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Anisotropic Wettability on Imprinted Hierarchical Structures Fengxiang Zhang and Hong Yee Low* Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602 ReceiVed February 2, 2007. In Final Form: April 27, 2007 A series of two-level hierarchical structures on polystyrene (PS) and poly(methyl methacrylate) (PMMA) were fabricated using sequential nanoimprinting lithography (NIL). The hierarchical structures consist of micrometer and sub-micrometer scale grating imprinted with varying orientations. Through water contact angle measurements, these surface hierarchical structures showed a wide range of anisotropic wettabilities on PMMA and PS, with PMMA having an anisotropic wettability from 6° to 54° and PS having an anisotropic wettability from 8° to 32°. At the same time, the water contact angle of PMMA and PS can be tuned to nearly 120° without modifying the surface chemistry. A tunable anisotropic wettability is beneficial for applications where controlling the direction of liquid flow is important, such as in microfluidic devices.
Introduction Surface wettability is an important property of materials, which is generally characterized by measuring the contact angle of a liquid droplet sitting on the surface. When water is used, a contact angle less than 90° is indicative of a hydrophilic surface while a contact angle greater than 90° is indicative of a hydrophobic surface. If a surface shows identical contact angles when measured from different directions, the surface is said to be isotropic in wettability, otherwise it is anisotropic. Modification of surface wettability is achieved through either chemical or physical means or through both. Chemical means such as silanization,1-3 fluorination,4-6 plasma treatment,7-10 and photolytic treatment11-13 have been widely used; some of these, however, suffer the drawback of a short-lived effect. Physical means of modifying surface wettability are typically achieved through surface roughening, which results in either ordered or disordered surface structures. Very often, surface roughening or patterning works together with chemical treatments to alter surface wettability.14,15 Meanwhile, the natural world has provided some inspirations for surface wettability modifications. For example, the hierarchical surface texture is responsible for the superhydrophobic and selfcleaning properties of the lotus leaf;16 the hierarchical structure * To whom correspondence should be addressed. E-mail: hy-low@ imre.a-star.edu.sg. (1) Ho´rvo¨lgyi, Z.; Ma´te´, M.; Da´niel, A.; Szalma, J. Colloids Surf., A 1999, 156, 501. (2) Araujo, Y. C.; Toledo, P. G.; Leon, V.; Gonzalez, H. Y. J. Colloid Interface Sci. 1995, 176, 485. (3) Almanza-Workman, A. M.; Raghavan, S.; Deymier, P.; Monk, D. J.; Roop, R. J. Electrochem. Soc. 2002, 149, H6. (4) Colorado, R.; Lee, T. R. Langmuir 2003, 19, 3288. (5) Chapman, T. M.; Marra, K. G. Macromolecules 1995, 28, 2081. (6) Marchand-Brynaert, J.; Pantano, G.; Noiset, O. Polymer 1997, 38, 1387. (7) Davies, J.; Nunnerley, C. S.; Brisley, A. C.; Sunderland, R. F.; Edwards, J. C.; Kruger, P.; Knes, R.; Paul, A. J.; Hibbert, S. Colloids Surf., A 2000, 174, 287. (8) Guruvenketa, S.; Raoa, G. M.; Komath, M.; Raichur, A. M. Appl. Surf. Sci. 2004, 236, 278. (9) Paynter, R. W. Surf. Interface Anal. 1998, 26, 674. (10) Larrieu, J.; Held, B.; Cle´ment, F.; Soulem, N.; Dubois, D. Eur. Phys. J.: Appl. Phys. 2004, 26, 113. (11) Athanassiou, A.; Lygeraki, M. I.; Pisignano, D.; Lakiotaki, K.; Varda, M.; Mele, E.; Fotakis, C.; Cingolani, R.; Anastasiadis, S. H. Langmuir 2006, 22, 2329. (12) Ichimura, K.; Oh, S. K.; Nakagawa, M. Science 2000, 288, 1624. (13) Raduge, C.; Papastavrou, G.; Kurth, D. G.; Motschmann, H. Eur. Phys. J. E 2003, 10, 103. (14) Wilkinson, C. D. W.; Riehle, M. O. Nano Lett. 2005, 5, 2097. (15) Han, J. T.; Zheng, Y.; Cho, J. H.; Xu, X.; Cho, K. J. Phys. Chem. B 2005, 109, 20773.
