Effects of Rugged Nanoprotrusions on the Surface Hydrophobicity and

rugged nanoprotrusions on the surface of microstripes provided ideal 3D roughness, which ... of the rugged nanostructures in enhancing the apparent CA...
0 downloads 0 Views 472KB Size
Download PDF
4886

Langmuir 2007, 23, 4886-4891

Effects of Rugged Nanoprotrusions on the Surface Hydrophobicity and Water Adhesion of Anisotropic Micropatterns Xuefeng Gao,*,†,‡ Xi Yao,†,‡ and Lei Jiang† Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, PR China, and Graduate School of the Chinese Academy of Sciences, Beijing 1000864, PR China ReceiVed October 16, 2006. In Final Form: February 13, 2007 A facile laser-etching method was used for the one-step creation of various controllable dimensions of anisotropic micropatterns consisting of an alternating arrangement of microgrooves and microstripes with rugged nanoprotrusions, which after modified with fluoroalkylsilane reagent, showed perfect isotropic superhydrophobicity without apparent CA hystereses, water adhesion, and drag resistance, other than the conventional view of anisotropic surface microstructures with anisotropic surface dewetting. The detailed experiments and analyses have indicated that the introduction of the rugged nanoprotrusions on the surface of microstripes provided ideal 3D roughness, which could not only enhance the apparent contact angles close to 180° by the “point” contact fashion to maximally reduce the liquid-solid contact area but, most importantly, make droplets easily roll off the surface without apparent CA hysteresis by regulating the triple-phase contact line (TCL) to become extremely discrete. These findings would be helpful in understanding the role of complex micro- and nanostructures on natural superhydrophobic biosurfaces and guiding the design of perfect artificial superhydrophobic materials for technological innovations such as the raindrop easy-cleaning, aquatic superfloating, and drag-reducing coatings.

1. Introduction Water repellence is an important property of solid materials and some biosurfaces that is governed by surface microstructures and chemical compositions.1 For example, the self-cleaning of lotus leaves2,3 and the aquatic super-floating of water strider legs4 are ascribed to the striking superhydrophobicity of special surface micro- and nanostructures, with an equilibrium contact angle (CA) above 150° and a tilted angle (TA) below 5°. Besides, we have revealed that the anisotropic dewetting of rice leaves results from the quasi-one-dimensional arrangement of micropapillae with nanoprotrusions along the direction parallel to the leaf edge.3 Inspired from these, scientists have paid increasing attention to regulating surface hydrophobicity by elaborate microscopic patterns due to the significant values in basic researches and technological applications. So far, various superhydrophobic surfaces that only satisfy the former criterion have been obtained by creating uniscale microscopic structures on hydrophobic surfaces5-7 and modifying rough micropatterns with materials of low surface energy.8-13 However, these are * To whom correspondence should be addressed. E-mail: gaoxf@ iccas.ac.cn. † Institute of Chemistry, Chinese Academy of Sciences. ‡ Graduate School of the Chinese Academy of Sciences. (1) Sun, T.; Gao, X.; Jiang, L. Acc. Chem. Res. 2005, 38, 644-652. (2) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1-8. (3) 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. (4) Gao, X.; Jiang, L. Nature 2004, 432, 64. (5) Guo, C.; Feng, L.; Zhai, J.; Wang, G.; Song, Y.; Jiang, L.; Zhu, D. ChemPhysChem 2004, 5, 750-753. (6) Feng, L.; Li, S.; Li, H.; Zhai, J.; Song, Y.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2002, 41, 1221-1223. (7) Feng, L.; Yang, Z.; Zhai, J.; Song, Y.; Liu, B.; Ma, Y.; Yang, Z.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2003, 42, 4217-4220. (8) O ¨ ner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777-7782. (9) Bico, J.; Marzolin, C.; Que´re´, D. Europhys. Lett. 1999, 47, 220-226. (10) Yoshimititsu, Z.; Nakajima, A.; Watanabe, T.; Hashimoto, K. Langmuir 2002, 18, 5818-5822. (11) Chen, Y.; He, B.; Lee, J.; Patankar, N. A. J. Colloid Interface Sci. 2005, 281, 458-464. (12) Sommers, A. D.; Jacobi, A. M. J. Micromech. Microeng. 2006, 16, 15711578.

