Preparation of Superhydrophobic Surfaces of Hierarchical Structure of

Jun 19, 2009 - The hierarchical structure silica surface of inlaying silica nanoparticles along a regular pillar-like pattern is fabricated by embossi...
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Preparation of Superhydrophobic Surfaces of Hierarchical Structure of Hybrid from Nanoparticles and Regular Pillar-Like Pattern† Kuan-Yu Yeh, Kuan-Hung Cho, and Li-Jen Chen* Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan Received April 30, 2009. Revised Manuscript Received June 1, 2009 The hierarchical structure silica surface of inlaying silica nanoparticles along a regular pillar-like pattern is fabricated by embossing silica sol-gel precursor mixed with silica nanoparticles on glass substrates with an elastomeric mold. The substrate is further modified by a self-assembled fluorosilanated monolayer to reduce its surface energy. The advancing/ receding contact angle measurements are performed to demonstrate that a water droplet on these surfaces can exhibit a transition from the Wenzel state to the Cassie state due to the addition of silica nanoparticles to enhance its surface roughness.

1. Introduction Superhydrophobicity of solid surfaces is an important property for a wide range of applications in medicine, coatings, selfcleaning, textiles, and microfluidics.1-16 The definition of a superhydrophobicity is a surface with the water contact angle larger than 150°. In addition, these surfaces also demonstrate a very low water roll-off angle. It is well-known that surface roughness and surface energy are the two dominant factors to fabricate the superhydrophobic surface. The lower surface energy is, the higher hydrophobicity would be. However, even the functional group -CF3 which has the lowest surface energy (6.7 mJ/m2) on the top of the surface can only provide a water contact angle of 120°.12 Therefore, in order to enhance the hydrophobicity, the surface roughness has to be increased substantially. † Part of the "Langmuir 25th Year: Wetting and superhydrophobicity" special issue. *Corresponding author. E-mail address: [email protected].

(1) Bico, J.; Marzolin, C.; Quere, D. Europhys. Lett. 1999, 47, 220. (2) Yoshimitsu, Z.; Nakajima, A.; Watanabe, T.; Hashimoto, K. Langmuir 2002, 18, 5818. (3) Ferrari, M.; Ravera, F.; Liggieri, L. Appl. Phys. Lett. 2006, 88, 203125. (4) Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watanabe, T. Adv. Mater. 1999, 11, 1365. (5) Erbil, H. Y.; Demirel, A. L.; Avci, Y.; Mert, O. Science 2003, 299, 1377. (6) Woodward, I.; Schofield, W. C. E.; Roucoules, V.; Badyal, J. P. S. Langmuir 2003, 19, 3432. (7) Callies, M.; Quere, D. Soft Matter 2005, 1, 55. (8) Oner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777. (9) Coffinier, Y.; Janel, S.; Addad, A.; Blossey, R.; Gengembre, L.; Payen, E.; Boukherroub, R. Langmuir 2007, 23, 1608. (10) Sun, M. H.; Luo, C. X.; Xu, L. P.; Ji, H.; Qi, O. Y.; Yu, D. P.; Chen, Y. Langmuir 2005, 21, 8978. (11) Fresnais, J.; Benyahia, L.; Poncin-Epaillard, F. Surf. Interface Anal. 2006, 38, 144. (12) (a) Yeh, K. Y.; Chen, L. J.; Chang, J. Y. Langmuir 2008, 24, 245. (b) Kuan, W. F.; Chen, L. J. Nanotechnology 2009, 20, 035605. (c) Chiou, D. R.; Chen, L. J.; Lee, C. D. Langmuir 2006, 22, 9403. (13) (a) Xiu, Y.; Zhu, L.; Hess, D. W.; Wong, C. P. Nano Lett. 2007, 7, 3388. (b) Li, Y.; Huang, X. J.; Heo, S. H.; Li, C. C.; Choi, Y. K.; Cai, W. P.; Cho, S. O. Langmuir 2007, 23, 216. (c) Gao, L.; McCarthy, T. J. Langmuir 2006, 22, 2966. (d) Ming, W.; Wu, D.; van Benthem, R.; deWith, G. Nano Lett. 2005, 5, 2298. (14) (a) Zhu, L.; Xiu, Y.; Xu, J.; Tamirisa, P. A.; Hess, D. W.; Wong, C. P. Langmuir 2005, 21, 11208. (b) Lee, Y.; Park, S. H.; Kim, K. B.; Lee, J. K. Adv. Mater. 2007, 19, 2330. (c) Cortese, B.; D'Amone, S.; Manca, M.; Viola, I.; Cingolani, R.; Gigli, G. Langmuir 2008, 24, 2712. (d) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Perry, C. C. Langmuir 2005, 21, 937. (15) (a) Nakajima, A.; Abe1, K.; Hashimoto, K.; Watanabe, T. Thin Solid Films 2000, 376, 140. (b) Liu, Y.; Tan, T.; Wang, B.; Zhai, R.; Song, X.; Li, E.; Wang, H.; Yan, H. J. Colloid Interface Sci. 320 (2008) 540. (16) Liao, K. S.; Wan, A.; Batteas, J. D.; Bergbreiter, D. E. Langmuir 2008, 24, 4253.