in a gecko’s foot gives rise to its ability to adhere to the wall and ceiling;17 and the heterogeneous surface on a Stenocara beetle’s back consisting of hydrophilic spots on a hydrophobic background endows the beetle with a unique water harvesting capability in the desert.18 These inspirations have led to a lot of efforts to mimic these biological structures, in particular the hierarchical structure of the lotus leaf and thus the remarkable superhydrophobic property.19-23 Most of these examples reported isotropic hierarchical structures with the aim to achieve superhydrophobicity on different materials such as on silicon and polymer substrates. Anisotropic wettability has attracted much interest more recently. Similar to the approaches taken on tuning the surface wettability, anisotropic wettability is also achieved either through chemical patterning24-26 or surface roughening.27,28 Surfaces with controlled anisotropic wettability have the advantage of restricting liquid flow to a desired direction, which has potential applications in microfluidic devices.29 For example, Sommers et al. reported drainage enhancement with the aid of wetting anisotropy on an aluminum surface.30 In nature, anisotropic wettability has been observed on the surface of the rice leaf, and it has been mimicked by growing aligned carbon nanotubes on a substrate.31 Anisotropic wettability was also reported on parallel PDMS grooves.32 While most literature addressed anisotropic wetting behavior on single level parallel line structures, there are relatively few papers on (16) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1. (17) Autumn, K.; Liang, Y. A.; Tonia Hsieh, S.; Zesch, W.; Chan, W. P.; Kenny, T. W.; Fearing, R.; Full, R. J. Nature 2000, 405, 681. (18) Parker, A. R.; Lawrence, C. R. Nature 2001, 414, 33. (19) Li, Y.; Cai, W.; Cao, B.; Duan, G.; Sun, F.; Li, S.; Jia, L. Nanotechnology 2006, 17, 238. (20) Zhai, L.; Cebeci, F.; Cohen, R.; Rubner, M. Nano Lett. 2004, 4, 1349. (21) Jeong, H. E.; Lee, S. H.; Kim, J. K.; Suh, K. Y. Langmuir 2006, 22, 1640. (22) Bormashenko, E.; Stein, T.; Whyman, G.; Bormashenko, Y.; Pogreb, R. Langmuir 2006, 22, 9982-9985. (23) Liu, H.; Feng, L.; Zhai, J.; Jiang, L.; Zhu, D. B. Langmuir 2004, 20, 5659. (24) Gau, H.; Herminghaus, S.; Lenz, P.; Lipowsky, R. Science 1999, 283, 46. (25) Morita, M.; Koga, T.; Otsuka, H.; Takahara, A. Langmuir 2005, 21, 911. (26) Brandon, S.; Haimovich, N.; Yegar, E.; Marmur, A. J. Colloid Interface Sci. 2003, 263, 237. (27) Gleiche, M.; Chi, L. F.; Fuchs, H. Nature 2000, 403, 173. (28) Higgins, A. M.; Jones, R. A. L. Nature 2000, 404, 476. (29) Ionov, L.; Houbenov, N.; Sidorenko, A.; Stamm, M.; Minko, S. AdV. Funct. Mater. 2006, 16, 1153. (30) Sommers, A. D.; Jacobi, A. M. J. Micromech. Microeng. 2006, 16, 1571. (31) Feng, L.; Li, S.; Lim, Y.; Li, H.; Zhong, L.; Zhai, J.; Song, Y.; Liu, A.; Jiang, L.; Zhu, D. AdV. Mater. 2002, 14, 1857. (32) Chen, Y.; He, B.; Lee, J.; Patankar, N. A. J. Colloid Interface Sci. 2005, 281, 458.
