UVO-Tunable Superhydrophobic to Superhydrophilic Wetting

2608

Langmuir 2007, 23, 2608-2614

UVO-Tunable Superhydrophobic to Superhydrophilic Wetting Transition on Biomimetic Nanostructured Surfaces Joong Tark Han, Sangcheol Kim, and Alamgir Karim* Polymers DiVision, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 ReceiVed October 3, 2006. In Final Form: NoVember 22, 2006

A novel strategy for a tunable sigmoidal wetting transition from superhydrophobicity to superhydrophilicity on a continuous nanostructured hybrid film via gradient UV-ozone (UVO) exposure is presented. Along a single wetting gradient surface (40 mm), we could visualize the superhydrophobic (θH2O > 165° and low contact angle hysteresis) transition (165 ° > θH2O > 10 °) and superhydrophilic (θH2O < 10° within 1 s) regions simply through the optical images of water droplets on the surface. The film is prepared through layer-by-layer assembly of negatively charged silica nanoparticles (11 nm) and positively charged poly(allylamine hydrochloride) with an initial deposition in a fractal manner. The extraordinary wetting transition on chemically modified nanoparticle layered surfaces with submicrometer- to micrometer-scale pores represents a competition between the chemical wettability and hierarchical roughness of surfaces as often occurs in nature (e.g., lotus leaves, insect wings, etc).

Introduction The wettability of naturally occurring solid surfaces is often governed by both the chemical composition and the geometric structure of the surface. The wettability can be decreased by creating a local geometry with a large geometric area relative to the projected area, and this effect can be observed in nature on lotus leaves.1,2 In addition, the chemical heterogeneity and topography can be modified (e.g., in the case of a desert beetle, this modification is helpful in capturing drinking water from fog-laden wind).3 These inspire us to consider novel biomimetic approaches for manipulating properties that combine both topological and chemical modifications to obtain potentially unique surface and material properties, such as with wettability, adhesion, and optical characteristics. The principles of hydrophobicity on rough surfaces were outlined decades ago by Wenzel4 and by Cassie and Baxter.5 Moreover, Que´re´ and co-workers6 have predicted that a liquid will invade the solid texture below the critical contact angle θc given as

cos θc )

1 - φs r - φs

where φs is the solid fraction remaining dry during a wicking process and r (g1) is the surface roughness factor (ratio of the actual surface area to the projected surface area). For a porous surface (r f ∞), this equation predicts that the surface texture will be fully invaded by any liquid having θc ) 90°. For rough surfaces (r > 1, φs < 1), the critical angle can be defined as intermediate between 0 and 90°. In the case of porous materials, switching between superhydrophobicity and superhydrophilicity * To whom correspondence should be addressed. E-mail: alamgir.karim@ nist.gov. Fax: +1-301-975-3928. Tel: +1-301-975-6588. (1) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1-8. (2) Furstner, R.; Barthlott, W.; Neinhuis, C.; Walzel, P. Langmuir 2005, 21, 956-961. (3) Parker, A. R.; Lawrence, C. R. Nature 2001, 414, 33-34. (4) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988-994. (5) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546. (6) Bico, J.; Thiele, U.; Quere, D. Colloids Surf., A 2002, 206, 41-46.

