat (a), concave (b) tips and nanotubes (c). (d-f) Schematic illustration of the interfaces between water and a single PS nanopillar corresponding to (a), (b), and (c), respectively. Yellow section means the water/PS contact fraction, gray shaded areas represents the water contacts with the sealed air, and the others is water contacts with open state air (continuous with atmosphere).
Figure 3. (a) AFM image (topography) of the PS nanopillars with concave tips. (b) Cross-sectional height profile of (a).
all the as-prepared PS films are superhydrophobic, and the contact angles are about 151°, 157°, and 158° for water on surfaces with flat, concave tips, and nanotube surface, respectively (Figure 4b; see Supporting Information Figure S3a,d). The reason for the superhydrophobicity of these surfaces is mainly ascribed to the peculiar nanostructures. For example, as to the surface with flat tips, the superhydrophobic property is resultant from the air trapped between the nanopillars, which can prevent the intrusion of water into the nanostructures, resulting in the large contact angle. The theoretical explanation of the superhydrophobicity of the PS surface can be expressed by eq 1, which was first proposed by Cassise and Baxter.50,51 cos θr ¼ rf f cos θ þ f - 1
Figure 4. (a) Behavior of a water droplet on a smooth PS film with a contact angle of 86°. Shapes of a water droplet on the as-prepared PS surface with concave tips, tilted at different angles: (b) 0°, (c) 90°, and (d) 180°.
confirms the special structure of such nanopillars, and is in good agreement with the SEM results. Furthermore, from the crosssectional image of AFM (Figure 3b), it is easy to find the average depth of the concave is ∼196.9 nm. As to a well-prepared flat PS surface, the contact angle for a water droplet is about 86° (Figure 4a).49 However, in this work, (49) Li, Y.; Pham, J.; Johnston, K. P.; Green, P. F. Langmuir 2007, 23, 9785– 9793.
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ð1Þ
Here θ and θr are the contact angles of the flat PS surface and the nanostructured PS surface, respectively, f is the fraction of the projected area of the solid surface that is wet by the liquid, and rf is the roughness ratio of the wet area. It is easy to find from eq 1 that the fraction of air in the surface is an important factor in determining the superhydrophobicity of the surface. Different from the nanopillars with flat tips, there are two kinds of air trapped between water and the aligned nanopillars with concave tips and nanotubes. One is trapped between individual nanopillars or nanotubes,52 and the other is the air sealed in the concaves or nanotubes under the water (Figure 5a-c). For these (50) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546–551. (51) Marmur, A. Langmuir 2003, 19, 8343–8348. (52) Wang, S.; Jiang, L. Adv. Mater. 2007, 19, 3423–3424.
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Figure 6. Force-distance curves recorded before and after the
water droplet (3 μL) contacted the as-prepared PS surface with different tip geometries. Process 1: PS surface approaches the water droplet. Process 2: PS surface leaves the water droplet after contact. Process 3: PS surface breaks away from the water droplet. The lines with different symbols represent the nanopillars with different tips: concave (9), flat (0), and nanotubes (2).
two surfaces, more air is trapped than the surface with flat tips and leads to higher contact angles. The adhesive property of the as-prepared PS surfaces is our major concern in this study. On most flat surfaces, a water droplet comes to rest at a local energy minimum due to either chemical structures or topography, and the three-phase contact line is fixed. Therefore, there are energy barriers for advancing and receding movements of a water droplet, which cause contact angle hysteresis (difference between advancing and receding contact angles).33,53-58 The energy barriers can be small if the contorted, discontinuous contact line with a water droplet is formed, inducing a water droplet to move easily on the surface. According to the SEM images of the obtained PS surfaces (Figure 2), the topography of the surfaces is discontinuous, which maybe leads to a discontinuous contact line and a low energy barrier, causing the water droplet to slide easily. However, for all these prepared surfaces, a water droplet does not slide even when the PS surfaces tilted vertically or turned upside down (Figure 4c,d; see Supporting Information Figure S3b,c,e,f), indicating that all these surfaces are high adhesive to water. Furthermore, this sticky property is stable; the water droplet can adhesive to the surface all the while without any external disturbance. For a 3 μL water droplet, after about 30 min, it will be vaporized and vanished in the atmosphere. By using a high-sensitivity micro-electromechanical balance system, the adhesive action was assessed accurately. Figure 6 displays the force-distance curves recorded before and after a water droplet contacted the as-prepared PS surface. First, the PS surface was placed on the plate of the balance system, a 3 μL water droplet was suspended on a metal ring, and the force of the balance system was initialized to zero (step 1). Then, the PS surface was brought into contact with the droplet while maintaining the balance force at zero. The surface was moved at a rate of 0.02 mm s-1. When the surface left the water droplet after contact, the balance force increased gradually and reached its maximum at the end of the step 2. Finally, the balance force decreased immediately when the surface broke away from the droplet in step 3 to finish one cycle of the measurement. (53) Quere, D.; Azzopardi, M.-J.; Delattre, L. Langmuir 1998, 14, 2213–2216. € (54) Chen, W.; Fadeev, A. Y.; Hsieh, M. C.; Oner, D.; Youngblood, J.; McCarthy, T. J. Langmuir 1999, 15, 3395–3399. (55) Yoshimitsu, Z.; Nakajima, A.; Watanabe, T.; Hashimoto, K. Langmuir 2002, 18, 5818–5822. (56) Extrand, C. W. Langmuir 2004, 20, 4017–4021. € (57) Oner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777–7782. (58) Gao, L.; McCarthy, T. J. Langmuir 2006, 22, 6234–6237.
