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) werefabricated using sequential nanoimprinting lithography (NIL). The hierarchical structures consist of micrometer andsub-micrometer scale grating imprinted with varying orientations. Through water contact angle measurements, thesesurface hierarchical structures showed a wide range of anisotropic wettabilities on PMMA and PS, with PMMA havingan 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. Atunable 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, whichis generally characterized by measuring the contact angle of aliquid droplet sitting on the surface. When water is used, a contactangle less than 90° is indicative of a hydrophilic surface whilea contact angle greater than 90° is indicative of a hydrophobicsurface. If a surface shows identical contact angles when measuredfrom different directions, the surface is said to be isotropic inwettability, otherwise it is anisotropic. Modification of surfacewettability is achieved through either chemical or physical meansor 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 thedrawback of a short-lived effect. Physical means of modifyingsurface wettability are typically achieved through surfaceroughening, which results in either ordered or disordered surfacestructures. Very often, surface roughening or patterning workstogether with chemical treatments to alter surface wettability.14,15

Meanwhile, the natural world has provided some inspirations forsurface wettability modifications. For example, the hierarchicalsurface texture is responsible for the superhydrophobic and self-cleaning properties of the lotus leaf;16 the hierarchical structure

in a gecko’s foot gives rise to its ability to adhere to the walland ceiling;17 and the heterogeneous surface on a Stenocarabeetle’s back consisting of hydrophilic spots on a hydrophobicbackground endows the beetle with a unique water harvestingcapability in the desert.18 These inspirations have led to a lot ofefforts to mimic these biological structures, in particular thehierarchical structure of the lotus leaf and thus the remarkablesuperhydrophobic property.19-23Most of these examples reportedisotropic hierarchical structures with the aim to achieve supe-rhydrophobicity on different materials such as on silicon andpolymer substrates.

Anisotropic wettability has attracted much interest morerecently. Similar to the approaches taken on tuning the surfacewettability, anisotropic wettability is also achieved either throughchemical patterning24-26or surface roughening.27,28Surfaces withcontrolled anisotropic wettability have the advantage of restrictingliquid flow to a desired direction, which has potential applicationsin microfluidic devices.29 For example, Sommers et al. reporteddrainage enhancement with the aid of wetting anisotropy on analuminum surface.30 In nature, anisotropic wettability has beenobserved on the surface of the rice leaf, and it has been mimickedby growing aligned carbon nanotubes on a substrate.31Anisotropicwettability was also reported on parallel PDMS grooves.32Whilemost literature addressed anisotropic wetting behavior on singlelevel parallel line structures, there are relatively few papers on

* To whom correspondence should be addressed. E-mail: hy-low@imre.a-star.edu.sg.

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J. C.; Kruger, P.; Knes, R.; Paul, A. J.; Hibbert, S.Colloids Surf., A2000, 174,287.

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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. B2005,

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Kenny, T. W.; Fearing, R.; Full, R. J.Nature2000, 405, 681.(18) Parker, A. R.; Lawrence, C. R.Nature2001, 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.Langmuir2006, 22, 1640.(22) Bormashenko, E.; Stein, T.; Whyman, G.; Bormashenko, Y.; Pogreb, R.

Langmuir2006, 22, 9982-9985.(23) Liu, H.; Feng, L.; Zhai, J.; Jiang, L.; Zhu, D. B.Langmuir2004, 20, 5659.(24) Gau, H.; Herminghaus, S.; Lenz, P.; Lipowsky, R.Science1999, 283, 46.(25) Morita, M.; Koga, T.; Otsuka, H.; Takahara, A.Langmuir2005, 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.Nature2000, 403, 173.(28) Higgins, A. M.; Jones, R. A. L.Nature2000, 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.

7793Langmuir2007,23, 7793-7798

10.1021/la700293y CCC: $37.00 © 2007 American Chemical SocietyPublished on Web 06/01/2007

the study of anisotropic wettability of surfaces with fabricatedhierarchical structures and none reported the combined effectsof 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 ofhierarchical structures on polymer films using the technique ofsequential imprinting,33 which is based on conventional nanoim-print lithography.34 These structures allow one to tune theanisotropic wettability on polymeric films without the use ofchemical treatment.