insufficient from the conventional viewpoint of raindrop selfcleaning application because water droplets are pinned on such surfaces, despite certain special application that the superhydrophobic polystyrene nanotubes with high adhesive force may be used as a “mechanic hand” for reversibly no-loss transport of microdroplets.14,15 Actually, Prof. Jiang’s group earlier reported that hierarchical micro- and nanostructures could induce not only high CA but low TA, whereas uniscale nanostructure only caused the former.3 Very recently, Prof. McCarthy’s group also found that the contact angle hysteresis could be eliminated by creating hydrophobic polymer network nanostructures on the tops of microscale posts.16 Thus, it is very important to explore the role of the rugged nanostructures in enhancing the apparent CA of micropatterns and regulating the profile, length, and continuity of the underlying three-phase contact line (TCL) to influence the motion of water despite several emerging superhydrophobic cases.16-24 Extensive studies on anisotropic surface wettability have indicated that the parallel array of grooves (or stripes; Figure 1a) at the micro- or nanoscale can induce distinctly directiondependent TCLs and CAs that make droplets move more easily (13) Morita, M.; Koga, T.; Otsuka, H.; Takahara, A. Langmuir 2005, 21, 911-918. (14) Jin, M.; Feng, X.; Feng, L.; Sun, T.; Zhai, J.; Li, T.; Jiang, L. AdV. Mater. 2005, 17, 1977-1981. (15) Hong, X.; Gao, X.; Jiang, L. J. Am. Chem. Soc. 2007, in press. (16) Gao, L.; McCarthy, T. J. Langmuir 2006, 22, 2966-2967. (17) Shibuichi, S.; Onda, T.; Satoh, N.; Tsujii, K. J. Phys. Chem. 1996, 100, 19512-19517. (18) Xie, Q.; Xu, J.; Feng, L.; Jiang, L.; Tang, W.; Luo, X.; Han, C. C. AdV. Mater. 2004, 16, 302-305. (19) Zhang, X.; Shi, F.; Yu, X.; Liu, H.; Fu, Y.; Wang, Z.; Jiang, L.; Li, X. J. Am. Chem. Soc. 2004, 126, 3064-3065. (20) Han, J. K.; Xu, X.; Cho, K. Langmuir 2005, 21, 6662-6665. (21) Jin, M.; Feng, X.; Xi, J.; Zhai, J.; Cho, K.; Feng, L.; Jiang, L. Macromol. Rapid Commun. 2005, 26, 1805-1809. (22) Ming, W.; Wu, D.; Benthem, R. V.; With, G. D. Nano Lett. 2005, 5, 2298-2301. (23) Zhang, L.; Zhou, Z.; Cheng, B.; DeSimone, J. M.; Samulski, E. T. Langmuir 2006, 22, 8576-8580. (24) Xiu, Y.; Zhu, L.; Hess, D. W.; Wong, C. P. Langmuir 2006, 22, 96769681.

10.1021/la0630357 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/22/2007

Anisotropic Micropatterns

Langmuir, Vol. 23, No. 9, 2007 4887

Figure 1. Schematic illustration of two kinds of anisotropic micropatterns. (a) The alternating array of microgrooves and smooth microstripes and (b) the alternating array of microgrooves and microstripes with rugged nanoprotrusion.

