Nanocasting Method for

Dec 13, 2010 - Abstract Image. An electric field-aided process was introduced for a curable casting process. .... It can be credited for a wave of inv...
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Pressure/Electric-Field-Assisted Micro/Nanocasting Method for Replicating a Lotus Leaf Gyuhyun Jin and GeunHyung Kim* Bio/Micro-fluid Lab, Department of Mechanical Engineering, Chosun University, Gwang-ju, 501-759, Korea Received October 12, 2010. Revised Manuscript Received November 23, 2010 An electric field-aided process was introduced for a curable casting process. As a micro/nanosized pattern mask, a lotus leaf, which has a hierarchical structure, was used. The process consists of two steps: (1) applying an electric field to a liquid polymer and solidifying the polymer for use as a negative mold, and (2) using the negative polymer mold to fabricate a replicated poly(ethylene oxide) (PEO) surface in the original shape of the lotus leaf. In this process, the applied electric field induces unstable vibration of the liquid polymer, due to electrokinetic phenomena. The electrokinetic fluid motion resulted in well-replicated PEO surfaces. The quality of the fabricated surface was highly dependent on the applied field and pressure. We believe that this technique improves the quality of the standard nanocasting method and will be useful for fabricating micro/nanosized structures.

Introduction Nanoimprinting lithography (NIL) was first suggested by Chou et al. as a replication technology on a nanometer scale.1 NIL has many advantages, such as low cost and the possibility of mass production, and it is a very simple process compared with photolithography-based processing2,3 or chemically treated surface techniques.4,5 This is the main reason why NIL has attracted wide attention within a few years of its inception.6 NIL uses a hard mold, which is fabricated by high-precision processing methods, such as semiconductor manufacturing processes and microelectromechanical systems (MEMS) processing technologies, to replicate micro/nanosized patterns by transferring the surface structure of the master to a polymer that is heated under pressure.7 In addition, NIL could be supplemented with ultraviolet (UV) light, and UV-assisted-NIL appears to be promising for the production of patterns in the sub-50 nm range due to the advantages of room temperature and low pressure compared to thermal NIL.8,9 For fabricating micro/nanoscale patterned features, other specific replicating processes are embossing and nanocasting. In embossing process, a desired pressure is applied to a mold, allowing the pattern on the mold to be fully transferred onto the polymer substrate over a set amount of time; after a certain contact time between the mold and the substrate, the substrate can be cured by UV irradiation and the heated substrate can be cooled down below softening temperature, followed by separating the mold from the substrate.10-12 In addition, nanocasting is one *To whom correspondence should be addressed. E-mail: gkim@ chosun.ac.kr. (1) Chou, S. Y.; Krauss, P. R.; Renstrom, P. J. Appl. Phys. Lett. 1995, 67, 3114. (2) Shiu, J. Y.; Kuo, C. W.; Chen, P.; Mou, C. Y. Chem. Mater. 2004, 16, 561. (3) He, B.; Patankar, N. A.; Lee, J. H. Langmuir 2003, 19, 4999. (4) Erbil, H. Y.; Demirel, A. L.; Avc, Y.; Mert, O. Science 2003, 299, 1377. (5) Youngblood, J. P.; McCarthy, T. J. Macromolecules 1999, 32, 6800. (6) Guo, L. J. J. Phys. D: Appl. Phys. 2004, 37, 123. (7) Lee, S. M.; Kwon, T. H. Nanotechnology 2006, 17, 3189. (8) Colburn, M.; Johnson, S.; Stewart, M.; Damle, S.; Bailey, T.; Choi, B.; Wedlake, M.; Michaelson, T.; Sreenivasan, S. V.; Ekerdt, J.; Willson, C. G. Proc. SPIE Int. Soc. Opt. Eng. 1999, 3676, 379. (9) Gilles, S.; Meier, M.; Prompers, M.; Hart, A. V. D.; Kugeler, C.; Offenhausser, A.; Mayer, D. Microelectron. Eng. 2009, 86, 661. (10) Lee, L. J. Ann. Biomed. Eng. 2006, 34, 75. (11) Goh, C.; Coakley, K. M.; McGehee, M. D. Nano Lett. 2005, 5, 1545. (12) Aura, S.; Jokinen, V.; Sainiemi, L.; Baumann, M.; Franssila, S. J. Nanosci. Nanotechnol. 2009, 9, 6710.