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Interestingly, it is observed that the maximum adhesive force between water droplet and these surfaces is different. The surface with concave tips has the largest adhesive force of 90.3 μN, then the surface terminating in flat tips, about 76.7 μN, and the aligned nanotube film has a relatively lower adhesive force of 57.5 μN (every datum is an average value of 20 samples with the same structure). As known, the adhesion of the surface can be governed by both the chemical composition and the geometrical microstructures. In this work, all surfaces were prepared with the same hydrophobic material PS and the same template. The factors that can influence the adhesive property such as chemical compositions, densities, and diameters of these surfaces are similar to each other, and the only difference is the tip geometry. Thus, it is easy to find that the different adhesions of these surfaces are mainly caused by their different tip geometries. As previously reported, the adhesion between polar water and nonpolar PS surface can be explained mainly by the dispersive adhesion caused by van der Waals forces.59 As to the surface with flat tips, the as-prepared PS surface is composed of about 6.81 106 nanopillars mm-2; a high van der Waals force will result, leading to a strong adhesion between water and the PS surface. This explanation can be understood further when we consider the mechanism of the gecko’s ability to climb on walls. Each hair of the gecko’s foot operates by van der Waals forces and produces just a miniscule force, but lots of hairs collectively create the formidable adhesion.1,2 The effect of the PS nanopillars is similar to that of gecko’s hair. Noticeably, compared with surface terminating in flat tips, surface with concave tips has a larger adhesion, while aligned nanotube surface has a lower adhesion, and this maybe due to the following two reasons. The first is the different van der Waals forces, and the second is the different negative pressures produced by the air sealed in the concaves and nanotubes. Similar to the surface with flat tips, the surface with concave tips and the nanotube surface are also capable of generating large van der Waals forces and high adhesion for their high densities (∼6.77 106 nanopillars mm-2 and ∼6.79 106 nanotubes mm-2 for nanopillars with concave tips and nanotube surface, respectively). However, both the water/PS contact areas for one nanopillar with concave tip (∼4.44 104 nm2, Figure 5e) and one nanotube (∼4.80 104 nm2, Figure 5f) are smaller than that for single nanopillar with flat tip (∼7.22 104 nm2, Figure 5d), and as mentioned above, the densities and diameters of these nanopillars with different shapes are similar between each other. Therefore, for the same water droplet, both the whole water/PS contact areas for the surface with concave tips and the nanotube surface should be smaller than that for the surface with flat tips; accordingly, the adhesive force produced by the van der Waals forces would be lower for both the surface with concave tips and the nanotube surface compared with the surface terminating in flat tips because the van der Waals forces are proportional to the contact areas.41 If just consider the van der Waals forces, the experimental results are apparently inconsistent, indicating that another important factor influences the adhesion. When a water droplet is placed on the PS surface with concave tips and the nanotube surface, there are two kinds of trapped air: the air pockets in the open state (continuous with the atmosphere) and the sealed pockets of air trapped in the concaves and nanotubes (Figure 5b,c).52 Here, since the open air pockets between nanopillars or nanotubes cannot form a closed system to generate the negative pressure, their contribution to the adhesion may be negligible. According to Boyle’s law,60 there is (59) Bargeman, D. J. Colloid Interface Sci. 1972, 40, 344–348. (60) West, J. B. J. Appl. Physiol. 1999, 87, 1543–1545.