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Consider a water droplet sitting on a rough surface. In some case, the water droplet would fill up the rough surface to form a completely wetted contact with the surface, and this phenomenon is known as the Wenzel state.17 On the other hand, the water droplet may sit on the apex of the nanostructure and air cushions fill in the valleys of the structure under the droplet. Then, the water droplet is known as the Cassie state.18 The observed equilibrium contact angles always fall between the advancing and receding contact angles. However, it is well-known that there is no unique contact angle to characterize any given surface. The contact angle hysteresis Δθ, defined as the difference between advancing (θa) and receding (θr) contact angles (Δθ = θa - θr), can be used to conclude the state of a liquid droplet. The contact angle hysteresis increases along with an increase in surface roughness when the liquid droplet is at the Wenzel state. On the other hand, the contact angle hysteresis decreases along with a decrease in solid fraction when the liquid droplet is at the Cassie state. Thus, the contact angle hysteresis of the Wenzel state is always larger than that of Cassie state.12a For a superhydrophobic surface, the contact angle hysteresis must be small. Superhydrophobic surfaces have been fabricated in many different studies.1-16 There are some other methods to fabricate superhydrophobic surfaces by constructing hierarchical structures in literature.12b,13,14 In this study, the hierarchical structure of inlaying nanoparticles along the regular pillar-like pattern is proposed to be fabricated by simply embossing silica sol-gel precursor mixed with silica nanoparticles with an elastomeric patterned mold. Then, the substrate is modified its chemical nature of surface with a self-assembled (tridecanfluoro-1,1,2,2tetrahydrooctyl)-1-trichlorosilaned (FTS) monolayer to lower the surface energy. It is generally understood that the hierarchical structure can substantially enhance the surface roughness as well as contact angles. Thus, the superhydrophobic surfaces could be easily prepared by simply using the soft embossing method. Scanning electron microscopy and advancing and receding contact angle measurements are applied in this study to verify the superhydrophobic surface of the hierarchical structure of hybrid from nanoparticles and regular pillar-like pattern. (17) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988. (18) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546. (19) Rao, K. S.; Khalil, E.-H.; Kodaki, T.; Matsushige, K.; Makino, K. J. Colloid Interface Sci. 2005, 289, 125.

Published on Web 06/19/2009

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Figure 1. Schematic illustration of fabrication of the regular pillar-like structrure cursor only.

a-d Pn-s (h)

by simply embossing the sol-gel pre-

2. Experimental Details 2.1. Fabrication of Silica Regular Pillar-Like Structures. Surfaces with regular pillar-like structures of silicon wafers were prepared by photolithography.12 The surface topology was observed by using either an atomic force microscope (Nanoscope IIIa, Digital Instruments) or a surface profiler (Dektak). The notation Pa-d n (h) is applied to describe the substrate number n with microstructure a μm  a μm square pillars separated by a distance d μm and the pillar height h μm, as schematically illustrated in Figure 5a of ref 12a. Six different patterned sub6-3 6-3 6-6 6-6 strates: P6-3 1 (1.00), P2 (1.41), P3 (3.05), P4 (1.50), P5 (2.76), 6-6 and P6 (5.02) were used in this study. The silica regular pillar-like structures were fabricated by soft embossing method,21 which simply applied an elastomeric polydimethylsiloxane (PDMS) (SylgardTM184, Dow Corning Co.) mold as often used in soft lithography to an imprinting process. The PDMS mold was fabricated by casting prepolymer against a silicon wafer as a master of regular pillar-like pattern.13,21 The glasses were cut into 1 2 cm2 pieces for fabrication of patterned surfaces. Figure 1 shows a schematic illustration of fabricated silica regular pillar-like structures by soft embossing process silica sol-gel precursor. The sol-gel precursor was prepared according to the well-known procedure20,21 by mixing TEOS/ethanol/HCl (37%)/water at weight ratio of 12/21/1.4/ 3.45. A drop of silica sol-gel precursor was spread onto the clean glass substrate. Then, the patterned PDMS mold (developed from the substrate Pa-d n (h)) was embossed on this substrate and followed by baking at 50 °C for 1 h. After peeling off the PDMS mold, the patterned silica regular pillar-like structures are (20) Ting, C. Y.; Ouyan, D. F.; Wan, B. Z. J. Electrochem. Soc. 2003, 150, F164. (21) Chiou, D. R.; Yeh, K. Y.; Chen, L. J. Appl. Phys. Lett. 2006, 88, 133123.