along the parallel direction than the perpendicular.9-13,25 For example, Yoshimitsu et al. found that the microgroove structure showed better water-shedding property in the parallel direction than the micropillar structure and the perpendicular due to its lower energy barrier to the movement of the TCL.10 Very recently, Chen et al. also reported that a droplet on a PDMS micropattern, with a stripe width of 23 µm, a groove width of 25.6 µm, and a groove depth of 30 µm, presents apparent CAs of 150.7° and 127.2° along the direction parallel and perpendicular to the grooves, respectively.11 Since the hierarchical micro-/nanostructures could induce superhydrophobicity,3,16-24 it would be more intuitive and easier to understand the role of nanostructures if the TCL was varied from an anisotropic quasicontinuous state into an isotropic discrete state. Thus, anisotropic micropatterns consisting of micrometer-scale grooves and stripes may be considered as a good model for introducing rugged nanostructures (Figure 1b) and exploring their effect on the apparent CAs and TCLs of micropatterns, which are still unexplored hitherto. Although the uniscale micro- or nanopatterns consisting of smooth stripes and grooves have been made by photolithography,10 soft lithography,11 Langmuir-Blodgett technique,25 and vapor deposition,26 how to prepare hierarchical micropatterns with rugged nanostructures in one-step remains a technical challenge nowadays. Here, we report a simple surface roughening technology that uses laser etching to prepare in one-step the customer-tailored anisotropic micropatterns, composed of the alternating microgrooves and microstripes covered with rugged nanoprotrusions. To explore the effect of the rough nanoprotrusions on the surface hydrophobic and adhesive properties of micropatterns, we chose a sort of low-energy fluoroalkylsilane (FAS) molecules used in Yoshimitsu et al.’s works10 to modify the surfaces. Detail measurements show that these FAS-modified micropatterns possess ideal direction-independent superhydrophobicity without apparent water adhesion and drag resistance for water droplets, different from the previous view of anisotropic micropatterns with anisotropic surface hydrophobicity and stronger water adhesion in the perpendicular direction than the parallel. In other words, the effect of a superhydrophobic surface can overwhelm the effect of an anisotropic surface. These remarkable phenomena and further theoretical analyses offer us insight into how to use the hierarchical nanostructures to regulate the surface hydrophobicity and water adhesion of micropatterns, which would be helpful in the design of prefect superhydrophobic materials for raindrop self-cleaning and aquatic super-floating coatings. (25) Gleiche, M.; Chi, L.; Gedig, E.; Fuchs, H. ChemPhysChem 2001, 3, 187-191. (26) Gau, H.; Herminghaus, S.; Lenz, P.; Lipowsky, R. Science 1999, 283, 46-49.

Figure 2. Schematic illustration of preparing superhydrophobic anisotropic multiscale micropatterns. (a) A pit forms when a quadrate laser pulse strikes the silicon surface, accompanying the molten SiO2 nanoparticles splashing around. (b) The alternating array of microgrooves and microstripes covered with rugged nanoprotrusions form when the applied laser beam scans along the surface in a line raster mode. (c) The surface was modified by FAS molecules.

2. Experimental Section The polished silicon wafers (General Research Institute for Nonferrous Metals, China) were cut into rectangular slices with the size of 1 cm × 2 cm. To remove organic contaminants, the samples were rinsed though the ultrasonication in acetone, ethanol and deionized water for 20 min, respectively. Then, the samples were dried with N2 airflow. The microstripe patterns with hierarchical nanoprotrusions on the Si surface were realized with a QuickLazeII Laser etching machine (New Wave Research, USA). The pulse laser used here was an Nd:YAG laser. The wavelength and repetition rate of the laser pulse were 532 nm and 20 Hz, respectively, and the energy was 5 J/cm2. The spot sizes of the laser were 2 × 25, 2.5 × 25, 3 × 25, and 10 × 25 µm2, respectively. The laser pulse may dredge micrometer-scale grooves on the silicon surface to form regular micropatterns when the substrates were set to move in a specific customer-tailored raster track. The moving speed of the controller was set at 4 mm/min. And the scanning intervals were 6, 9, 15, and 30 µm to form four representative stripe widths, respectively, as shown in Figure 3. The sample surfaces were chemically modified with heptadecafluorodecyltrimethoxysilane (FAS: CF3(CF2)7CH2CH2Si(OCH3)3, Shin-Etsu Chemical Co., Japan).27 First, these samples were submerged in a freshly prepared mixture of H2SO4 (98%) and H2O2 (30%) at a volume ratio of 7:3. The solution was heated to 80 °C for 1 h to remove organic contaminants, unbound SiO2 nanoparticles, and dust particles adsorbed on the surface of anisotropic micropatterns. Simultaneously, such treatment also endowed the surface with fresh hydroxyl groups. Then, the samples were rinsed thoroughly with deionized water and dried with N2 airflow. Then, these samples were immediately immersed in a 1.0 wt % ethanol solution of hydrolyzed FAS-17 for 3 h and heated at 140° for 1 h. The morphology of micropatterns was examined in a field-emission scanning electronic microscope (FE-SEM, JSM-6700F, JEOL, Japan) at 3 kV. The root-mean-square roughness of the nanoprotrusions (27) Tadanaga, K.; Katata, N.; Minami, T. J. Am. Ceram. Soc. 1997, 80, 10401042.