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of soft lithography and it is widely used to fabricate nanoscale pattern by using soft and deformable materials and directly replicating templates.13-15 According to several researchers, to obtain complex topographic patterns of lotus leaf, the nanocasting process was applied, and the replicated surface showed superhydrophobic property like a natural lotus leaf.14,16 Recently, to accelerate the transfer of complex micro/nanosized patterns to a polymer, NIL technology has been improved. Mekaru et al.17 combined the NIL technique with an ultrasonic generator connected to the mold, which pushed the mold patterns into and out of a thermoplastic at high speed. Ultrasonic vibration at high frequency (over 5 kHz) and large amplitude promotes thermal deformation and improves the molding accuracy in ultrasonic nanoimprint technology. To achieve large-area imprinting, roll-to-plate and roll-to-roll mode imprinting methods have been applied to various substrates to produce 300-nm linewidth grating patterns.18 To imprint an area selectively, a stepand-repeat thermal NIL process has been proposed that uses an infrared lamp to heat the selected portion of the polymer layer.19 Additionally, nano/microsized patterns have been fabricated under various electrical field conditions. Using a DC electric field, Schaffer et al.20 generated an interfacial electrostatic pressure gradient and this eventually changed a thin polymer layer into a regular, self-assembled pillar array. This technology is based on the ability of dielectric materials to produce a force in an electric field gradient. Similar work using an electric field has been conducted by various researchers.21-25 The replicated surface can (13) Taguchi, A.; Smatt, J. H.; Linden, M. Adv. Mater. 2003, 15, 1209. (14) Sun, M.; Luo, C.; Xu, L.; Ji, H.; Ouyang, Q.; Yu, D.; Chen, Y. Langmuir 2005, 21, 8978. (15) Lu, A. H.; Sch€uth, F. Adv. Mater. 2006, 18, 1793. (16) Yuan, Z.; Chen, H.; Zhang, J. Appl. Surf. Sci. 2008, 254, 1593. (17) Mekaru, H.; Takahashi, M. J. Micromech. Microeng. 2009, 19, 125026. (18) Ahn, S. H.; Guo, L. J. ACS Nano 2009, 3, 2304. (19) Yoon, H. S.; Cho, H. S.; Suh, K. Y.; Char, K. Nanotechnology 2010, 21, 105302. (20) Schaffer, E.; Thurn-Albrecht, T.; Russell, T. P.; Steiner, U. Nature 2000, 403, 874. (21) Boltau, M.; Walheim, S.; Mlynek, J.; Krausch, G.; Steiner, U. Nature 1998, 391, 877. (22) Kim, G. H. Compos. Sci. Technol. 2005, 65, 1728. (23) Kim, G. H.; Shkel, Y. M. J. Micromech. Microeng. 2007, 17, 2522. (24) Lee, S. H.; Kim, P.; Jeong, H. E.; Suh, K. Y. J. Micromech. Microeng. 2006, 16, 2292. (25) Jiang, L.; Zhao, Y.; Zhai, J. Angew. Chem., Int. Ed. 2004, 43, 4338.

Published on Web 12/13/2010

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Figure 1. Schematic of the field-aided casting technique showing (a) the first casting process used to produce a negative PDMS mold and (b) the second casting process used to produce the positive PEO surface.