Experimental Section

Mold Treatments and Polymer Film Preparations.Silicon (Si)grating molds (supplied by the Institute of Microelectronics,Singapore) of 2µm pitch (1:1 duty cycle, 2µm in height) and 250nm pitch (1:1 duty cycle, 250 nm in height) were used. The moldswere cut into suitable sizes to allow for different alignments of thesecondary imprint relative to the primary one. The patterned areaand mold area of the molds after cutting were exactly the same. Themolds were cleaned in an ultrasonic bath using isopropanol, rinsedwith acetone, treated with oxygen plasma (80 W, 250 mTorr for 2min), and subsequently treated with perfluorodecyltrichlorosilane(FDTS, 5 mM in heptane) for 20 min in a nitrogen glove box wherethe relative humidity was kept at 15-18%; finally, they weresonicated in heptane for 5 min to remove physisorbed FDTS, rinsedwith acetone, and then blown dry. FDTS treatment on the Si moldis an effective method to reduce the surface energy of the mold tofacilitate mold release after imprinting.

Thin films of polystyrene (PS, Aldrich, averageMw ≈ 280 000g/mol, Tg ≈ 100 °C) approximately 1.7µm thick (based onprofilometry) were obtained by spinning a 13% by weight PS solutionin toluene on well-cleaned Si substrates at 3000 revolutions perminute (RPM) for 40 s, followed by baking at 150°C for 10 minfor removal of the residual solvent. A 15% by weight poly(methylmethacrylate) (PMMA, Aldrich, averageMw ≈ 15 000 g/mol,Tg ≈105 °C) solution in toluene was spun onto well-cleaned Si wafersat 2000 RPM for 30 s and then baked at 150°C for 10 min; theresulting PMMA thin films were∼1.3 µm in thickness.

Imprinting Processes. Imprinting was performed using anObducat nanoimprinter. All the FDTS treated molds first underwenta self-cleaning imprint on PS or PMMA thin films to further removethe physisorbed silane (if any) that escaped sonication; the self-cleaning imprints were made at 120°C and 40 bar for 300 s. Thecleaned molds were then used to carry out imprinting on samplesfor wetting property studies. By doing so, any possible silane transferfrom the molds to samples was eliminated or minimized.

On both PS and PMMA samples, the primary 2µm grating wasimprinted under a recipe consisting of 130°C, 40 bar, and 600 s,while the secondary imprints (both 250 nm and 2µm) were madeat 90°C and 40 bar for 900 s with different alignments relative tothe primary grating. All the molds used in the secondary imprintinghave the same history, so that the imprinted structures are comparablein terms of their exposure to the silane layer on the molds; this wasto ensure that any differences among the wetting properties of thehierarchically imprinted films originate predominantly from structuraldifferences instead of chemical effects.

Static Contact Angle Measurements.A Rame-Hart digitalcontact angle (CA) goniometer was used to measure the surfacewetting properties of imprinted polymer films. A deionized (DI)water droplet (0.5µL for PMMA and 1µL for PS) was depositedgently on the sample surface using an automatic pipet, and aphotograph of the water droplet was taken immediately with thegoniometer camera. CA values were given by the softwaremeasurement; we also cross-checked the CA values obtained fromthe software with the CA values measured manually on the printedphotograph of the water droplet. For each sample, three to six points

were examined in two directions: orthogonal to and parallel withthe longitudinal axis of the primary grating.

Results and Discussion

Hierarchical Structure Fabrication. Sequential imprinting33

was employed to fabricate different hierarchical structures. Theprocess is schematically shown in Figure 1. Basically, it involvessequential steps of imprints, with the primary imprint made abovethe glass transition temperature (Tg) of the polymer and thesubsequent one(s) well belowTg. By varying the alignmentbetween different imprints and/or by using different combinationsof molds, a variety of sophisticated hierarchical structures canbe fabricated.

In this work, five types of hierarchical grating structures wereimprinted on PMMA and PS films, whose primary imprint wasmade using a 2µm grating mold and secondary imprint wasmade using a 250 nm or 2µm grating mold. The structurenomenclatures and details are listed in Table 1. Figures 2 and3 show representative microscope and scanning electron mi-

(33) Zhang, F. X.; Low, H. Y.Nanotechnology2006, 17, 1884.(34) Chou, S. Y.; Krauss, P. R.; Renstrom, P. J.Appl. Phys. Lett.1995, 67,

3114.