10.1021/la700293y CCC: $37.00 © 2007 American Chemical Society Published on Web 06/01/2007
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the study of anisotropic wettability of surfaces with fabricated hierarchical structures and none reported the combined effects of wettability tuning and directional wetting on a single surface. Inspired by the hierarchical structures reported for the lotus leaf, rice leaf, and water strider’s leg, we fabricated a series of hierarchical structures on polymer films using the technique of sequential imprinting,33 which is based on conventional nanoimprint lithography.34 These structures allow one to tune the anisotropic wettability on polymeric films without the use of chemical treatment. Experimental Section Mold Treatments and Polymer Film Preparations. Silicon (Si) grating molds (supplied by the Institute of Microelectronics, Singapore) of 2 µm pitch (1:1 duty cycle, 2 µm in height) and 250 nm pitch (1:1 duty cycle, 250 nm in height) were used. The molds were cut into suitable sizes to allow for different alignments of the secondary imprint relative to the primary one. The patterned area and mold area of the molds after cutting were exactly the same. The molds were cleaned in an ultrasonic bath using isopropanol, rinsed with acetone, treated with oxygen plasma (80 W, 250 mTorr for 2 min), and subsequently treated with perfluorodecyltrichlorosilane (FDTS, 5 mM in heptane) for 20 min in a nitrogen glove box where the relative humidity was kept at 15-18%; finally, they were sonicated in heptane for 5 min to remove physisorbed FDTS, rinsed with acetone, and then blown dry. FDTS treatment on the Si mold is an effective method to reduce the surface energy of the mold to facilitate mold release after imprinting. Thin films of polystyrene (PS, Aldrich, average Mw ≈ 280 000 g/mol, Tg ≈ 100 °C) approximately 1.7 µm thick (based on profilometry) were obtained by spinning a 13% by weight PS solution in toluene on well-cleaned Si substrates at 3000 revolutions per minute (RPM) for 40 s, followed by baking at 150 °C for 10 min for removal of the residual solvent. A 15% by weight poly(methyl methacrylate) (PMMA, Aldrich, average Mw ≈ 15 000 g/mol, Tg ≈ 105 °C) solution in toluene was spun onto well-cleaned Si wafers at 2000 RPM for 30 s and then baked at 150 °C for 10 min; the resulting PMMA thin films were ∼1.3 µm in thickness. Imprinting Processes. Imprinting was performed using an Obducat nanoimprinter. All the FDTS treated molds first underwent a self-cleaning imprint on PS or PMMA thin films to further remove the physisorbed silane (if any) that escaped sonication; the selfcleaning imprints were made at 120 °C and 40 bar for 300 s. The cleaned molds were then used to carry out imprinting on samples for wetting property studies. By doing so, any possible silane transfer from the molds to samples was eliminated or minimized. On both PS and PMMA samples, the primary 2 µm grating was imprinted under a recipe consisting of 130 °C, 40 bar, and 600 s, while the secondary imprints (both 250 nm and 2 µm) were made at 90 °C and 40 bar for 900 s with different alignments relative to the primary grating. All the molds used in the secondary imprinting have the same history, so that the imprinted structures are comparable in terms of their exposure to the silane layer on the molds; this was to ensure that any differences among the wetting properties of the hierarchically imprinted films originate predominantly from structural differences instead of chemical effects. Static Contact Angle Measurements. A Rame´-Hart digital contact angle (CA) goniometer was used to measure the surface wetting properties of imprinted polymer films. A deionized (DI) water droplet (0.5 µL for PMMA and 1 µL for PS) was deposited gently on the sample surface using an automatic pipet, and a photograph of the water droplet was taken immediately with the goniometer camera. CA values were given by the software measurement; we also cross-checked the CA values obtained from the software with the CA values measured manually on the printed photograph of the water droplet. For each sample, three to six points (33) Zhang, F. X.; Low, H. Y. Nanotechnology 2006, 17, 1884. (34) Chou, S. Y.; Krauss, P. R.; Renstrom, P. J. Appl. Phys. Lett. 1995, 67, 3114.
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Figure 1. Schematic for sequential imprinting: (a) A polymer film is coated onto a substrate, and a primary imprint is made by pressing the polymer with a hard mold at a temperature above the Tg of the polymer and at an elevated pressure. (b) A second mold is aligned to the primary imprint and pressed at below the Tg of the polymer and at an elevated pressure. (c) A two-level hierarchical structure is created on the polymer film.