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Figure 2. Schematic illustration of fabrication of the hierarchical structure Pa-d n-ps(h) by simply embossing the coating solution prepared by mixing the monodisperse nanoparticles with the sol-gel precursor. fabricated. The slow heat treatment of these patterned silica surfaces were then applied up to 180 °C. These silica substrates 0 -d 0 0 are then identified by Pan-s (h ), where the subscript n is used to trace back to its originality from the substrate number n. 2.2. Preparation of Silica Nanoparticles. The monodisperse uniform-sized silica nanoparticles were prepared by closely following a sol-gel process proposed by Rao et al.19 The reagents ammonia hydroxide (Acros, 28-30%), ethanol (Merck), and tetraethyl orthosilicate (TEOS) (Merck) were used as received for preparing silica nanoparticles. Water was purified by double distillation and then followed by a PURELAB Maxima Series (ELGA, LabWater) purification system with the resistivity always better than 18.2 MΩ-cm. First, ethanol and water were well mixed and then kept in a sonication bath (Branson, model 1510) for 10 min. A prescribed amount of TEOS was then added into the system for hydrolysis for 20 min, followed by adding ammonium hydroxide as a catalyst to promote the condensation reaction for another 60 min to get the white turbid (nanoparticles) solution. All the above-described experiments were performed in the sonication bath in the presence of ice, that is, the temperature was kept at 0 °C. The suspended nanoparticles were separated by centrifugation (15000 rpm, 10 min) and then washed three times by ethanol.

2.3. Fabrication of Hierarchical Structures of Silica Nanoparticles on Top of Pillar-Like Pattern. The hierarchical structures of silica nanoparticles on top of a pillar-like pattern were fabricated by soft embossing a coating solution (silica sol-gel precursor mixed with monodisperse silica nanoparticles) on the clean glass substrate with a PDMS mold, as schematically illustrated in Figure 2. Note that the fabrication procedure is exactly the same as the one described in Figure 1 except the coating solution. Here, the sol-gel precursor was first prepared according to the recipe Langmuir 2009, 25(24), 14187–14194

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Figure 3. Schematic illustration of fabrication of the hierarchical a-d structure Pn-ps-s (h) by simply embossing the sol-gel precursor only. described in section 2.1. The coating solution was then prepared by mixing the sol-gel precursor with monodisperse silica nanoparticles at weight ratio (the sol-gel precursor/silica nanoparticles) of 4. It should be noted that the sol-gel precursor was applied as a glue to stick all the silica nanoparticles together to fabricate the hierarchical structures of inlaying nanoparticles onto the pillarlike pattern, as illustrated in Figure 2. These substrates are then 00 -d0 0 00 identified by Pan-ps (h ), where the subscript ps stands for the nanoparticle size in nm and the subscript n is used to trace0 0 back to -d 0 0 00 its originality. That is, the PDMS mold, used to generate Pan-ps (h ), was fabricated by using the substrate number n (Pa-d n (h)) as the template. Once the substrate of the hierarchical structure was fabricated, 00 -d 0 0 00 this substrate Pan-ps (h ) was applied as a master to further develop a PDMS mold, as illustrated in Figure 3. This PDMS mold is different from the one used previously (as shown in Figures 1 and 2), and identified by the PDMSn-ps mold hereafter. This negative template of the hierarchical structure was further a*-d* applied to fabricate the hierarchical structure surface Pn-ps-s (h*) by soft embossing the PDMSn-ps mold onto the sol-gel precursor (without nanoparticles), as illustrated in Figure 3.

2.4. Surface Chemical Modification and Surface Characterization. Isooctane (Merck, >99%) was purified by percolating through a column of anhydrous aluminum oxide (Merck). This percolating process removes a certain amount of water and impurity in isooctane. Isooctane was then used as a solvent to prepare the (tridecanfluoro-1,1,2,2-tetrahydrooctyl)-1-trichlorosilane (FTS) (Gelest) solutions. The FTS solution in isooctane was freshly prepared right before the silanation reaction. The patterned (silica) substrates were cleaned by Piranha solution (a 7:3 mixture (v/v) of 98% H2SO4 (Merck) and 30% H2O2 (Merck)) at Langmuir 2009, 25(24), 14187–14194