can occur by slight changes in surface chemistry.7 Accordingly, many authors have been exploring the fabrication and understanding of superhydrophobic and superhydrophilic surfaces.8-22 Here we demonstrate the delicate interplay that controls a transition from one to the other using a gradient chemical modification approach of the top surface. In this regard, if we can fabricate a 3D-like porous surface by a simple process and make a surface energy gradient on that surface, then we can presume that the surface wettability can be changed from superhydrophobicity to superhydrophilicity below the critical contact angle on a single substrate. Such an extreme gradient wetting surface can be useful in understanding the wettability trends on rough or porous surfaces, with ramification for many important biological and physical processes. Recently, Yu et al. have also reported a gradient surface from superhydrophobicity to superhydrophilicity based on the controlled selfassembly of a thiol monolayer on a rough gold surface.23 However, (7) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Perry, C. C.; Roach, P. Chem. Commun. 2005, 3135-3137. (8) Onda, T.; Shibuichi, S.; Satoh, N.; Tsujii, K. Langmuir 1996, 12, 21252127. (9) Miwa, M.; Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watanabe, T. Langmuir 2000, 16, 5754-5760. (10) O ¨ ner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777-7782. (11) Li, H.; Wang, X.; Song, Y.; Liu, Y.; Li, Q.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2001, 40, 1743-1746. (12) Lafuma, A.; Que´re´, D. Nat. Mater. 2003, 2, 457-460. (13) Feng, X.; Feng, L.; Jin, M.; Zhai, J.; Jiang, L.; Zhu, D. J. Am. Chem. Soc. 2004, 126, 62-63. (14) 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, 17011705. (15) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Chabrol, G.; Perry, C. C. AdV. Mater. 2004, 16, 1929-1932. (16) Erbil, H. Y.; Demirel, A. L.; Avci, Y.; Mert, O. Science 2003, 299, 13771380. (17) Patankar, N. A. Langmuir 2003, 19, 1249-1253. (18) Marmur, A. Langmuir 2004, 20, 3517-3519. (19) Han, J. T.; Lee, D. H.; Ryu, C. Y.; Cho, K. J. Am. Chem. Soc. 2004, 126, 4796-4797. (20) Krupenkin, T. N.; Taylor, J. A.; Schneider, T. M.; Yang, S. Langmuir 2004, 20, 3824-3827. (21) Han, J. T.; Xu, X.; Cho, K. Langmuir 2005, 21, 6662-6665. (22) Martines, E.; Seunarine, K.; Morgan, H.; Gadegaard, N.; Wilkinson, C. D. W.; Riehle, M. O. Nano Lett. 2005, 5, 2097-2103. (23) Yu, X.; Wang, Z.; Jiang, Y.; Zhang, X. Langmuir 2006, 22, 4483-4486.

10.1021/la0629072 CCC: $37.00 © 2007 American Chemical Society Published on Web 02/02/2007

From Superhydrophobicity to Superhydrophilicity

their method may be limited in application because of the specific deposition condition (electrodeposition) and rough control of the wettability (fast linear transition from the Cassi-Baxter state to the Wenzel state) on microstructured fractal surfaces. For this approach, facile control of the surface roughness and agradientsurfaceenergyviacombinatorialfabricationapproaches,24-29 which result in continuous specimens that exhibit many cases on a single substrate, present a considerable advantage because they enable the rapid and thorough analysis of the effect of surface chemistry. Additionally, the layer-by-layer (LBL) assembly of polyelectrolytes or nanoparticles is a potentially powerful candidate for control of the surface roughness or porosity.30-39 Recently, Han et al.34 have reported that the surface roughness and porosity can be controlled by the deposition of polyelectrolytes and nanoparticles by the LBL assembly for obtaining superhydrophobicity. Cebeci et al.35 have also fabricated multifunctional antifogging (superhydrophilic) nanoporous films from LBLassembled silica nanoparticles and a polycation. The nanostructure surface by the LBL assembly process promises to be a model testbed for the study of gradient wetting behavior on nanostructured surfaces and for the study of both fundamental measurements and practical applications involving coatings. The present work has two principle objectives: to improve the fundamental understanding of competition between opposing factors controlling wettability on nanostructured solid surfaces and to use such findings to inspire biomimetic processing strategies required for well-controlled wetting surfaces from superhydrophobicity to superhydrophilicity. As a testbed for wettability, nanostructured organic-inorganic films having controlled surface roughness and porosity were fabricated by the LBL process and hydrophobized with a monochlorosilane self-assembled monolayer (SAM) via vapor deposition. We investigated the gradient wetting behavior on these nanostructured solid surfaces through a simple gradient UV-ozone (UVO) treatment process of hydrophobized nanostructured films. Experimental Section Materials and Chemicals. Poly(allylamine hydrochloride) (average MW ) 70 000 g/mol, GPC vs PEG std.), poly(acrylic acid) (25% aqueous solution, average MW ) 250 000 g/mol), and fumed (24) Chaudhury, M. K.; Whitesides, G. M. Science 1992, 256, 1539-1541. (25) Bhat, R. R.; Genzer, J.; Chaney, B. N.; Sugg, H. W.; Liebmann-Vinson, A. Nanotechnology 2003, 14, 1145-1152. (26) Genzer, J.; Fischer, D. A.; Efimenko, K. Appl. Phys. Lett. 2003, 82, 266-268. (27) Sehgal, A.; Ferreiro, V.; Douglas, J. F.; Amis, E. J.; Karim, A. Langmuir 2002, 18, 7041-7048. (28) Roberson, S. V.; Fahey, A. J.; Sehgal, A.; Karim, A. Appl. Surf. Sci. 2002, 200, 150-164. (29) Julthongpiput, D.; Fasolka, M. J.; Zhang, W.; Nguyen, T.; Amis, E. J. Nano Lett. 2005, 5, 1535-1540. (30) Rouse, J. H.; Ferguson, G. S. J. Am. Chem. Soc. 2003, 125, 1552915536. (31) Soeno, T.; Inokuchi, K.; Shiratori, S. Appl. Surf. Sci. 2004, 237, 539543. (32) Jisr, R. M.; Rmaile, H. H.; Schlenoff, J. B. Angew. Chem., Int. Ed. 2005, 44, 782-785. (33) Zhai, L.; Cebeci, F. C¸ .; Cohen, R. E.; Rubner, M. F. Nano Lett. 2004, 4, 1349-1353. (34) Han, J. T.; Zheng, Y.; Cho, J. H.; Xu, X. R.; Cho, K. J. Phys. Chem. B 2005, 109, 20773-20778. (35) Cebeci, F. C¸ .; Wu, Z.; Zhai, L.; Cohen, R. E.; Rubner, M. F. Langmuir 2006, 22, 2856-2862. (36) Takeshita, N.; Paradis, L. A.; O ¨ ner, D.; McCarthy, T. J.; Chen, W. Langmuir 2004, 20, 8131-8136. (37) Zhang, X.; Shi, F.; Yu, X.; Lin, H.; Fu, Y.; Wang, Z.; Jiang, L.; Li, X. J. Am. Chem. Soc. 2004, 126, 3064-3065. (38) Zhao, N.; Shi, F.; Wang, Z.; Zhang, X. Langmuir 2005, 21, 4713-4716. (39) Shi, F.; Wang, Z.; Zhang, X. AdV. Mater. 2005, 17, 1005-1009.