4888 Langmuir, Vol. 23, No. 9, 2007

Gao et al.

Figure 3. SEM images of anisotropic multiscale micropatterns created by the laser-etching method. The widths of microstripes and microgrooves are 20 and 10.5 µm (a), 9.2 and 2.5 µm (b), 6.6 and 2.0 µm (c), and 3.8 and 1.5 µm (d), respectively. The inset in panel a displays rugged nanoparticle conglomerations. These nanoprotrusions on the microstripes become increasingly rough with the sizes of grooves and stripes decreasing, especially scaling down to 2 and 8 µm, respectively. on the microstripes was measured using atomic force microscopy (AFM, Seiko Instruments Inc., Tokyo, Japan). The water-repellent and adhesive properties of a water droplet on these FAS-modified anisotropic micropatterns with the rough nanoprotrusions were characterized in an optical contact angle meter system (Dataphysics Inc, OCA20, Germany).

3. Results and Discussions Figure 2 shows a schematic illustration for elucidating the basic principle of laser-etching for one-step fabrication of anisotropic patterns of microgrooves and microstripes covered with nanoprotrusion on silicon substrates. It is known that, when a quadrate laser pulse strikes the silicon surface, a pit will form, accompanying the molten SiO2 nanoparticles splashing around (Figure 2a). By skillfully combining it with the line raster scanning mode of the sample holder, we may create various sizes of anisotropic multiscale micropatterns. In other words, the alternating array of microgrooves and microstripes covered with rugged nanoprotrusions may form as the laser beam is applied on silicon substrates and scans along the surface in a line raster mode (Figure 2b). Various anisotropic multiscale micropatterns of controllable sizes have been achieved by setting the laser pulse in continuous shots under certain pulse energies (spot sizes) and moving the substrate in a custom-tailored track and speed. Figure 3 shows four representative scanning electronic microscopic (SEM) images of anisotropic multiscale micropatterns. The average widths of microstripes and microgrooves are about 20 and 10.5 µm (Figure 3a), 9.2 and 2.5 µm (Figure 3b), 6.6 and 2.0 µm (Figure 3c), and 3.8 and 1.5 µm (Figure 3d), respectively. Under a high-resolution SEM, numerous rugged nanoparticle conglomerations, exhibiting the roughness at the pseudo-threedimensional (P-3D) levels, are evident on the surface of microstripes (the inset in Figure 3a). These data and lots of unreported here have clearly demonstrated that the facile laseretching method can effectively construct rough nanoprotrusions on the anisotropic microstripe patterns separated by nonplanar microgrooves on the silicon surface. It was found that the smaller the size of the laser spot and the higher of the pulse energy, the narrower and the deeper the as-prepared microgrooves, whereas

the width of the microstripes may be well-tailored by the laserscanning track, i.e., tuning the speed of the laser beam moving along lengthways, the direction perpendicular to the grooves. Besides, the microstripe surface would become increasingly rough when the dimensions of microgrooves and microstripes were gradually decreasing. As shown in Figure 3b-d, the splashing SiO2 particles easily got together to form larger spherical conglomerations at the edges as the dimensions of microgrooves and microstripes scale down to less than 2 and 8 µm, respectively. To investigate the effects of nanostructures combined with anisotropic micropatterns on the surface hydrophobicity and water adhesion, the surfaces of the as-prepared micropatterns with various sizes (Figure 3 and the Supporting Information) were chemically modified with FAS reagent, as shown in Figure 2c. It was reported that such FAS self-assembled films on the smooth surfaces own lower surface energy and make an intrinsic water CA of 115° on a flat surface (Figure S1).10,27 Surprisingly, our experimental measurements indicated that the introduction of rugged nanoprotrusions on the surface of microstripes almost induced identically perfect water-repellent properties independent of the directions although it is well-known that the anisotropic patterns of smooth microstripes and microgrooves usually present the distinct surface water-shedding and adhesive properties along the directions parallel and perpendicular to microgrooves.9-13 Due to their so perfect superhydrophobicity and extremely low water adhesion, the conventional measuring technique of sessile droplets became invalid in obtaining accurate quantitative data in this case. To display their remarkable properties, we took the micropattern sample of Figure 3a with the root-mean-square roughness of the nanoprotrusions on the surface of microstripes at a range of 473.2-601.6 nm as a representative. As shown in Figure 4 and Movie S1, a 3.5-µL droplet is suspended on a microsyringe, and then the sample (Figure 4a) is lifted to tightly contact the droplet (Figure 4b). Interestingly, we found that the suspending droplet was difficult to be pulled down to the surface of micropatterns in all cases; that is, the surface has almost no apparent water adhesion to the suspending droplet. Even though the droplet is tightly contacted with the surface and deformed