Article 90 °C for 30 min before use. These substrates were then exposed to steam for around 30 s until water drops formed on the surface and then blown dry with dry nitrogen. The hydrated substrates were immersed in a 1 wt % FTS solution for 10 min at room temperature and then removed and rinsed by dichloromethane (Merck, 99.5%) and trichloromethane (Merck, 99-99.4%) to remove any unadsorbed FTS molecules. All the preparations of the adsorbate (FTS) solution and silanation reactions were conducted in a glovebag filled with dry nitrogen to exclude the amount of water traces in the surrounding atmosphere. A flat substrate with no pattern was prepared in the same way as the reference surface. All the substrates were modified by the selfassembled FTS monolayer to reduce its surface energy for further characterization. The experimental detail can also be found in our previous studies.12,22 All the samples were modified by the self-assembled FTS monolayers and then characterized by using contact angle measurements, atomic force microscopy (AFM, Nanoscope IIIa, Digital Instrument, Santa Barbara), and scanning electron microscopy (SEM, JEOL JSM-5600). The advancing/receding contact angles of water were measured by a homemade enhanced videomicroscopy system incorporating digital image analysis. The details of the methodology and its experimental setup can be found elsewhere.12,13,21,22 The rate of water pumping and suction through the needle to perform the advancing and receding, respectively, contact angle measurement was kept smaller than 0.3 mL/min. The volume of the water droplet was always smaller than 4.5 μL. The advancing/receding contact angles were measured at more than five different positions on each substrate. It should be pointed out that the advancing and receding contact angles of water on the flat silicon wafer modified by the selfassembled FTS monolayer are 120° and 81°, respectively. It should be noted that our enhanced videomicroscopy system remains having a problem to measure the contact angle larger than 160°, since the liquid droplet is rather flat near three-phase (gas-liquid-solid) contact point and the drop profile near the contact point is not sharp enough to be easily located.12,21 It is not an easy task of contact angle measurements in the limit of angles larger than 160°. Callies and Quere7 also pointed out that there is no commonly accepted method for measuring such large angles (>160°) with high precision.

3. Results and Discussion There are six different regular pillar-like structures of silicon 6-3 6-3 6-6 6-6 wafers: P6-3 1 (1.00), P2 (1.41), P3 (3.05), P4 (1.50), P5 (2.76), and P6-6 (5.02) used in this study. The advancing/receding contact 6 angles of water on these six substrates are listed in Table 1, consistent with the theoretical predictions of Wenzel state and of Cassie state, also given in Table 1 for comparison. In addition, the surface roughness is calculated on the basis of the dimension of pillars determined by AFM, also listed in Table 1. The water 6-6 droplets on the substrates P6-3 1 (1.00) and P4 (1.50) are at the Wenzel state, and the water droplets on the other four substrates are at the Cassie state due to the increase in pillar height, consistent with our previous findings.12 The variation of contact angles as a function of surface roughness based on the data in Table 1 are plotted in Figure 4 for clarity. It is obvious in Figure 4 that the system falls in the Cassie state for r > 1.4 and in the Wenzel state for r < 1.4. These substrates Pa-d n (h) were used as templates to fabricate the PDMS molds, which were further applied to soft embossing the silica sol-gel0 precursor to generate regular pillar-like silica -d 0 0 (h ). The dimensions of the patterned substrates substrates, Pan-s 0 0 -d 0 (h ) and the advancing/receding contact angles of water on Pan-s (22) Chen, L. J.; Tsai, Y. H.; Liu, C. S.; Chiou, D. R.; Yeh, M. C. Chem. Phys. Lett. 2001, 346, 241.

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Table 1. Advancing (θa)/Receding (θr) Contact Angles and Hysteresis (Δθ) of a Drop on Patterned Surfaces at the Wenzel State and Cassie State of a a- d a-d Substrates Pa-d n (h), Pn-s (h), and Pn-s2(h) Regular Pillar-Like Structures of Silicon Wafers Pa-d n (h)

θa

θr

Δθ

roughness

state

θw,a

θw,r

θc,a

θc,r

P6-3 1 (1.00) P6-3 2 (1.41) P6-3 3 (3.05) P6-6 4 (1.50) P6-6 5 (2.76) P6-6 6 (5.02)

136 153 155 135 154 153

83 123 125 79 128 134

53 30 30 56 26 19

1.30 1.42 1.90 1.25 1.46 1.84

Wenzel Cassie Cassie Wenzel Cassie Cassie

130 135 162 129 137 157

78 77 73 79 77 73

141 141 141 151 151 151

119 119 119 135 135 135

Soft Embossing the Sol-Gel Precursor by Using the PDMS Mold Fabricated from the Substrate Pa-d n (h) Pa-d n-s (h) P5.7-3.3 (0.61) 1-s (1.25) P5.0-4.0 2-s (1.97) P5.8-3.2 3-s 5.2-6.8 P4-s (1.05) 5.9-6.1 P5-s (2.15) (4.58) P4.9-7.1 6-s

θa

θr

Δθ

θc,a

θc,r

128 81 47 1.17 Wenzel 126 79 143 133 79 54 1.31 Wenzel 131 78 148 154 126 28 1.56 Cassie 141 76 142 125 80 45 1.15 Wenzel 125 80 155 134 77 57 1.35 Wenzel 132 78 152 151 130 21 1.62 Cassie 144 75 156 (h) Soft Embossing the Sol-gel Precursor by Using the PDMS Mold Fabricated from the Substrate Pa-d n-s

122 130 121 142 136 144

roughness

state

θw,a

θw,r

Pa-d n-s2 (h)