Langmuir, Vol. 23, No. 5, 2007 2609 silica nanoparticles (11 nm diameter) were obtained from SigmaAldrich. SAM molecules, n-octyldimethylchlorosilane (ODS), and (tridecafluoro-1,1,2,2-tetrahydrooctyl)diemthyl chlorosilane (FDS) were purchased from Gelest, Inc. All materials were used as received without further purification. A silica nanoparticle colloid solution was prepared by dispersion in deionized water and sonication for 2 h. Deionized water was obtained by using a Millipore RiOs/ Milli-Q system and had a resistivity greater than 18 MΩ cm. Glass slides were cleaned with piranha solution to remove any organic residues from the slide surface and to prepare the hydrophilic surface. Fabrication and Characterization of Gradient Wetting Surfaces. Rough and nanoporous films were easily prepared by the electrostatic assembly method at pH 4.5 for SiO2 nanoparticle deposition and at pH 7.0 for PAH deposition at room temperature. Prior to the first deposition of SiO2, three bilayers of PAH and PAA ((PAH/PAA)3) were deposited onto the glass slide or Si wafer (cleaned with piranha solution (7:3 H2SO4/H2O2)) in order to prepare the dense first SiO2 layer. The (PAH/PAA)3-coated glass slide was immersed in 40 mmol/L PAH solution for 5 min and then rinsed three times with ultrapure water for 1 min each time. The substrate was then dipped into the SiO2 colloid solution for another 5 min and rinsed three times with ultrapure water. Then, nanostructured substrates were allowed to hydrophobize in a vacuum desiccator along with a shallow Teflon boat containing ∼0.5 mL of ODS or FDS. A low vacuum, applied to the desiccator, helps saturate the chamber with ODS or FDS vapor, which reacts with hydroxylfunctionalized SiO2 nanoparticles on top of the nanostructured film. After 12 h of vapor exposure, the specimen is removed from the desiccator and then rinsed thoroughly with n-hexane and ethanol and dried with a stream of dry nitrogen. Gradient specimens are prepared by conditioning the nanostructured film with a graded UVO exposure. Our process uses a custombuilt device that employs a motorized stage to translate the specimen beneath a UV source (192 nm) projected through a 2-mm-wide slit aperture operating at 3000 V and 30 A. Computer-driven acceleration of the stage produces an exposure gradient. Atomic force microscopy (Dimension 3100 microscope with a Nanoscope IV control unit, Digital Instruments, Inc.) and scanning electron microscopy (Hitachi 4700) were used to characterize the surface roughness and porosity of prepared surfaces. Scanning electron micrographs were obtained with uncoated samples, mounted on aluminum stubs. The use of a lower beam current and a short working distance allowed image contrast. Water contact angles were measured along a single specimen on a Kru¨ss G2 contact angle measuring system at ambient temperature. The 2D fractal dimension, Df, was determined by measuring the perimeter L and area A of the fractal clusters of the AFM image as follows:40 L ∝ ADf/2 The 2D fractal dimension of the nanostructured film surface can be calculated from a plot of log L versus log A.