Anisotropic Micropatterns

Figure 4. Anisotropic multiscale micropatterns with striking water repellence. A 3.5 µL water droplet is suspended on a syringe (a), tightly contacted with the lifting surface (b), even deformed (c) due to the increasing up-pushing force, and finally departs from the lowering surface (d). The red arrows represent the substrate’s moving direction.

severely (Figure 4c), by elevating the sample to produce an increasingly up-pushing force, it may also depart from the gradually lowering surface easily without any of the droplet remaining (Figure 4d). Thus, the adhesive force between the sample and the droplet along the vertical orientation is extremely feeble and may be ignored. It was noted that the apparent CA of these micropatterns with rugged nanoprotrusions may be considered to be close to 180° due to the perfect sphericity of beads although the conventional sessile droplet method for the measurement of CAs is invalid here. More interestingly, these anisotropic micropatterns have no apparent water adhesion and drag resistance as a droplet slides along the direction perpendicular to the grooves due to the introduction of nanoprotrusions. As the sample was lifted to tightly contact a droplet (2.0 µL) suspended on a microsyringe and then moved reversibly from one end (Figure 5a) to another (Figure 5c) along the perpendicular direction, the droplet remains intact without any apparent deformation, preserving the perfect sphericity and behaving just like a solid ball sliding on a smooth glass surface, besides the slight turning as denoted by black arrows in the middle of each droplet (see Movie S2). Further, as the dose of water reaches 4.0 µL, the droplet would fall on the sample under the gravitation and instantaneously rolled off without specific directions (Figure 6). The detail may be obtained from Movie S3 in the Supporting Information. Thus, these anisotropic micropatterns with hierarchical rough nanoprotrusions have almost no CA hystereses with TA f 0°, showing remarkable isotropic superhydrophobicity, instead of the previous viewpoints of anisotropy microstructures with anisotropy surface hydrophobicity and water adhesion.9-13 It was found in the experiments that such strikingly isotropic superhydrophobicity was independent of the size of micropatterns whether the widths of microgrooves and microstripes varied from 4 to 38 µm or the depth of microgrooves from 5 to 17 µm. It is noted that the nanoprotrusions on the surface of microstripes would become increasingly rough with the dimensions gradually decreasing. Anyway, these have clearly demonstrated that the introduction of the rough nanostructure on the micropatterns can greatly enhance the surface hydrophobicity to the extreme, which offers us an insight into understanding why natural superhydrophobic biosurfaces such as lotus leaves2,3 and water strider

Langmuir, Vol. 23, No. 9, 2007 4889

Figure 5. Anisotropic multiscale micropatterns with extremely low drag resistance. After being lifted to tightly contact with a 2.0 µL of droplet suspending on a syringe, the surface is moved from one end (a) to the midst (b) to another end (c) without any deformation along the direction perpendicular to the grooves. The red arrow represents the substrate’s moving direction. (d) A schematic illustration for describing the local interaction of the sort water interface with the top of rugged nanoprotrusions on the microstripes by the typical “point” contact fashion.