Figure 2. (a) Schematic of the unstable flow at the parallel plate electrodes under various electric field conditions. The temporal sequences show the movement of black ink in liquid PDMS under electric fields of (b) 0.5, (c) 1.5, and (d) 2.5 kV/mm at 1 Hz.

be used on satellite dishes, self-cleaning glass, fog-resisting surfaces, and in microfluidics.26 Sun et al. first built up a simple nanocasting method to fabricate an artificial superhydrophobic surface using a lotus leaf as a template, poly(dimethylsiloxane) (PDMS) solution for casting the lotus leaf, and antistick monolayer trimethylchlorosilane (TMCS) on the template.14 The replicated surface showed the similar superhydrophobic properties (high hydrophobic angle and low rolling-off angle) as a natural lotus leaf. However, the PDMS material has some hydrophobic nature, so that the pure quality of micro/nanopattern of the lotus leaf was not precisely analyzed. In this work, we fabricated the negative PDMS mold with an electric field under pressure, and we tried to obtain hydrophobic poly(ethylene oxide) (PEO) films replicating a lotus leaf, although the PEO is hydrophilic nature. In previous work, we simply introduced a replicated negative PDMS surface of taro plant leaf by using an imprinting technique supplemented with an electric field.27 In the study, we only showed the possibility of an imprinting process supplemented with an electric field. Here, we expand the electric field-aided technique to a curable casting process with various electric field strengths and pressures. During this process, the electric field is used to vibrate the liquid polymer, via electrokinetic phenomena, and this induces flow motion to the surface of the micro/nanosized (26) Roach, P.; Shirtcliffe, N. J.; Newton, M. I. Soft Matter 2008, 4, 224. (27) Kim, G. H.; Jeon, H. J.; Yoon, H. Macromol. Rapid Commun. 2009, 30, 991.

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pattern on the plant and mold. To observe the motion of the liquid polymer caused by the electric field, a series of photographs of an ink droplet was taken. In this process, the micro/ nanosized pattern of a plant structure, a lotus leaf, was used as a mask, and the final surface was PEO. The technique consists of two steps. First, an electric field is applied to a liquid PDMS under the lotus leaf, and the polymer is solidified to produce a negative mold. Then, the negative mold is used to fabricate a positive PEO surface in the original shape of the lotus leaf. The fabricated surface structures on the PEO were compared with a structure manufactured using the standard nanocasting technique (NCT) with pressure. The surface morphology was examined using scanning electron microscopy (SEM) and a roughness measurement machine, using an optical method. Finally, the water contact angles (WCAs) were measured for various negative PDMS and positive PEO surfaces.

Experimental Section Materials. The liquid polymer PDMS was obtained from Dow-Corning. The liquid PDMS had a dielectric constant, density, and viscosity of 2.65, 1.03 g/mL, and 3900 cps, respectively. During the process, two different pressures of 10 and 25 kPa were maintained, because too high a pressure can disrupt the micro/nanosized pattern of the plant leaf. Using the applied electric field, the liquid PDMS was cured for 6 h at 50 °C. The curing process involves hydroxyl-terminated polymers, alkyltriacetoxysilane cross-linkers, and a tin catalyst. These components vulcanize when exposed to ambient humidity. DOI: 10.1021/la1040954

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Figure 3. (a) Movement of a droplet from the midpoint between the parallel electrodes on the liquid PDMS versus time and (b) moving speed of the droplet at various electric field strengths and 1 Hz.

Figure 4. SEM images of (a) a lotus leaf; (b,c) magnified views of the area within the rectangle in panel a; and (d) further magnification of the image in (c). To fabricate a positive surface, like a lotus leaf, PEO (Mw = 900 000) from Aldrich-Sigma was used. To produce a stable structure on the PEO surface, 7 wt % PEO was prepared in distilled water. Using the applied electric field, the PEO solution was hardened for 24 h at 50 °C. The electrical conductivity of the PEO solution was 115 μs 3 cm-1. Characterization. To observe the quality of the replicated surface, the roughness, and the microsized shapes of the PDMS and PEO, we used SEM (Hitachi S-4800, Japan), atomic force microscopy (AFM; Nanowizard AFM; JPK), and a surface roughness tester (Nanoview-m4151p, Korea). The threedimensional (3D) profile of the average surface roughness values was measured using the roughness tester, using phase shifting interferometry, a common optical technique for noncontact surface profilometry. The data represent the mean of at least 30 independent points; the error bars indicate the standard deviation (SD). The electrical conductivity of the PEO solution was measured using a multiparameter analyzer (model C861; Montreal Biotech, Kirkland, Quebec, Canada). The WCAs of the replicated surface (positively replicated PEO film) were measured using a contact angle analyzer (SEO Phoenix 300, South Korea), and the contact angles at five independent points were averaged. A water droplet of 10 μL was placed on the surfaces, and the contact angle was 830 DOI: 10.1021/la1040954