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an inverse relationship between the volume (V0) and pressure (P) for ideal gas under the conditions of constant temperature and quality. Once the droplet on the surfaces is drawn, the meniscus on each concave or nanotube would be changed from concave to convex, and the negative pressure would be produced by the expansion of the sealed air. As to the surface with concave tips, because the volume of the sealed air is very small (assuming a concave is a hemiellipse, the volume of the air sealed in one concave is about 7.23 106 nm3), when the water is drawn, the increased volume (ΔV) is close to the initial volume (V0) of the sealed air, which leads to a large air-expansion ratio (ΔV/V0) and a large negative pressure. Thus, compared with the surface terminating in flat tips, the surface with concave tips has a smaller van der Waals force, but the negative pressure would be large enough to endow the surface with higher adhesion. Similarly, there is also sealed air in the nanotubes; however, the initial volume (V0) of the sealed air in nanotubes is much larger than the increased volume (ΔV) (the volume of the sealed air is about 1.59 109 nm3 in one nanotube), resulting in a low air-expansion ratio (ΔV/V0) and a small negative pressure. When the water is drawn, the adhesive force induced by the small negative pressure cannot offset the difference value of van der Waals forces between the aligned nanotube surface and the surface with flat tips; as a result, the aligned nanotube surface has a lower adhesion. Therefore, the adhesive forces for these surfaces not only depend on the van der Waals forces but also are relative with the air sealed between the surface and the water, indicating that the tip geometries of these surfaces are important for their adhesive properties. Furthermore, from the above results, one can find that the initial volume of the sealed air is crucial for the adhesion; briefly, larger volume of sealed air would be expect to have a lower airexpansion ratio and lesser negative pressure. So it can be concluded that the adhesive force can be controlled easily by change the volume of the air sealed in the concaves or nanotubes, and this may be an effective way to fabricate surfaces with different adhesions. In order to have a thorough understanding of the negative pressure produced by the sealed air, theoretical considerations are necessary. First, the existence of air sealed in the concaves and nanotubes need to be confirmed. As to the surface with concave tips, if the water permeates into the concaves, the contact angle of water on the concave inner surface would first get to the advancing contact angle (θa); at the moment, the Laplace pressure (PL) produced by the liquid/air interface between the water and the concave can be described as (Figure 7a) PL ¼
2γ r=cosðπ - θa Þ
ð2Þ
where γ is the tension of water (72.8 10-5 N/cm), r is the inner radius of the concave (93.65 nm), and θa is the advancing contact angle (101°, see Supporting Information Figure S4a). Therefore, PL can be calculated to be about 2.97 105 Pa. The pressure produced by the gravity of water can be obtained by the following formula:61 PG ¼ 2
FVg
32=3
7 6 V 7 6 7 π6 4 1 - cos θ 5 2 ð1 - cos θÞ 1 3
sin2 θ
ð3Þ
where F is the density of water (1.0 g/cm ), V is the volume of water (3 μL), g is the acceleration of gravity (9.8 N/kg), and θ is 3
(61) Ishino, C.; Okumura, K.; Quere, D. Europhys. Lett. 2004, 68, 419–425.
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Figure 7. (a) Schematic illustration of the interface between water and a single nanopillar with concave shape. (b) Schematic illustration of the volume change of the sealed air in one PS nanopillar with concave tip upon the action of external force.
the contact angle of a water droplet on the surface with concave tips (157°). So the PG can be calculated to be about 77.9 Pa. Take into account of the external atmospheric presser (P0 = 1.013 105 Pa), it can be found PL . PG þ P0
ð4Þ
Therefore, it can be concluded that the contact angle of water on the PS concave inner surface cannot get to the advancing angle; that is, water cannot permeate along the inner surface. At the same time, on the assumption that the contact angle of water on the PS concave inner surface gets to the advancing angle, the maximal depth of water below the PS surface (h, Figure 7a) under this limiting condition can be obtained as h ¼
r ½1 - sinðπ - θa Þ cosðπ - θa Þ
ð5Þ
where r is the inner radius of the concave (93.7 nm) and θa is the advancing contact angle (101°, see Supporting Information Figure S4a). So h is about 9 nm, which is far less than the average depth of the concave ∼196.9 nm. Thus, it is sure that some air can be sealed between water and the concaves. As to the naonotubes, because of its height as large as 53.8 μm, so similar to the surface with concave tips, the air can also be sealed in the nanotubes. As mentioned above, the air can be sealed in the concaves and nanotubes; then how does the sealed air influence the adhesive properties of these surfaces? Once the water on the PS surface is drawn, the meniscus on each concave (or nanotube) would be changed from concave to convex. This could result in an increased volume of air sealed in each concave (or nanotube), and the negative pressure would be produced for the increase of the volume of the sealed air (Figure 7b). Assuming the sealed air is ideal gas, the negative pressure ΔP could be described as Δp ¼ - P0
1 V0 þ1 ΔV
ð6Þ
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Article
Cheng et al.