θa

θr

Δθ

roughness

state

θw,a

θw,r

θc,a

θc,r

P5.3-3.7 1-s2 (0.46) P4.6-4.4 2-s2 (0.96) P5.2-3.8 3-s2 (1.77) P4.6-7.3 4-s2 (0.98) P4.8-7.2 5-s2 (1.70) P4.0-8.0 6-s2 (2.96)

124 125 151 124 126 152

84 84 135 79 83 142

40 41 16 45 43 10

1.12 1.22 1.45 1.13 1.23 1.33

Wenzel Wenzel Cassie Wenzel Wenzel Cassie

124 128 137 124 128 132

80 79 77 80 79 78

146 150 146 158 157 161

127 134 128 146 145 151

a The symbols θw,a and θw,r stand for the advancing and receding, respectively, contact angles based on the Wenzel model. The symbols θc,a and θc,r stand for the advancing and receding, respectively, contact angles based on the Cassie model.

Figure 4. Variation of the advancing/receding contact angle of a-d a-d water droplet on pattern surfaces Pa-d n (h), Pn-s (h), and Pn-s2(h) as a function of the surface roughness. The filled symbols stand for the advancing contact angles and open symbols for the receding contact angles. The black symbols stand for the experimental a-d a-d results of substrates Pa-d n (h), Pn-s (h), and Pn-s2(h), green symbols for the theoretical predictions based on the Wenzel model, and red symbols for that of the Cassie model. For the substrate number n, n = 1 (circle), 2 (square), 3 (triangle), 4 (inverted triangle), 5 (diamond), and 6 (star).

these substrates are listed in Table 1. Note that the pillar shrinks in both size and height and the pillar period remains intact. It is believed that the shrinkage of pillars is simply due to the solvent evaporation of the sol-gel precursor. The water droplets on the (1.97) and P4.9-7.1 (4.58) are at the Cassie state substrates P5.8-3.2 3-s 6-s and the water droplets on the other four substrates are at the Wenzel state due to the decrease in the roughness (i.e., the shrinkage in pillar height and size). There is an obvious transition from the Cassie state to the Wenzel state for the substrate number 6-6 2 (and 5) switching from the substrate P6-3 2 (1.41) (and P5 (2.76)) 14190 DOI: 10.1021/la9015492

to the substrate P5.0-4.0 (1.25) (and P5.9-6.1 (2.15)), as the blue 2-s 5-s arrow shown in Figure0 4.0 -d 0 (h ) were further used as templates to These substrates Pan-s fabricate the another PDMS molds, which were again applied to soft embossing the silica sol-gel precursor to0 0 generate “secon-d 0 0 00 dary” regular pillar-like silica substrates, Pan-s2 (h ). Figure 5 demonstrates the SEM images of substrates P5.7-3.3 (0.61) and 1-s P5.3-3.7 1-s2 (0.46). It should be noted that that there is no cracking of the patterned structures after the slow heat treatment stage as shown in Figure 5. It is quite obvious that the dimensions of the pillar decrease as one can see in Figure 5. In other words, both the surface roughness and the solid fraction reduce in the process of 0 0soft0 0 embossing. The dimensions of the patterned substrates -d Pan-s2 (h00 ) and the advancing/receding contact angles of water on these substrates are also listed in Table 1. Note that the dimensions of pillars decrease and the pillar period remains constant in a0 -d 0 0 a0 0 -d 0 0 00 the series of substrates, Pa-d n (h) f Pn-s (h ) f Pn-s2 (h ). That is, a > a0 > a00 , h > h0 > h00 and a þ d = a0 þ d 0= a00 þ d 00 under the condition of a fixed n. It should be noted that the decrease in the dimensions of pillars would reduce the surface roughness, and consequently, the system has a tendency to move toward the Wenzel state, as shown in Table 1. It is well-known that the increase in the roughness would drive the system from the Wenzel state to the Cassie state under the condition of a fixed chemical nature. Therefore, we propose to prepare the coating solution, instead of using the silica sol-gel precursor only, by mixing the monodisperse nanoparticles into the silica sol-gel precursor. Then, the procedure illustrated in Figure 2 is closely followed to generate the hierarchical structure of inlaying nanoparticles onto the pillar-like pattern. The bumpy surface would certainly increase the surface roughness that could lead the system to approach the Cassie state. Langmuir 2009, 25(24), 14187–14194

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Article Table 2. Average Particle Sizes at Three Different Reaction Conditions experiment ethanol TEOS [H2O]/ NH4OH average particle SD no. (mol /L) (mol/L) [TEOS] (mol /L) size (nm) ((nm) sample-1 sample-2 sample-3

5.7-3.3 Figure 5. The SEM images of the substrates (a,b) P1-s (0.61)

and (c,d) P5.3-3.7 (0.46). (a,c) Low magnification (2000). (b,d) 1-s2 High magnification (10 000). Langmuir 2009, 25(24), 14187–14194