measured under the atmospheric temperature of 23.2 °C and humidity of 37%. Since the PEO is hydrophilic in nature, the WCA of the PEO surface was measured below 5 s. Sliding angle is very essential to study the superhydrophobic surface. Therefore, the sliding angle measurement of a water droplet (10 μL) was carried out. The sliding angle was defined as the critical angle at which the droplet suddenly starts to slide down an inclined surface. Field-Aided Casting Technique. Figure 1a is a schematic diagram of the field-aided casting technique used to replicate the micro/nanostructure surface of the lotus leaf. The process has two steps. First, a negative PDMS mold of the lotus leaf is fabricated, and then a positive patterned PEO surface like the lotus leaf is made from the PDMS mold. To fabricate the patterned surface, an alternating current (AC) electric field (sinusoidal waveform and no DC bias field) was applied to the parallel electrodes under constant pressure (10 or 25 kPa). To increase the solidification rate, this was conducted in a chamber at 50 °C. During the thermal curing, an electric field of 0.06-2.5 kV/mm and 1-100 Hz was applied using parallel plate electrodes, fabricated using a copper layer deposited on a poly(methyl methacrylate) (PMMA) substrate. To prevent current from flowing between the electrodes because of the strong electrical field, the electrode was laminated Langmuir 2011, 27(2), 828–834

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Figure 5. SEM images of the replicated negative PDMS surfaces fabricated using the FA-NCT at electric field strengths and frequencies of (a) 0.5 kV/mm and 100 Hz, (b) 2.5 kV/mm and 10 Hz, (c) 2.5 kV/mm and 100 Hz, and 2.5 kV/mm and 100 Hz under pressure of (d) 10 kPa and (e) 25 kPa. The insets show magnified images. with a 50-μm-thin poly(ethylene terphthalate) film. The lotus leaf was attached to the electrode with adhesive tape. After the PDMS solidified, the leaf was removed to produce a negative PDMS mold. This negative PDMS mold was used to fabricate a positive PEO surface replicating the lotus leaf in the same manner as used to fabricate the PDMS mold (Figure 1b).

Results and Discussion To observe the field-induced interactions of liquid PDMS under an AC electric field with a 1 Hz frequency and electric field strengths of 0.5, 1.0, 1.5, and 2.5 kV/mm, black ink and silicone oil were dropped on the surface of the liquid polymer to show the flow movement of the PDMS. The droplets of the black ink and silicone oil in liquid PDMS exhibited similar movements under the electric field, although they possess different dielectric constants. A schematic of measurement of the flow movement is shown in Figure 2a. The viscosity of the liquid PDMS was 3900 cps. The movement of the droplet was observed using an optical microscope. As shown in Figure 2b-d, the droplet in liquid PDMS was observed under the three different electric field strengths. Series of images illustrate the motion of a droplet under the applied field. The first frame was taken before applying the electric field, and the subsequent images show the motion of the droplet caused by the electric field. As shown in the figure, the droplet moved with the surrounding liquid via an electromechanical interaction through the induced dipole moment of the polymer chains under an electric field, in which a polymer chain acts like charged beads (dipoles), connected by an elastic Hookean spring.28 The induced force of the chains describes physical phenomena resulting from the interaction between the induced (28) Nath, S.; Siddiqui, R. J. Phys. Chem. 1995, 103, 3212.