Figure 6. Anisotropic multiscale microstructures without apparent contact angle hysteresis and water adhesion. A 4.0 µL of drop falls on the surface due to the gravitation and instantaneously rolls off without specific directions. In top left of each image shows the time sequence.

legs4 evolve the complex hierarchical micro- and nanostructures. However, lots of works have reported that the anisotropy of surface microstructures consisting of alternating microgrooves andmicrostripescaninducetheanisotropyofsurfacedewetting.3,9-13 For example, Prof. Patankar’s group reported that a droplet on a PDMS micropattern with the stripe widths of 23 µm and groove widths of 25.6 µm exhibited apparent CAs of 150.7° and 127.2° along the direction parallel and perpendicular to grooves, respectively.11 The conventional views suggest that the oriented arrangement of microstructures may influence the contour, length, and continuity of the triple-phase (liquid-air-solid) contact line (TCL) and control the way a droplet tends to move.3,9-13 As clearly shown in Figure 7, panels a and b, there exists a more continuous and shorter TCL along the parallel direction than that along the perpendicular direction due to the lower energy barrier for the movement of the TCL in this sliding direction. Thus, the droplet is impelled to take an elliptic shape with a higher static

4890 Langmuir, Vol. 23, No. 9, 2007

Gao et al.

Cassie’s principle for surface wettability,29 the anisotropic micropatterns consisting of parallel microgrooves and microstripes with rugged nanoprotrusions may be considered as a composite surface composed of air and solid, and their apparent CA θCB may be given by

cos θCB ) -1 + fs(cos θ + 1)

Figure 7. Schematic illustration of the triple-phase contact line (TCL) from the top view. Anisotropic micropatterns consisted of alternating microgrooves and smooth microstripes show the more continuous and shorter TCL along the parallel direction (a) than that along the perpendicular direction (b). The anisotropic multiscale micropatterns consisting of alternating microgrooves and microstripes with rugged nanoprotrusions show the isotropic extremely discrete TCL for the parallel (c) and the perpendicular (d) direction. Back arrow: sliding direction; blue: water; gray: stripe parts; white: groove parts; yellow: the top parts of nanoprotrusions; circle: contact part of water and solid surface at the base.

CA along the direction parallel to the microgrooves. Besides, the anisotropic micropattern shows a better water-shedding property with a lower TA along the parallel direction. Despite this, the surfaces continue to have very strong adhesion or CA hystereses (TA > 20°),9-13 especially along the perpendicular direction due to the higher energy barrier. In this case, we wonder why the emergence of rugged nanoprotrusions made the surface of anisotropic micropatterns present perfect superhydrophobic and nonadhesive properties independent of the directions parallel and perpendicular to the microgrooves, whereas the surface of rice leaves shows apparently anisotropic CA and adhesive properties due to the quasi-one-dimensional arrangement of micropapillae with rough nanoprotrusions.3 It is well-known that the Wenzel’s equation28 and the Cassie’s equation29 are two basic principles that are successively developed on the basis of the classical Young’s equation only applicable for ideal smooth surfaces and widely adopted for solving the textured surface wettability problem. Although both models believe that the surface roughness can enhance the surface hydrophobicity, their intrinsic mechanisms are distinct. This is because the former performs the amplification of apparent contact angles by the way of increasing the solid-liquid contact area, which causes water droplets to be firmly pinned on the surface with a higher adhesion, whereas the latter enhances apparent contact angles by availably trapping air among the voids of submicrometer-scale structures to dramatically reduce the solidliquid contact area, which makes water droplets easily roll off the surface with a extremely low adhesion.29 Clearly, the latter mode may be used here to thoroughly analyze the effects of rough nanoprotrusions in tuning the surface hydrophobicity and water adhesion of anisotropic micropatterns. According to the (28) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988-994. (29) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546-561. (30) Que´re´, D.; Lafuma, A.; Bico, J. Nanotechnology 2003, 14, 1109-1112.

(1)