Figure 2. Optical microscope images for different hierarchical structures: (A) 2 µm ⊥ 250 nm and (B) 2 µm ∠ 2 µm. The scale bar in both images represents 10 µm. were examined in two directions: orthogonal to and parallel with the longitudinal axis of the primary grating.
Results and Discussion Hierarchical Structure Fabrication. Sequential imprinting33 was employed to fabricate different hierarchical structures. The process is schematically shown in Figure 1. Basically, it involves sequential steps of imprints, with the primary imprint made above the glass transition temperature (Tg) of the polymer and the subsequent one(s) well below Tg. By varying the alignment between different imprints and/or by using different combinations of molds, a variety of sophisticated hierarchical structures can be fabricated. In this work, five types of hierarchical grating structures were imprinted on PMMA and PS films, whose primary imprint was made using a 2 µm grating mold and secondary imprint was made using a 250 nm or 2 µm grating mold. The structure nomenclatures and details are listed in Table 1. Figures 2 and 3 show representative microscope and scanning electron mi-
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Figure 3. SEM micrographs for different hierarchical structures: (A) 2 µm ⊥ 250 nm; (B) 2 µm ∠ 250 nm; (C) 2 µm // 250 nm; (D) 2 µm ⊥ 2 µm; and (E) 2 µm ∠ 2 µm. Micrographs (A) and (B) are from PMMA films; (C), (D), and (E) are from PS films. The scale bar in all images is 1 µm. Table 1. Details for the Hierarchical Structures structure 2 µm ⊥ 250 nm 2 µm ∠ 250 nm 2 µm // 250 nm 2 µm ⊥ 2µm 2 µm ∠ 2µm a
mold for pri pattern
mold for sec pattern
sec/pri alignment
pri protr width (µm)
pri tren width (µm)
2 µm grating
250 nm grating
90° 45° 0° 90° 45°
2.5 2.5 3 2.8a 2.2
1.2 1.2 1 0.9a 1.4
2 µm grating
Averaged over the positions where the width is largest and where it is smallest. Table 2. Measured Water Contact Angles and Degrees of Wetting Anisotropy PS
PMMA
sample
θx (°)
θy (°)
∆θ (°)
θx (°)
θy (°)
∆θ (°)
bare 2 µm 250 nm 2 µm ⊥ 250 nm 2 µm ∠ 250 nm 2 µm // 250 nm 2 µm ⊥ 2 µm 2 µm ∠ 2 µm
91 77 ( 4 92 ( 1 108 ( 5 101 ( 4 100 ( 2 109 ( 8 90 ( 1
91 115 ( 5 109 ( 1 135 ( 2 133 ( 4 127 ( 1 117 ( 3 125 ( 1
0 38 17 27 32 27 8 35
68 61 ( 2 53 ( 3 67 ( 2 61 ( 3 62 ( 1 113 ( 1
69 112 ( 3 95 ( 4 121 ( 2 103 ( 1 109 ( 2 119 ( 2
1 51 42 54 42 47 6
croscopy (SEM) images of the various hierarchical structures. (SEM images were obtained on a high-resolution field-emission JEOL JSM6700F system.) It is noted here that there is a small degree of flattening in the primary structure which was taken into consideration in the contact angle calculations to be shown later in this paper. It is also observed that the structural dimensions of the hierarchical structures are virtually the same for PS and PMMA based on the SEM data. Anisotropic Wetting Characterization. For the anisotropic wettability study, we define X-direction and Y-direction as the directions orthogonal to and parallel with the longitudinal axis of the primary 2 µm grating, respectively, θx as the static contact angle (CA) measured in the X-direction, and θy as the static CA measured in the Y-direction. These definitions are schematically shown in Figure 4. We further define ∆θ () θy - θx) as the degree of wetting anisotropy. The volumes of the water droplets used in the CA measurements are 0.5 µL for PMMA and 1 µL