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polarization moment (μ) and the gradient of the electric field (rE), so that the field-induced force (F) can be expressed as F=(μ 3 r)E, where E, F, and μ are vectors.21 In Figure 2d, the ink droplet is slightly stretched in the direction of the electric field between the electrodes at 35 s due to the strong electric field, which deflects the ink droplet according to its surface charges. This deflection is affected by the surface tension and electrostatic force.29 According to Kim et al.,23 the flow movement was highly dependent on the applied electric field strength, frequency, and dielectric constant and viscosity of the fluid medium. Their work showed that the velocity of the moving fluid was linearly dependent on the applied field strength and dielectric constant of the fluid. Figure 3a shows the movement of the droplet versus time for various electric field strengths. In agreement with theory,23 the velocity of the flow movement under electric field was almost linearly related to the electric field strength (Figure 3b). These results confirm that increasing the electric field strength increases the flow movement of the liquid polymer. To examine the feasibility of the field-aided nanocasting technique (FA-NCT), we used a lotus leaf as a replicating mask. In Figure 4a-d, the lotus leaf was characterized using SEM. As shown in the figures, the protruding hierarchical surface structures ranged from 15 to 35 μm in height and consisted of micro- and nanosized protruding shapes. Figure 5a-e shows SEM images of negative PDMS surfaces replicated from a lotus leaf using various FA-NCT conditions. A replica (negative mold) of a lotus leaf was fabricated using various electric fields (0.5 and 2.5 kV/mm under 10 and 100 Hz) at an applied pressure of 10 and 25 kPa and curing the liquid PDMS for 6 h including 2 h with an applied electric field. The entire (29) Pohl, H. A. Dielectrophoresis; Cambridge University Press: New York, 1978.

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Figure 6. The 3D topography of surface roughness of (a) a lotus leaf and (b-d) the replicated PDMS using the FA-NCT at 2.5 kV/mm and 100 Hz under various pressures ((b) 0, (c) 10, and (d) 25 kPa). (e) Average roughness data for a lotus leaf, and negative PDMS replicates using the FA-NCT. (f) The depths of holes in the replicated surfaces and the height of the microsized knolls on the surface of the lotus leaf.

procedure was carried out in a heat chamber to accelerate the curing of the PDMS. As shown in Figure 5, the replicated surface contained various holes, because the plant leaves had protruding shapes and a hexagonal pattern. The replicated surfaces under the electric field without pressure were much smoother than the actual lotus leaf and surfaces fabricated the electric field with pressure. However, the PDMS surface fabricated using the high electric fields (2.5 kV/mm and 100 Hz) and pressures (10, 25 kPa) were rougher, as shown in Figure 5d,e. Because the SEM images provide qualitative results, we measured the surface morphology using a nanoroughness tester using an optical method. Figure 6a-d show the results using a nanoroughness tester. The figure shows the 3D topography of the surface roughness of a lotus leaf and surfaces fabricated using various FA-NCT at 2.5 kV/mm and 100 Hz under various pressures. As the figure shows, the scanned topographic images of the lotus leaf and the replicas include convex and concave shapes, indicating that their dimensional features are qualitatively similar to the SEM images. Using the data from the scanned picture, the roughness and depth of the holes on the surface can be quantified. The average roughness and depths of the holes on the surface are shown in Figure 6e,f, respectively. The surface produced using FA-NCT and pressure had a good average roughness and depth than the surface obtained using FA-NCT without pressure. From these results, we can find that the applied pressure on this FA-NCT was an important parameter in replicating roughness of the lotus leaf. Nevertheless, the values of the replica made using FA-NCT were 832 DOI: 10.1021/la1040954