where fs and 1 - fs are the area fractions of the nanoprotrusions contacted with water and the air trapped in the spacing among the nanoprotrusions and the microgrooves, respectively, and θ is the equilibrium CA of FAS on a flat surface. Replacing the value of θCB and θ into eq 1, we deduced fs f 0. As clearly shown in Figure 5d, the apparent CA can be dramatically enhanced closely to be a maximal value of 180° through making the liquidsolid contact infinitesimal when the rugged nanoprotrusions were introduced on the microstripes. Macroscopically, it seems that an air cushion may form at the solid/liquid interface as an effective water barrier on which the only suffered surface tension is enough to contract a droplet in a spherical shape, whereas the interaction between the solid (the top of nanoprotrusions) and the bead that might result in the collapse of water droplet;31,32 that is, the switch from the stable superhydrophobic state (Cassie’s state) toward the adhesive superhydrophobic state may be ignored here, even for the case of droplets impacting the samples under the gravitation.33 In this case, the contact fashion of soft droplet with the anisotropic micropatterns covered with rugged nanoprotrusions (Figures 1b, 3, and 5d) may be considered as a sort of typical “point” contact, whereas the interface of droplets contacting with smooth micropatterns (Figure 1a) such as those reported in the refs 10 and 11 should belong to typical “area” contacts as shown in Figure 7a, which makes the water film move more easily along the parallel direction than the perpendicular, thus inducing the asymmetry of droplets. Apparently, it was such “point” contacts and the consequent air cushion that greatly reduce the water droplet drag resistance. Despite all that, numerous nanoprotrusions still have a very weak impact on the drag resistance when the sample elevated to tightly contact the droplet suspended on the microsyringe was moved back and forth. When the soft water interface was sliding over the top of the nanoprotrusions (Figure 5d), a very weak drag resistance would arise, which drives the suspended bead to produce a very slight turn as shown in the black arrows of Figure 5a-c. Recently, Kim et al. reported that engineering the surface into superhydrophobic structures with water CA over 175° may dramatically reduce the droplet flow resistance over 99%.34 Indeed, the perfect superhydrophobic surfaces show no apparent water droplet drag resistance in contrast to the ordinary hydrophobic surfaces. Besides, such nanostructures on the anisotropic micropatterns can eliminate the CA hysteresis through making an extremely discrete TCL on the random and rugged asperities as shown in Figure 7c. It is known that the CA hysteresis frequently occurs on real solid surfaces due to the heterogeneity of surface microstructures or chemical compositions.35,36 The TA is a crucial parameter to characterize the rolling ability of a droplet on a hydrophobic surface, which is closely related to the CA hystereses (i.e., the difference between advancing and receding CA).4,8,16 The smaller TA, the smaller the CA hysteresis and the easier (31) Patankar, N. A. Langmuir 2004, 20, 7097-7102. (32) Marmur, A. Langmuir 2004, 20, 3517-3519. (33) Lafuma, A.; Que´re´, D. Nat. Mater. 2003, 2, 457-460. (34) Kim, J.; Kim, C. J. IEEE 2002, 479, 479-482. (35) McHale, G.; Shirtcliffe, N. L.; Newton, M. I. Langmuir 2004, 20, 1014610149. (36) Furmidge, C. G. L. J. Colloid Sci. 1962, 17, 309-324.

Anisotropic Micropatterns

Langmuir, Vol. 23, No. 9, 2007 4891

droplets roll off the surface. Their relationship may be described by36

mg sin R ) wγLV(cos θR - cos θA)

(2)