for PS; these sizes are small enough such that the effect of gravitational force on the wetting behavior will be negligible when it resides on the structured surfaces. The measured CA results and degree of anisotropy on PS and PMMA films with various surface structures are summarized in Table 2. The bare PS film showed an isotropic wettability, and the CA measured (91°) was in good agreement with the values reported in the literature.35-37 Such an isotropic hydrophobiciy was turned into a strongly anisotropic wetting behavior by the imprint of a single 2 µm and a single 250 nm grating, with the resultant θy value being larger and θx value being smaller than the intrinsic CA on the bare film. Upon introduction of a secondary grating (250 nm or 2 µm) with different alignments relative to the primary grating, the surface hydrophobicity was enhanced to different extents, indicated by the different increases of the CA in both directions. The most hydrophobic surface was achieved with the “2 µm ⊥ 250 nm” structure, whose CA reached 135° in the Y-direction. As far as we know, the largest CA reported in the literature for PS, without any low-surface-energy treatment, was 162°, which was accomplished on a porous microsphere/nanofiber composite film.38 Compared with the 250 nm grating, the 2 µm grating as the secondary imprint resulted in a lower degree of hydrophobicity enhancement. The various hierarchical structures (35) Kwok, D. Y.; Lum, C. N. C.; Li, A.; Zhu, K.; Wu, R.; Neumann, A. W. Polym. Eng. Sci. 1998, 38, 1675. (36) Marie, H.; Jerome, L.; Matthieu, H.; Laurent, H.; Jacques, P. J. Surf. Interface Anal. 2006, 38, 1266. (37) Johnson, W. C.; Wang, J.; Chen, Z. J. Phys. Chem. B 2005, 109, 6280. (38) Jiang, L.; Zhao, Y.; Zhai, J. Angew. Chem., Int. Ed. 2004, 43, 4338.
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Figure 4. Schematic for the directional measurement of contact angles on grating structures. X and Y represent the directions in which the contact angles are measured.
Figure 6. DI water contact angles on differently imprinted PMMA surfaces.
Figure 5. DI water contact angles on differently imprinted PS surfaces.
resulted in anisotropic wettabilities from 8° to 32°. The variations in the CA and the degree of wetting anisotropy can also be seen in Figure 5. The wetting behaviors of PMMA with different surface structures are shown in Figure 6. The measured CA for bare PMMA is consistent with the literature reported values.39-40 Single gratings, both 2 µm and 250 nm, resulted in a high degree of wetting anisotropy on PMMA, which is characterized by θy values larger and θx values smaller than the intrinsic CA. The “2 µm ⊥ 250 nm” and “2 µm ⊥ 2 µm” hierarchical structures rendered the film more hydrophobic, with CAs of 121° and 119°, respectively, in the Y-direction. These values are very close to the highest literature reported CA, which was ∼120° and was obtained by surface patterning and chemical treatment with perfluorodecyltriethoxysilane.41 The “2 µm ∠ 250 nm” and “2 µm // 250 nm” structures, however, resulted in lower θy values as compared to the samples where the secondary imprint was in perpendicular direction to the primary imprint. As can be seen from Table 2 and Figure 6, the various hierarchical structures resulted in anisotropic wettabilities from 6° to 54°. Analyses on Anisotropic Wetting Behaviors. There are several ways a liquid droplet wets or de-wets a roughened surface.
When the droplet wets both the peak and valley of the surface (homogeneous wetting), its CA is governed by the Wenzel equation:42 cos θ ) r cos θ0, where r is the roughness ratio, or the ratio between the actual surface area over the projected area, and θ0 is the intrinsic CA of the material on its flat surface. According to this equation, surface roughness will amplify the hydrophilicity or hydrophobicity, depending on the chemistry of the material itself. If the droplet sits only on the peaks of the roughened surface and leaves the air trapped below (heterogeneous wetting), its contact angle follows the Cassie-Baxter (CB) equation:43 cos θ ) fs(cos θ0 + 1) - 1, where fs is the fraction of the liquid droplet surface in contact with the solid and θ0 is the intrinsic CA of the material on its flat surface. In the cases of the lotus leaf44 and water strider’s leg,45 the hierarchical
(39) Lim, H.; Lee, Y.; Han, S.; Cho, J.; Kim, K. J. J. Vac. Sci. Technol., A 2001, 19, 1490. (40) Briggs, D.; Chan, H.; Hearn, M. J.; McBriar, D. I.; Munro, H. S. Langmuir 1990, 6, 420. (41) Jung, Y. C.; Bhushan, B. Nanotechnology 2006, 17, 4970.