still lower than those of the original lotus leaf. According to several researchers, the efficiency replicating micro/nanostructures can be manipulated by controlling the mechanical property of the stamping PDMS.12,30 In particular, Aura et al. fabricated UV-embossed micro/nanopillars, which consisted of microsized cylinders 5 μm high and 2 μm in diameter and cone-shaped nanopillars (500 nm wide and 1 μm high).12 In their work, a common PDMS was not rigid enough to emboss the nanopillar, while the mixture of PDMS/hard-PDMS provided clear replication of the nanopillars. From these references, we can estimate that the low replication of a lotus leaf can be improved by using the mixture of PDMS/hard-PDMS. To compare the normal NCT and FA-NCT (2.5 kV/mm and 100 Hz) under the same pressure (25 kPa), we fabricated a negative PDMS mold and replicated PEO surfaces. The normal NCT process was completely the same as the FA-NCT under a pressure of 25 kPa, except for the applied electric field. Figure 7a,b shows the replicated PDMS molds of normal NCT and FA-NCT under the same pressure, respectively. Figure 7c,d shows the AFM images of PDMS surfaces fabricated with normal NCT (Figure 7a) and FA-NCT (Figure 7b), respectively. From the images, we can find that the surface morphologies fabricated with and without the electric field are significantly different. To replicate the PDMS surface, a 7 wt % PEO solution in distilled water was used, and the solidifying time was 24 h under (30) Schmid, H.; Michel, B. Macromolecules 2000, 33, 3042.

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Figure 7. SEM images of PDMS and PEO surfaces: (a) a PDMS surface fabricated with a normal NCT with pressure, 25 kPa, (b) a PDMS surface with an electric field strengths (0.19 kV/mm) and a frequency (1 Hz) with 25 kPa. AFM images of (c) the PDMS surface of panel a, and (d) the PDMS surface of panel b. SEM images of (e) a PEO surface replicated with the negative PDMS of panel a, and (f-h) PEO surfaces replicated with the PDMS mold of panel b and FA-NCT with and without pressure (25 kPa). The insets show magnified images.

25 kPa, including 6 h with an applied electric field, at 50 °C; the other procedures were the same as those used to fabricate the PDMS mold. Figure 7e,f-h shows the SEM images of the replicated PEO surfaces fabricated with the normal NCT and FA-NCT method with the PDMS mold of Figure 7a,b, respectively. When replicating the negative PDMS surface of Figure 7a,b, the same pressure (25 kPa) was applied to both processes. However, for the PEO surface fabricated with FA-NCT, an electric field of 0.19 kV/mm at 1 Hz was applied. When using a frequency above 50 Hz and high electric field strength above 0.19 kV/mm, a spark occurred between the electrodes due to the high electrical conductivity (115 μs 3 cm-1) of the PEO solution; thus, we fixed the frequency at 1 Hz and the electric field strength at 0.19 kV/mm. As predicted, the FA-NCT with a pressure of 25 kPa enabled fine replication of the pattern compared to the normal NCT and FA-NCT without pressure. To evaluate the surfaces quantitatively, the roughness and height of the protruding shapes Langmuir 2011, 27(2), 828–834