4 8 8

0.045 0.11 0.33

311 27 27

14 14 14

200 378 593

41 15 14

Figure 6. The SEM images of silica nanoparticles (a) 200 ( 41 nm, (b) 378 ( 15 nm, and (c) 593 ( 14 nm in diameter. DOI: 10.1021/la9015492

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Figure 8. Variation of the advancing/receding contact angle of a-d water droplet on pattern surfaces Pn-ps (h) prepared by using three different coating solutions as a function of the surface roughness. The filled symbols stand for the advancing contact angles and open symbols for the receding contact angles. The black symbols stand a-d for the experimental results of substrates Pn-ps (h), green symbols for the theoretical predictions based on the Wenzel model, and red symbols for that of the Cassie model. For the substrate number n, n = 1 (circle), 2 (square), 3 (triangle), 4 (inverted triangle), 5 (diamond), and 6 (star).

Figure 7. The SEM images of the substrates (a) P6.0-3.0 1-378 (0.90) and (b) P5.6-3.5 1-378-s(0.68).

The monodisperse nanoparticles of three different sizes were prepared according to the recipe given in Table 2. The size of the particles was averaged over 100 particles from the SEM pictures, as shown in Figure 6. The particle size and its standard deviation are also given in Table 2, consistent with the previous study.19 It is obvious that the silica nanoparticles in three samples are essentially monodisperse. Figure 7a shows the SEM image of the substrate P6.0-3.0 1-378 (0.90) fabricated by soft embossing the coating solution composed of the sol-gel precursor and the monodisperse nanoparticles of size 378 nm onto a glass substrate with the PDMS mold developed from the substrate P6-3 1 (1.00) as the template. Note that the dimensions of the pillars remain intact, except for the pillar height, which was reduced from 1 to 0.90 μm. The existence of the silica nanoparticles on top of the silica regular pillar-like structures could effectively enhance the surface roughness as one can see in Figure 7a. The advancing (or receding) contact angles of these substrates Pa-d 1-ps(h) as well as the theoretical Wenzel and Cassie contact angles are listed in Table 3, consistently larger (or smaller) than that of the substarte (0.61). Note that the theoretical Wenzel and Cassie conP6.0-3.0 1-s tact angles are determined on the basis of the dimensions of pillar structure without considering the roughness introduced by nanoparticles in Table 3. The surface roughness is also calculated on 14192 DOI: 10.1021/la9015492

the basis of the dimensions of pillars without considering the roughness introduced by nanoparticles, also listed in Table 3. There is almost no difference in the advancing/receding contact angles among the substrates Pa-d 1-ps(h) using nanoparticles of different sizes, as shown in Figure 8 and in Table 3. As one can see in Figure 8, the system falls in the Cassie state for for 1.3 < r and in the Wenzel state for 1.3 > r. Under the condition of the same PDMS mold prepared from the patterned silicon wafer (substrate Pa-d n (h)), it is interesting to examine the effect of the coating solution of adding the monodisperse nanoparticles. When no0 nanoparticles are added in the coating solutions, the substrate -d 0 0 (h ) is fabricated. While the sol-gel precursor is mixed with Pan-s the monodisperse nanoparticles to form the coating solution, the 0 -d 0 0 (h ) is0 fabricated. In general, the surface roughness substrate Pan-ps -d 0 0 (h ) is larger than that of the substrate of 0 the0 substrate Pan-ps -d 0 (h ) due to the bumpy surface. As a consequence, the Pan-s a0 -d 0 0 (h ) is larger than that of hydrophobicity of the substrate P n-ps 0 -d 0 0 (h ), as one can see the advancing/receding the substrate Pan-s contact angles given in Tables 1 and 3, especially the receding contact angle or hysteresis. For example, when no nanoparticles are added in the coating solutions, a water droplet on the substrate (1.25) (and P5.9-6.1 (2.15)) would be at the Wenzel state. On P5.0-4.0 2-s 5-s the other hand, a water droplet on the substrate Pa-d 2-ps(h) (and (h)) would be at the Cassie state, while the sol-gel precursor is Pa-d 5-ps mixed with the monodisperse nanoparticles to form the coating solution. There is no difference among the particle sizes applied to the system, as shown in Figure 8. The occurrence of a transition between the Wenzel and Cassie states could be attributed to not only surface roughness but also the size and topography of nanoparticles appearing on the pillar-like structure. The procedure illustrated in Figure 3 is closely followed to develop the PDMS1-378 mold from the substrate P6.0-3.0 1-378 (0.90) as the template, and then, the substrate P5.6-3.5 1-378-s(0.68) with hierarchical structure of bumpy surfaces along the pillar-like pattern is fabricated by simply embossing the PDMS1-378 mold on the silica sol-gel precursor (with no nanoparticles) directly. Figure 7b shows the SEM image of the substrate P5.6-3.5 1-378-s(0.68). The bumpy surface along the pillar-like pattern is duplicate from the substrate P6.0-3.0 1-378 (0.90), and the dimension of pillar structure is reduced as expected. The advancing/receding contact angles Langmuir 2009, 25(24), 14187–14194