(42) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988. (43) Cassie, A. B. D. Discuss. Faraday Soc. 1948, 3, 11. (44) Cheng, Y. T.; Rodak, D. E.; Wong, C. A.; Hayden, C. A. Nanotechnology 2006, 17, 1359. (45) Gao, X.; Jiang, L. Nature 2004, 432, 36.
Figure 7. Diameters of water droplets sitting on (A) PS and (B) PMMA surfaces with different surface structures, observed in the direction orthogonal to the primary grating.
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Table 3. Comparison between Calculated and Measured Contact Angles PS
PMMA
sample
fs
θCB (°)
meas θy (°)
r
θW (°)
meas θx (°)
fs
θCB (°)
meas θy (°)
r
2 µm 2 µm ⊥ 250 nm 2 µm // 250 nm 2 µm ∠ 2 µm
0.50 0.34 0.40 0.44
120 131 127 124
115 ( 5 135 ( 2 127 ( 1 117 ( 3
2 5 2.4 1.8
92 96 93 92
77 ( 4 108 ( 5 100 ( 2 109 ( 8
0.50 0.34 0.40 0.44
109 128 117 114
112 ( 3 121 ( 2 109 ( 2 119 ( 2
2 5 2.4 1.8
a
θW (°) 44 noa 31 48
meas θx (°) 61 ( 2 67 ( 2 62 ( 1 113 ( 1
Not applicable since θ0 ) 69° and cos θW ) 5cosθ0 > 1.
structures result in a rough surface where the apparent contact angles are explained by the CB equation. In the present work, we use both equations to study the wetting behaviors of PS and PMMA with different surface structures. The solid fraction fs and the roughness factor r for four representative structures were calculated from the structural dimensions obtained from the SEM images and are listed in Table 3. The evaluation details are available in the Supporting Information. With fs and r being known, the CB equation and Wenzel equation were solved for the apparent contact angles, which are shown also in Table 3. It can be seen from Table 3 that the experimental contact angles of various PS surface structures do not agree with the Wenzel equation; the calculated θW value does not vary significantly with the varying structures, and it shows no appreciable enhancement of hydrophobicity compared with that of bare PS (CA ) 91°). The above results are best explained by recognizing that the cosine of the intrinsic CA of bare PS has a value very close to zero. As a result, the roughness factor will expectedly have an insignificant effect on the cosine value and thus the apparent CA. Similar results were obtained for PMMA where the CA values calculated from the Wenzel equation, though they vary substantially with different surface structures, are not in agreement with the experimental θx or θy values. On the other hand, the CA values calculated using the CB equation (θCB) are within 10% of the experimentally measured θy values for both PS and PMMA. Morita et al. reported similar results: the apparent CA calculated from the CB equation well agreed with the measured value in one direction on a chemically line-patterned surface.25 Table 3 also shows that the surfaces with hierarchical structures have lower solid fractions and consequently higher calculated CAs than the surface with a pure 2 µm grating. In addition, different dimensions and alignments of the secondary gratings result in different solid-air fractions, which further determined the wettability of the roughened surfaces. The above findings are exactly where the tuning effect from the hierarchical structures comes into play. The measured θy value on PS well demonstrated this tuning effect and followed the order “2 µm ⊥ 250 nm” > “2 µm // 250 nm” > “2 µm ⊥ 2 µm” > “2 µm”, which is exactly the opposite order of their solid-air fractions. In the case of PMMA, the tuning effect was demonstrated on the “2 µm ⊥ 250 nm” and “2 µm ⊥ 2 µm” structures; the measured θy results on the “2 µm ∠ 250 nm” and “2 µm // 250 nm” structures, however, are lower than that on the pure 2 µm grating structure, which is not consistent with the calculated results. At this point, we do not have a good explanation for this inconsistency, although it may have been caused by the changes in the dimensions of the imprinted structures. The wetting anisotropy on line or groove structures has been explained in the literature by the line-tension effect using a modified Cassie model,46 by the squeezing effect of a groove on a liquid droplet,32 and by the energy barrier effect.25,47 In the