Results and Discussion To fabricate a wettability testbed of films having a controlled roughness and nanoporosity on a negatively charged glass slide or Si wafer, we used the LBL technique with a commercial cationic polyelectrolyte, poly(allylamine hydrochloride) (PAH), and negatively charged silica nanoparticles (SN) with a particle diameter of 11 nm (according to the manufacturer). Prior to the first deposition of SN, three bilayers of PAH and poly(acrylic acid) (PAA) were deposited onto the negatively charged glass slide as an adhesive under layers, with PAH at the glass surface. Additionally, an SN colloid solution with a slightly high concentration (0.05% by mass) and a low pH (4.5) was intentionally (40) Voss, R. F.; Laibowitz, R. B.; Allessandrini, E. I. Phys. ReV. Lett. 1982, 49, 1441-1444.

2610 Langmuir, Vol. 23, No. 5, 2007

Han et al.

Figure 1. SEM images of nanostructured films having n PAH/SN bilayers: (a-f) 2, 3, 4, 5, 8, and 12 bilayers, respectively (scale bar ) 2.5 µm). Inset images are level-setting images (scale bar ) 1 µm) in which the black region indicates the flat region and pores.

used to have more facile control over the roughness and porosity for surface wettability control. To understand the surface roughness change and nanopore generation in (PAH/SN)n bilayer films (n ) number of bilayers), the surface morphologies of these films were analyzed by atomic force microscopy (AFM) and scanning electron microscopy (SEM). The surface roughness was enhanced in a step-by-step manner, and nanopores formed with increasing deposition of PAH/SN bilayers as shown in Figures 1 and 2. By increasing the number of PAH/SN bilayers from 3 to 12, the peak-peak height, Sy (height difference between the highest and lowest pixels) increased from (118 ( 7) to (300 ( 25) nm, whereas the uncovered flat region ((PAH/PAA)3) decreased from the cross-section profile (Figure 2). After eight deposition cycles, the glass surface appears to be almost covered with the PAH/SN bilayer as shown in Figure 1. Interestingly, the uncovered flat region below four bilayers shows a continuous flat PAA/PAH background, which adversely affects the enhancement of hydrophobicity. We also analyzed the 2D fractal dimensions of the deposition pattern of nanoparticles on the surface on the basis of the relationship between the perimeter L and area A of the digitized 2D AFM images as shown in Figure 3.40 From the curve fitting in Figure 3c, the values of the fractal dimensions Df of three- and fivebilayers films are obtained to be 1.28 and 1.52, respectively. This means that the nanoparticles aggregate has the property of fractal island-like growth on the positively charged surface.

Therefore, we can draw the scheme of roughness enhancement and nanopore formation as shown in Figure 4a,b; the fractal island-like SN domains become interconnected, and a combination of micropores and nanopores finally forms with an increasing number of PAH/SN bilayers. In regard to the assembly condition, we used different pH conditions from those in ref 35 because we control the roughness and porosity only for the superhydrophobicity and superhydrophilicity and do not care about the optical properties of multilayer films. Moreover, when the pH of polyelectrolytes and nanoparticle solutions are well adjusted, we can get well-packed nanoparticle layers as reported previously.34 Therefore, we used mismatched pH conditions to control the roughness and porosity easily. In detail, as can be seen in the morphology image (Figure 2), the roughness scale is different from the result in ref 35. Comparing 12 bilayer films, we find that the peak-peak height in our system is about 300 nm but only 90 nm in ref 35. For the optical coating, our system is not suitable because multilayer films become progressively cloudy as mentioned in ref 35. For wettability, a more rough and multilevel-structured surface is efficient in controlling the wettability from superhydrophobicity to superhydrophilicity. Immediately, after bilayer deposition processes were completed, hydrophobization of the nanostructure surfaces was performed by the deposition of SAM molecules, n-octyldimethylchlorosilane (ODS), and (tridecafluoro-1,1,2,2-tetrahy-