where R is the TA value, θR and θA are the advancing and receding CA, m and w are the mass and width of the droplet, and g is the gravitational acceleration. Apparently, as w f 0 or ∆ ) (θR θA)f 0, R f 0. As discussed above, due to the formed air cushion between the soft water interface and the solid surface (Figure 5d), the liquid-solid contact may be considered to be infinitesimal so that the only suffered surface tension is sufficient to contract the droplet into a perfect sphericity with a minute width w of droplets at the base, which is also related to the weight of the water droplet.10 In this case, the value is far less than the diameter of beads so as to be in an unstable state and easily roll off under a very slight disturbance. Besides, the typical “point” contact fashion makes the TCL become extremely discrete as shown in Figure 7c. Very recently, Gao and McCarthy found that introducing rough nanostructures on the micropost tops could eliminate the CA hysteresis and kinetic barrier by greatly enhancing the receding CA to make (θR - θA) f 0.16 Thus, the initial anisotropy of TCL and CA hysteresis induced due to the oriented array of microgrooves and microstripes as shown in Figure 7a would disappear as the rugged nanoprotrusions were introduced on the surface of microstripes. Compared with the anisotropic micropatterns of microgrooves and smooth microstripes with 2D roughness prepared by the widely used photolithography and soft lithography,9-13 the anisotropic micropatterns of alternating microgrooves and microstripes covered with the hierarchical rugged nanoprotrusions one-step created by our facile laser-etching method could provide the ideal 3D roughness as shown in Figure 5d, which made an extremely discrete TCL and drove the bead easily off the surface without any specific orientation by moving the front from a metastable point (the top of nanoprotrusions) to another on an air cushion. The above results could offer an inspiration for designing perfect artificial superhydrophobic surfaces and solving the engineering fluid drag resistance problem. First, to synthesize perfect superhydrophobic surfaces without apparent CA hystereses and water adhesion, the hierarchical micro- and nanostructures with ideal 3D roughness must be required. Although uniscale micrometer- or nanometer-size patterns may produce high static equilibrium CAs larger than 150°, they continue to have a strong CA hysteresis and adhesion, which hold back the technique applications in some fields such as easy-cleaning coatings and stain-resistant textiles. Actually, our group has verified this by constructing the uniscale aligned carbon nanotube (ACNT) films and the hierarchical micro-nanoscale lotus-like ACNT films and comparing their macroscopic properties that the TA of the latter is far lower (∼3°) than that of the former (>30°).3 Moreover, comparing with the previous opinion that more continuous TCL could result in better water-shedding properties with less adhesion,9-13 we believe that a new strategy may be proposed for widely solving the engineering drag-reducing problem although ideal smooth surfaces were long considered as an approach to reduce the fluidic drag resistance. It is known that the absolute ideal smooth surfaces at the atom and molecule scale usually require production conditions that are too strict,

and the endurance ability to resist the defect is poor in the actual application. However, the extremely rough surfaces with ideal 3D roughness provided by the hierarchical micro- and nanostructures such as natural lotus leaves and water strider legs may not only overcome these problems but greatly reduce the water or fluid drag resistance by fully introducing the third phase (air) at the solid-liquid interface to effectively form an extremely discrete TCL. Finally, it was worth noting that the common sense of so-called rough surfaces containing certain undulations, pores, and other asperities introduced in industrial production or daily use is not only invalid but may increase the droplet and fluid drag resistance compared with the smooth surfaces.

4. Conclusions In summary, a facile laser-etching method was used here for the one-step preparation of various controllable sizes of anisotropic micropatterns of alternating arrangement of microgrooves and microstripes covered with rugged nanoprotrusions, which opens a door for exploring the effects of the rugged nanostructures on the surface hydrophobicity, water adhesion, and drag-resistance of micropatterns. The detailed experiments and analyses have indicated that introducing the rugged nanoprotrusion on the microstripe surface of anisotropic micropatterns may provide ideal 3D roughness, which not only helps to maximally reduce the liquid-solid contact by forming numerous “point” contacts and thus greatly enhance the CA value but, most importantly, may regulate the TCL to become extremely discrete, which drives a droplet easily off the surface without apparent CA hysteresis and water adhesion. The effect of a superhydrophobic surface can overwhelm the effect of an anisotropic surface. The perfect superhydrophobicity of the as-prepared micropatterns independent of the widths of microstripes and microgrooves implies that the interspaces of microscale block units would become insensitive in certain ranges to obtain the perfect superhydrophobicity with extremely low adhesion as long as the nanostructures with enough roughness exist on the surface of micropatterns. However, it is worth noting that the combinational use of hierarchical microand nanostructures like these of rice leaves do not necessarily induce perfect superhydrophobicity and extremely low adhesion.3 Besides, high water-repellence and direction moving ability are two competing factors, which are of potential value for designing novel open microfluidic devices. One would not achieve directional water-shedding properties if the nanostructures are too rough although they could greatly enhance the surface hydrophobicity. These findings would offer us insight into understanding the evolution of complex hierarchical micro- and nanostructures on natural superhydrophobic biosurfaces and guiding the design of perfect artificial superhydrophobic materials: the easy-cleaning coatings for windows, traffic indicators and textiles; the aquatic super-floating and drag-reducing coatings for miniature aquatic robots and hulls. Acknowledgment. This research was supported by the Special Research Foundation of the National Natural Science Foundation of China and the Chinese Academy of Sciences. Supporting Information Available: Figure S1; text; Movies S1-3. This material is available free of charge via the Internet at http://pubs.acs.org. LA0630357