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Table 3. Advancing/Receding Contact Angles and Hysteresis of a Drop on Patterned Surfaces at the Wenzel State and Cassie State of Substrates a Pa-d n-ps(h) Prepared by Using Three Different Coating Solutions Coating Solution = Sol-Gel Precursor þ Nanoparticles of 200 nm in Diameter Pa-d n-200(h) 5.7-3.3 P1-200 (0.74) P5.0-4.0 2-200 (1.32) P5.7-3.3 3-200 (1.98) P5.2-6.7 4-200 (1.24) P5.9-6.2 5-200 (2.21) P5.2-6.9 6-200 (5.15)

θa

θr

Δθ

roughness

state

θw,a

θw,r

θc,a

θc,r

134 144 154 132 151 154

79 116 129 80 138 130

55 28 25 52 13 24

1.21 1.33 1.56 1.18 1.36 1.73

Wenzel Cassie Cassie Wenzel Cassie Cassie

127 132 141 126 133 150

79 78 76 79 78 74

143 148 143 155 152 155

122 130 122 141 136 142

θw,r

θc,a

θc,r

79 78 76 79 77 74

141 146 143 155 151 154

119 128 122 142 135 141

Coating Solution = Sol-Gel Precursor þ Nanoparticles of 378 nm in Diameter Pa-d n-378(h) P6.0-3.0 1-378 (0.90) P5.2-3.8 2-378 (1.46) P5.7-3.3 3-378 (2.09) P5.1-6.9 4-378 (1.32) P6.0-5.8 5-378 (2.30) P5.3-6.7 6-378 (5.04) Pa-d n-593(h) P5.9-3.2 1-593 (0.82) P5.2-3.8 2-593 (1.51) P5.9-3.2 3-593 (2.18) P5.1-6.9 4-593 (1.21) P5.9-6.2 5-593 (2.19) P5.2-6.9 6-593 (5.18)

θa 132 146 157 132 150 155

θr

Δθ

roughness

state

θw,a

80 52 1.27 Wenzel 129 118 28 1.37 Cassie 133 130 27 1.59 Cassie 143 79 53 1.19 Wenzel 126 137 13 1.40 Cassie 134 130 25 1.74 Cassie 151 Coating Solution = Sol-Gel Precursor þ Nanoparticles of 593 nm in Diameter

θa

θr

Δθ

roughness

state

θw,a

θw,r

θc,a

θc,r

134 144 155 133 153 154

80 114 128 79 136 134

54 30 27 54 17 20

1.23 1.39 1.62 1.17 1.35 1.74

Wenzel Cassie Cassie Wenzel Cassie Cassie

128 134 144 126 133 150

79 77 75 79 78 74

142 146 142 155 152 155

121 128 121 142 136 142

a The subscript ps stands for the size of nanoparticles used to prepare the coating solutions. The coating solution was prepared by mixing monodisperse silica nanoparticles into the silica sol-gel precursor. In this study, three different particle sizes are used, that is, ps = 200, 378, and 593 nm.

Table 4. Advancing/Receding Contact Angles and Hysteresis of a Drop on Patterned Surfaces at the Wenzel State and Cassie State of Substrates a Pa-d n-ps-s(h) Fabricated by Using the Soft Embossing Method with Three Different PDMSps Molds: ps = 200, 378, and 593 nm Soft Embossing the Sol-Gel Precursor by Using PDMSn-200 Mold (h)

θa

θr

Δθ

roughness

state

θw,a

θw,r

θc,a

θc,r

P5.3-3.6 1-200-s(0.69) P4.5-4.5 2-200-s(0.97) P4.9-4.1 3-200-s(1.80) P5.0-7.0 4-200-s(1.15) P4.9-7.1 5-200-s(1.80) P4.2-7.8 6-200-s(3.27)

126 131 152 129 134 156

82 82 130 78 82 141

44 49 22 51 52 15

1.18 1.22 1.44 1.16 1.25 1.38

Wenzel Wenzel Cassie Wenzel Wenzel Cassie

126 127 136 125 128 134

79 79 77 80 79 78

145 151 148 156 156 160

126 135 131 143 144 149

θw,a

θw,r

θc,a

θc,r

79 46 1.18 Wenzel 127 83 47 1.26 Wenzel 129 133 19 1.45 Cassie 137 77 49 1.13 Wenzel 124 81 55 1.25 Wenzel 129 145 11 1.39 Cassie 134 Soft Embossing the Sol-Gel Precursor by Using PDMSn-593 Mold

79 79 77 80 79 77

144 152 150 158 157 158

124 137 134 146 144 147

Pa-d n-200-s

Soft Embossing the Sol-Gel Precursor by Using PDMSn-378 Mold Pa-d n-378-s(h)