From Superhydrophobicity to Superhydrophilicity

Figure 2. Three-dimensional AFM tapping mode height images of (PAH/SN)n films and AFM cross-sections of the line shown in the image: (a) (PAH/SN)3, (b) (PAH/SN)5, and (c) (PAH/SN)12.

Figure 3. Fractal analysis of (PAH/SN)n films: (a, b) Level-setting images of the tapping mode AFM image of three and five bilayers, respectively (scale bar ) 200 nm). The white regions represent the stacked silica nanoparticles. (c) Plot of perimeter L versus area A of (PAH/SN)n films on a log-log scale.

drooctyl)dimethyl chlorosilane (FDS) through the versatile vapor deposition method.29 By using the vapor-phase method, we not only modify the silica nanoparticle surface well but also do it in a way that protects the polyelectrolyte/nanoparticle multilayer film from potential damage from solvents used in SAM treatment and the washing process such as with toluene. Furthermore, vapordeposited chlorosilane SAMs on nanostructured surfaces are more robust than thiol SAMs on microstructured gold surfaces.23,36

Langmuir, Vol. 23, No. 5, 2007 2611

The water contact angles (hereafter CA) on hydrophobized surfaces were enhanced in a sigmoidal manner with increasing number of PAH/SN bilayers as shown in Figure 4c. In the case of three bilayers, the CA increased by approximately 10°, which may be affected by the continuous flat region having a CA of ca. 70° (ODS-treated (PAH/PAA)3 film) as shown in Figure 2a. Interestingly, the CA on four bilayers treated with ODS dramatically increased and reached a superhydrophobic state (CA > 155° and low contact angle hysteresis) after five bilayers had been deposited. This CA transition can be explained by the surface roughness enhancement, nanopore generation, and decrease of the continuous flat region with increasing number of bilayers. The enhanced roughness and air trapped in nanopores can reduce the water contact area, which cause the increase in CA as explained by the Cassie-Baxter model.5 Additionally, the CA on the FDS-treated surface was enhanced to 140° and even up to 170° with two and five bilayers, respectively, which means that the surface roughness effect can be amplified by a surface modification with a much lower surface energy molecule. Next, UVO exposure, applied along the specimen, gradually modifies the chemistry of the SAM on the flat and nanostructured surfaces along one direction. As reported previously, during this UVO processing, methyl-terminated alkyl chain monolayers (hydrophobic) are converted to hydrophilic chains containing carboxylate, ether, and carbonyl species.28 As UVO exposure time is varied along the length of the specimen, the density of hydrophobic molecules decreases, whereas the density of hydrophilic chains increases. These tunable chemical modifications, both in terms of choice of SAM and its UVO modification, provide very simple approaches to modifying the superhydrophobicity response of nanostructured surfaces. To verify the gradient change of the hydrophobicity and hydrophilicity along the specimen, the CA measurement was conducted at different positions along the length of the specimen. Figure 5 shows the CA measurements of ODS-treated (PAH/ SN)n films collected along a single substrate subjected to a linear UVO exposure gradient ranging from 0 to 5 s. CA measurements along the gradient flat ODS surface varied gradually from 97 to 41°, corresponding to a surface energy from 26 to 58 mJ/m2, in contrast to the UVO-exposed nanostructured surfaces with over four bilayer films treated with ODS. The CA along the gradient surface decreases with a sigmoidal shape after UVO exposure processing as shown in Figure 5a (in which solid lines indicate the fit from the Origin program). It is noticeable that the CAs of nanostructured surfaces having over eight PAH/SN bilayers decrease dramatically from the superhydrophobic state (CA > 165° and low contact angle hysteresis) to the superhydrophilic (CA < 10° within 0.5 s or less) with UV exposure processing time as schematically illustrated in the inset of Figure 5a. Additionally, as the number of PAH/SN bilayers deposited increases from 4 to 12, the wettability of films more dramatically changed from superhydrophobicity to superhydrophilicity as clearly shown from the exponential decay of the fwhm in the UV processing time derivatives of CA fits (Figure 5b). This trend tracks the decrease in the black region in the level-setting 2D projection images (insets in Figure 1) as shown in Figure 5b. (The actual structure is quasi-3D, so deviations can be expected.) Moreover, the irradiation time of a sharp wettability transition gradually increased with more PAH/SN bilayers (Supporting information, Figure S1). We can therefore speculate that the sharpness of the gradient wetting transition will not become significantly narrower upon increasing the number of PAH/SN bilayers as extrapolated with the dotted line of the fwhm. In the case of FDS-treated specimens, five bilayers were enough to