current work, since PS and PMMA are both more hydrophilic than air, a water droplet tends to flow or spread along the grating; when it spread orthogonal to the longitudinal direction of the grating, the three-phase contact line had to overcome the energy barrier exerted by the air in the trenches of the macro- or nanoscale structures until the trenches were bridged by the droplet. As a result, the droplet was elongated with the three-phase contact line pinned alongside the grating when reaching an equilibrium state and thus anisotropic wetting behavior was observed. On hierarchical structures, the above-described elongation event was disturbed by the secondary gratings, which serve as redirecting channels for the wetting liquid. The primary grating and secondary grating compete with each other to elongate the droplet along their own longitudinal directions, resulting in different dimensions of droplets. For instance, on the “2 µm ⊥ 250 nm” structure, the perpendicular 250 nm grating tends to elongate the droplet in the direction orthogonal to the longitudinal direction of the 2 µm grating; as a result, the diameter of the droplet was reduced relative to that on the pure 2 µm grating when observed in the X-direction. The secondary grating induced changes in the dimensions of water droplets sitting on various imprinted surfaces are shown in Figure 7. For PS, the diameter of the water droplet varied between 2% and 25% compared to the diameter of the water droplet on the bare PS surface, while, for PMMA, the changes in the water droplet diameter were 4-39% of the bare PMMA. In addition to tuning the directional wettability on polymer surfaces, the tuning of the liquid droplet diameter could potentially generate interest in applications where liquids are used as a focusing lens.48 We noted here that the mechanism of anisotropic wettability is still not well understood. The use of the CB equation to support the experimental observation is still a simplification of a complex wetting behavior. As pointed out by Gao et al., the interaction of the water with the solid at the three-phase contact line determined the wetting behavior of a liquid on a solid, but not the actual contact area between the liquid and the solid phase underneath the liquid droplet.49 While a detailed study is still ongoing to understand anisotropic wettability, the current study shows that the use of two-level hierarchical structures obtained by nanoimprint lithography is a relatively easy method to tune the anisotropic wettability of polymer films.
(46) Drelich, J.; Wilbur, J. L.; Miller, J. D.; Whitesides, G. M. Langmuir 1996, 12, 1913. (47) Youngblood, J. P.; McCarthy, T. J. Macromolecules 1999, 32, 6800.
(48) Moran, P. M.; Dharmatilleke, S.; Khaw, A. H.; Tan, K. W.; Chan, M. L.; Rodriguez, I. Appl. Phys. Lett. 2006, 88, 041120. (49) Gao, L.; McCarthy, J. Langmuir 2007, 23, 3762.
Conclusions Imprint lithography is a highly versatile technique for the fabrication of 2D and 3D surface structures. In this work, we have reported a series of two-level hierarchical structures imprinted on PS and PMMA films. Learning from the hierarchical structures shown by biological systems, we have demonstrated that two-level hierarchical structures consisting of grating patterns can be an effective and potentially low cost method to modify the wettability of polymeric films. By using different formats
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and alignments of patterns in the secondary imprints, various two-level hierarchical structures were formed, which resulted in tunable anisotropic wettabilities. The wetting behaviors of the two polymers carrying various surface structures were better modeled by the CB equation than the Wenzel equation. The anisotropic wettability tuning effect by hierarchical structures may result from the redirecting effect from the secondary gratings, as suggested by the different dimensions of a droplet on those
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structures. The technique may find potential applications in fields such as antifouling, microfluidics, micro- or nano-optics, and so forth. Supporting Information Available: Calculations of fs and r in the Cassie-Baxtar and Wenzel equations. This material is available free of charge via the Internet at http:// pubs.acs.org. LA700293Y