θa

P5.6-3.5 1-378-s(0.68) P4.4-4.7 2-378-s(1.21) P4.7-4.4 3-378-s(1.99) P4.6-7.5 4-378-s(1.03) P4.9-7.2 5-378-s(1.88) P4.5-7.5 6-378-s(3.13)

125 130 152 126 136 156

Pa-d n-593-s(h)

θa

θr

Δθ

roughness

state

θw,a

θw,r

θc,a

θc,r

P5.3-3.7 1-593-s(0.68) P4.6-4.5 2-593-s(1.26) P4.9-4.2 3-593-s(1.83) P4.4-7.7 4-593-s(1.02) P4.9-7.2 5-593-s(1.82) P4.4-7.6 6-593-s(3.17)

125 131 154 127 134 156

81 82 134 78 79 145

44 49 20 49 55 11

1.18 1.28 1.43 1.12 1.24 1.39

Wenzel Wenzel Cassie Wenzel Wenzel Cassie

126 130 136 124 128 134

79 78 77 80 79 77

146 151 149 159 157 159

127 135 132 148 144 148

a

θr

Δθ

roughness

state

The PDMSn-ps mold is fabricated from the substrate Pa-d n-ps(h).

and its theoretical contact angles of Wenzel and Cassie states for the substrates Pa-d n-ps-s(h) are given in Table 4. Note that the theoretical Wenzel/Cassie contact angles and surface roughness are determined on the basis of dimensions of pillar structure Langmuir 2009, 25(24), 14187–14194

without considering the roughness enhanced by bumpy surfaces and reported in Table 4. As one can see in Figure 9, the system falls in the Cassie state for 1.35 < r and in the Wenzel state for 1.3 > r. DOI: 10.1021/la9015492

14193

Article

Figure 9. Variation of the advancing/receding contact angles of water droplet on pattern surfaces as a function of the surface a-d roughness. The patterned substrates Pn-ps-s (h) are fabricated by using the soft embossing method with three different PDMSps molds: ps = 200, 378, and 593 nm. The filled symbols stand for the advancing contact angles and open symbols for the receding contact angles. The black symbols stand for the experimental a-d results of substrates Pn-ps-s (h), green symbols for the theoretical predictions based on the Wenzel model, and red symbols for that of the Cassie model. For the substrate number n, n = 1 (circle), 2 (square), 3 (triangle), 4 (inverted triangle), 5 (diamond), and 6 (star).

It should be pointed out that the advancing and receding contact angles of water on the flat silicon wafer modified by the self-assembled FTS monolayer are 120° and 81°, respectively. Consider the system at a constant solid fraction of silicon wafer that is regular pillar-like: the advancing contact angle increases along with the surface roughness, and the receding contact angle decreases along with the surface roughness when the system falls into the Wenzel state. While the surface roughness is further raised beyond its transition roughness (from Wenzel state to the Cassie state), the receding contact angle discontinuously jumps

14194 DOI: 10.1021/la9015492

Yeh et al.

from a value smaller than 90° to a value higher than 90°. When a water drop is at the Cassie state, its contact angle hysteresis strongly depends on the solid fraction and has nothing to do with the surface roughness. The result is consistent with our previous study.12a Moreover, the dimensions of the pillar structure are reduced after the soft embossing process, and thus, the surface hydrophobicity of the substrate varies accordingly. For example, there exists a transition from the Cassie state to the Wenzel state when a-d (h)) is applied to prepare the substrate 0 Pa-d 2-ps(h) (or P5-ps a -d 0 a0 -d 0 0 the substrateP2-ps-s(h ) (or P5-ps-s(h0 )) via the soft embossing method. It should be noted that the hierarchical structure can be easily fabricated by simply embossing the sol-gel precursor with the PDMS3-ps (or PDMS6-ps) mold, as the procedure illustrated in Figure 3. In summary, two methods of fabrication of the hierarchical structure of inlaying the nanoparticles along the regular pillar-like pattern are proposed in this study. (1) The coating solution (composed of the sol-gel precursor and the monodisperse nanoparticles) is spread onto glass substrate and then embossed by the PDMS mold, as illustrated in Figure 2. (2) The PDMSn-ps mold, fabricated from the substrate Pa-d n-ps(h), embosses the solgel precursor spread on the glass substrate, as illustrated in Figure 3. After the thermal treatment, the hierarchical structure can be easily generated. Soft embossing is attractive because it is simple, inexpensive, flexible, and suitable for forming patterns over large areas in a single impression. The elastomeric PDMS mold and the surface chemistry for the formation of the selfassembled FTS monolayers can be manipulated to reduce the feature size. It can be used for many micro- and nanofabrication tasks of hierarchical structure and is a low-cost and robust process. Acknowledgment. This work was supported by the National Science Council of Taiwan.

Langmuir 2009, 25(24), 14187–14194