2612 Langmuir, Vol. 23, No. 5, 2007

Han et al.

Figure 4. Schematic illustration of the roughness enhancement and nanopore generation during LBL assembly with poly(allylamine hydrochloride) and silica nanoparticles: (a) top view and (b) side view. (c) Water contact angles on the hydrophobized (PAH/SN)n films as a function of the number of PAH/SN bilayers.

create an extreme gradient wetting surface because of the high surface energy of FDS as shown in Figure 6. The high CA of the (PAH/SN)12 film even after 2 s of UV irradiation can be explained by the intensified roughness and existence of nanopores after silica nanoparticle deposition based on the Cassie-Baxter model. Moreover, the sharp sigmoidal wetting transition from superhydrophobicity to superhydrophilicity while the CA of flat gradient surface changed from 95 to 48° with UVO exposure illustrates the prediction of Que´re´ and co-workers;6 the rough or porous structure will be fully invaded by any liquid having a contact angle (as measured on a flat surface) of less than 90°. Additionally, to visualize the contact angle hysteresis of the gradient surface, we captured the drag movement of a water droplet on the gradient surfaces.41 As shown in Figure 6, without UVO irradiation the water droplet on the FDS-treated (PAH/ SN)12 film can be moved freely without any adhesion to the surface, indicating very low contact angle hysteresis; the difference between advancing and receding CAs (∆θ) is below 5°. However, it is remarkable that with UVO irradiation the water droplet sticks to nanostructured surfaces even having a CA over 155°, which translates to an increase in the contact angle hysteresis (∆θ ) ca. 80°). This transition from a low adhesion to a high adhesion state on an ODS-treated surface occurred faster with UVO exposure than with an FDS coating because of the lower surface energy of the FDS SAM molecules. Recently, it has also been reported that a sticky water drop having a high contact angle can form on textured surfaces. Jin et al.42 have reported that a polystyrene nanotube layer has a strong adhesive force for water droplets as a result of a high van der Waals force even though its CA is over 160°. Moreover, Cheng (41) Sun, M.; Luo, C.; Xu, L.; Ji, H.; Ouyang, Q.; Yu, D.; Chen, Y. Langmuir 2005, 21, 8978-8981. (42) Jin, M.; Feng, X.; Feng, L.; Sun, T.; Li, T.; Jiang, L. AdV. Mater. 2005, 17, 1977-1981. Nature 2004, 432, 36.

et al.43 have also reported that the behavior of water droplet on lotus leaves that have experienced water vapor condensation on their surfaces can be sticky to drops with a high contact angle that do not roll off the surface or drops with a CA of less than 90° (e.g., the carnauda wax on lotus leaves is intrinsically relatively hydrophilic, CA ≈ 74.0 ( 8.5 °). In our case, the presence of nanopores and a slight increase in the number of hydrophilic moieties after UVO exposure on the surface trigger the sticky behavior of a superhydrophobic surface. The optical images in Figure 7 illustrate the gradient wetting from superhydrophobicity to superhydrophilicity after spraying water on the FDS-treated gradient surface tilted to 30°. A, B, and C regions indicate the superhydrophobic, transition, and superhydrophilic regions, respectively. Water droplets easily roll off in region A (superhydrophobicity) and fully invade and rapidly wet the nanostructured surface in region C (superhydrophilicity). An enlarged image of the transition region (B) shows the deviation from homogeneous hemispherical droplets near the hydrophobic end to interconnected heterogeneous droplets with odd shapes having a contact angle of below 90° as a direct influence of the substrate wettability change. The observation of homogeneous spherical droplets on the tilted surface demonstrates the hydrophobic sticky behavior of water droplets having high CAs (Figure 6). Moreover, the ellipsoidal shape of interconnected water droplets in the transition region indicates the directional (x-axis) chemical heterogeneity of the surface.