air under the action of external force. From eq 6, one can find the relationship of ΔV and V0 is important for ΔP, that is, when ΔV , V0, ΔP would be rather small, while ΔV ≈ V0, ΔP would be very large. In other words, larger V0 would result in smaller ΔP. From Figure 7b, one can observe that ΔV can be described as ΔV ¼ ΔV1 þ ΔV2
ð7Þ
Because the depth of water below the PS surface (h) produced by the PG is too small, the volume of water in the concave (ΔV2) can be negligible; therefore ΔV ≈ ΔV1 ¼ π
d þ 2x 2 sin θr
3
1 þ cos θr ð1 þ cos θr Þ2 1 3
ð8Þ
where d is the inner diameter of concave (or nanotube), x is the distance between liquid/air interface and the inner surface of concave (or nanotube), and θr is the receding contact angle of a water droplet on the flat PS surface (78°, see Supporting Information Figure S4b). Here, we are not sure at which point the water would leave the PS tip (Figure 7b), and we cannot give a fixed value of the ΔV, but we can give a range of the ΔV. As to one PS nanopillar with concave shape: 0 nm < x < 57.8 nm, 2.4 106 nm3 < ΔV < 10.1 106 nm3. The initial volume of sealed air is V0 = 7.23 106 nm3; ΔV and V0 have the same order magnitude, so according to eq 6, ΔP would be very large, which would be responsible for the largest adhesion of the surface with concave tips. While for one PS nanotube: 0 nm < x < 64.5 nm, 1.8 106 nm3 < ΔV < 10 106 nm3. The initial volume of sealed air is V0 = 1.59 109 nm3, ΔV , V0, so according to eq 6, one can find that the nanotube film has a lower air-expansion ratio (ΔV/V0); thus, ΔP would be rather small, that is, the force produced by this small negative pressure is not large enough to offset the difference value of van der Waals forces between the aligned nanotube surface and the aligned nanopillars with flat tips (the aligned nanotube surface has a low van der Waals force for the smaller water/PS contacted areas compared with the surface with flat
8238 DOI: 10.1021/la904510n
tips), and this may be the reason for the relative lower adhesion of the aligned nanotube surface.
Conclusions In summary, aligned PS nanopillars with flat, concave tips, and nanotubes were fabricated by a simple and reproducible method. All the as-prepared surfaces show both the superhydrophobicity with contact angles larger than 150° and high adhesion to water. Interestingly, the tip geometry plays an important role in determining the adhesive property. Surface with the concave tips has the highest adhesion, and then the surface with flat tips, whereas the aligned nanotube surface has a relative lower adhesion. Besides different van der Waals forces, different negative pressures produced by the sealed air may be the crucial factor for their different adhesions. Furthermore, the initial volume of the air sealed between water and the surface is important for the adhesion, that is, larger volume would result in the lower adhesion; thus, superhydrophobic surfaces with different adhesions can be obtained by simply tuning the volume of the sealed air. This work reported about the influence of the tip geometry on the adhesion of the superhydrophobic surface, and the results presented here is interesting and helpful for us to further understand the biological attachment systems and to design new nanomaterials with custom-built surface hydrophobicity and adhesion. Acknowledgment. The authors thank the National Research Fund for Fundamental Key Projects (2007CB936403) and the National Natural Science Foundation of China (20571077). Supporting Information Available: SEM images of the alumina membrane, cross-sectional SEM images of the nanostructured PS surfaces, behaviors of a water droplet on surface with flat tips and nanotube film, the advancing and receding contact angles of a water droplet on the flat PS surface. This material is available free of charge via the Internet at http://pubs.acs.org.
Langmuir 2010, 26(11), 8233–8238