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were measured. Figure 8a-d shows optical images of a lotus leaf and three replicated PEO surfaces fabricated using three different conditions: a normal NCT, FA-NCT, and FA-NCT assisted with pressure, respectively. As shown in the magnified images of Figure 8b,d, when using the electric field with pressure, the micro/nanosized roughness on the surface was more easily replicated than those of the normal NCT and FA-NCT without pressure. Figure 8e,f shows the average roughness and height of the protruding shape. As shown in the figures, although the normal NCT showed good replication, the roughness and height of the PEO surface replicated with an electric field with the pressure were more improved than those fabricated by the normal NCT. In addition, the replication efficiency of FA-NCT (without pressure) was slightly higher than that of the normal NCT (with pressure). From the result, we can derive one important advantage of FA-NCT. That is, if the negative mold is too soft and fragile, the pressure cannot be applied to the mold. In this case, the FA-NCT without pressure can be more effective to replicate the mold than the normal NCT. Since the WCA has been used as a good indicator of hydrophobicity, the WCA of the replicated PDMS and PEO surfaces were measured. The WCA of the lotus leaf was 148 ( 2°, showing superhydrophobic property, which was generated by air entrapment in the micro/nanopatterned surface.31 In general, the WCA of a lotus leaf is up to 160°. However, in our case, the lotus leaf was dried in a deep freezer (-78 °C) to research regardless of the season and also to stably store the hierarchical structure, so that the WCA was slightly decreased. Similar results were obtained in the work of Cheng et al.32 Figure 9a-i shows water drops indicating the WCA on the surface of lotus leaf, smooth PDMS, smooth PEO film, and replicated PDMS molds and PEO surfaces fabricated by normal NCT and FA-NCT and FA-NCT with a pressure of 25 kPa. The values of WCA for the smooth PEO and the replicated PEO surfaces fabricated using normal NCT, FA-NCT, and FA-NCT with pressure were 50 ( 4°, 110 ( 5°, 112 ( 5°, and 128 ( 3°, respectively. The comparisons of WCAs of PDMS and PEO for each process are described in Figure 9j. These results correspond well with the SEM images and roughness and height of replicated surfaces. Threshold sliding angle is an important parameter for determining superhydrophobic surface. We measured the threshold sliding angles for the dried lotus leaf and the replicated PDMS surfaces of Figure 7a,b. The sliding angles of the final PEO surfaces could not be measured because they are soluble during the measuring time. To measure the sliding angle, a 10 μL water droplet was used. The sliding angle of the lotus leaf was 9 ( 1°; for the PDMS surfaces of Figure 7a,b, we could not measure the sliding angles because the water droplet did not slide at 90°. In general, to generate a low sliding angle similar to the lotus leaf, a nanostructure should be needed.32 In our cases, the PDMS surfaces did not provide low sliding angles due to the insufficiently replicated nanostructures of the lotus leaf. Solving this problem will be our next research goal. On the basis of these results, although we cannot exactly produce the complicated hierarchical micro/nanostructure of the lotus leaf, we can stably fabricate a hydrophobic surface with microprotruding structure. The results demonstrate that the electric field-aided technique is a good supplementary method to improve the quality of the standard NCT, which can be useful in replicating micro/nanopatterns. (31) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546. (32) Cheng, Y. T.; Rodak, D. E.; Wong, C. A.; Hayden, C. A. Nanotechnology 2006, 17, 1359.

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Figure 8. The 3D topography of the surface roughness of (a) a lotus leaf, (b) a PEO surface replicated by normal NCT, and PEO replicates using FA-NCT (c) without and (d) with pressure (25 kPa). (e) The average roughness and (f) height of the protruded parts on the replicated PEO surfaces and the surface of the lotus leaf.

Figure 9. WCAs of the PDMS and PEO for each process. WCAs for (a) lotus leaf, (b) smooth PDMS, (c) smooth PEO, (d) PDMS fabricated by NCT, (e) PDMS fabricated by FA-NCT, (f) PDMS fabricated by FA-NCT with pressure (25 kPa), (g) PEO fabricated by NCT, (h) PEO fabricated by FA-NCT, and (i) PEO fabricated by FA-NCT with pressure (25 kPa). (j) Comparisons of the WCAs of PDMS and PEO for each process.

Conclusions This study used a casting method with an applied electric field to replicate a superhydrophobic lotus leaf. The FA-NCT consists of two steps: (1) the application of an electric field to a liquid polymer, which is used to make a negative mold of the lotus leaf, and the solidification of the polymer under the same applied electric field and pressure, and (2) fabrication of a positive surface in the original shape of the lotus leaf from the negative PDMS mold. The fine surface of the lotus leaf was appropriately reproduced in 834 DOI: 10.1021/la1040954

the negative PDMS mold and replicated PEO surface using FANCT with an appropriate pressure compared to a normal NCT. This was the result of electrokinetic forces that induced flow movement at the surface of the micro/nanosized pattern on the plant. Through the flow motion, we achieved high-quality replication of the micro/nanostructure surfaces using appropriate electric field strength and frequency. We believe that the FA-NCT process is a good technique for improving the quality of NCT, which can be useful to fabricate micro/nanosized structures. Langmuir 2011, 27(2), 828–834