Conclusions We present a simple surface chemistry and topology modification strategy to enable well-controlled gradient surfaces ranging from superhydrophobicity to superhydrophilicity with a controlled hysteresis transition zone along a single specimen (43) Cheng, Y.-T.; Rodak, D. E. Appl. Phys. Lett. 2005, 86, 144101.

From Superhydrophobicity to Superhydrophilicity

Langmuir, Vol. 23, No. 5, 2007 2613

Figure 6. Water contact angles along gradient surfaces treated with ODS and FDS plotted as a function of position. Optical images of slowly moving water droplets on gradient wetting surfaces; top images are from the FDS-treated (PAH/SN)12 film, and bottom images are from the ODS-treated (PAH/SN)12 film. The red arrow indicates the moving direction of a water droplet. Solid lines in advancing contact angles indicate the fit from the Origin program.

Figure 5. (a) Water contact angles along an ODS-treated specimen after UVO exposure. Values indicate the number of PAH/SN bilayers. Inset schemes show the water wetting transition on the nanostructured surface; the slight change in the surface hydrophilicity triggers the extreme wetting transition from superhydrophobicity to superhydrophilicity. Solid lines indicate the fit from the Origin program. CAs along an FDS-treated flat surface after UVO exposure are also shown for comparison. (b) Full width at half-maximum of derivatives of CA fits and the fraction of the black region in level-setting images of SEM images in Figure 1 as a function of the number of PAH/SN bilayers.

when the hydrophobized nanostructure surface was irradiated by UVO in a gradient manner. It was possible not only to fabricate nanostructured surfaces through layer-by-layer assembly with controlled roughness and nanoporosity but also to modify these surfaces sensitively with different monochlorosilane monolayer molecules via vapor deposition. This unusual wetting transition is dependent on the surface roughness and nanoporosity generated after LBL assembly of silica nanoparticles and polycations. This method provides a reproducible, well-controlled wetting surface in a range from over 170° to below 10° (with the liquid spreading rapidly) and a recipe that can also be used to produce large homogeneous or patterned reference calibrated surfaces with any desired contact angle in the extreme wettability range. Acknowledgment. This work used gradient UVO and contact angle instrumentation in the NIST Combinatorial Methods Center (NCMC; www.nist.gov/combi). We acknowledge helpful discussions with Dr. Jason Benkoski at NIST and Professor Kilwon Cho at POSTECH, South Korea. Certain commercial equipment,

Figure 7. Image of a glass slide coated with a nanostructured film (40 mm) consisting of 12 FDS-treated PAH/SN bilayers, illustrating the gradient wettability from (A) superhydrophobicity to (C) superhydrophilicity after UVO exposure across the 40 mm length, denoted by triangles in Figure 3. This image was taken after spraying water on a specimen tilted 30°. The lower image shows the magnified image of the (A) hydrophobic to (B) transition wetting region. The pink dotted line indicates the border of the superhydrophobic region (CA > 165°, ∆θ < 5°). The yellow dotted region shows the hydrophobic sticky region (165° > CA > 10°).

instruments, or materials are identified in this article in order to specify the experimental procedure adequately. Such identification is not intended to imply recommendation or endorsement by the

2614 Langmuir, Vol. 23, No. 5, 2007

National Institute of Standards and Technology, nor is it intended to imply that the materials or equipment identified are necessarily the best available for the purpose. Official contribution of the National Institute of Standards and Technology; not subject to copyright in the United States.

Han et al.

Supporting Information Available: Derivatives of CA fits and the irradiation time of a transition from hydrophobic to hydrophilic from (PAH/SN)4 to (PAH/SN)12 treated with ODS, illustrating the sharpness of the transitions. This material is available free of charge via the Internet at http://pubs.acs.org. LA0629072