Figure 1. Schematic for sequential imprinting: (a) A polymer filmis coated onto a substrate, and a primary imprint is made by pressingthe polymer with a hard mold at a temperature above theTg of thepolymer and at an elevated pressure. (b) A second mold is alignedto the primary imprint and pressed at below theTg of the polymerand at an elevated pressure. (c) A two-level hierarchical structureis created on the polymer film.

Figure 2. Optical microscope images for different hierarchicalstructures: (A) 2µm ⊥ 250 nm and (B) 2µm ∠ 2 µm. The scalebar in both images represents 10µm.

7794 Langmuir, Vol. 23, No. 14, 2007 Zhang and Low

croscopy (SEM) images of the various hierarchical structures.(SEM images were obtained on a high-resolution field-emissionJEOL JSM6700F system.) It is noted here that there is a smalldegree of flattening in the primary structure which was takeninto consideration in the contact angle calculations to be shownlater in this paper. It is also observed that the structural dimensionsof the hierarchical structures are virtually the same for PS andPMMA based on the SEM data.

Anisotropic Wetting Characterization. For the anisotropicwettability study, we defineX-direction andY-direction as thedirections orthogonal to and parallel with the longitudinal axisof the primary 2µm grating, respectively,θx as the static contactangle (CA) measured in theX-direction, andθy as the static CAmeasured in theY-direction. These definitions are schematicallyshown in Figure 4. We further define∆θ () θy - θx) as thedegree of wetting anisotropy. The volumes of the water dropletsused in the CA measurements are 0.5µL for PMMA and 1µL

for PS; these sizes are small enough such that the effect ofgravitational force on the wetting behavior will be negligiblewhen it resides on the structured surfaces. The measured CAresults and degree of anisotropy on PS and PMMA films withvarious surface structures are summarized in Table 2.

The bare PS film showed an isotropic wettability, and the CAmeasured (91°) was in good agreement with the values reportedin the literature.35-37Such an isotropic hydrophobiciy was turnedinto a strongly anisotropic wetting behavior by the imprint of asingle 2µm and a single 250 nm grating, with the resultantθy

value being larger andθx value being smaller than the intrinsicCA on the bare film. Upon introduction of a secondary grating(250 nm or 2µm) with different alignments relative to the primarygrating, the surface hydrophobicity was enhanced to differentextents, indicated by the different increases of the CA in bothdirections. The most hydrophobic surface was achieved with the“2 µm ⊥ 250 nm” structure, whose CA reached 135° in theY-direction. As far as we know, the largest CA reported in theliterature for PS, without any low-surface-energy treatment, was162°, which was accomplished on a porous microsphere/nanofibercomposite film.38 Compared with the 250 nm grating, the 2µmgrating as the secondary imprint resulted in a lower degree ofhydrophobicity 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. B2005, 109, 6280.(38) Jiang, L.; Zhao, Y.; Zhai, J.Angew. Chem., Int. Ed. 2004, 43, 4338.

Figure 3. SEM micrographs for different hierarchical structures: (A) 2µm ⊥ 250 nm; (B) 2µm ∠ 250 nm; (C) 2µm // 250 nm; (D) 2µm ⊥ 2 µm; and (E) 2µm ∠ 2 µm. Micrographs (A) and (B) are from PMMA films; (C), (D), and (E) are from PS films. The scale barin all images is 1µm.

Table 1. Details for the Hierarchical Structures

structuremold for pri

patternmold for sec

patternsec/pri

alignmentpri protr

width (µm)pri tren

width (µm)

2 µm ⊥ 250 nm 2µm grating 250 nm grating 90° 2.5 1.22 µm ∠ 250 nm 45° 2.5 1.22 µm // 250 nm 0° 3 12 µm ⊥ 2µm 2 µm grating 90° 2.8a 0.9a

2 µm ∠ 2µm 45° 2.2 1.4

a Averaged over the positions where the width is largest and where it is smallest.

Table 2. Measured Water Contact Angles and Degrees ofWetting Anisotropy

PS PMMA

sample θx (°) θy (°) ∆θ (°) θx (°) θy (°) ∆θ (°)

bare 91 91 0 68 69 12 µm 77( 4 115( 5 38 61( 2 112( 3 51250 nm 92( 1 109( 1 17 53( 3 95( 4 422 µm ⊥ 250 nm 108( 5 135( 2 27 67( 2 121( 2 542 µm ∠ 250 nm 101( 4 133( 4 32 61( 3 103( 1 422 µm // 250 nm 100( 2 127( 1 27 62( 1 109( 2 472 µm ⊥ 2 µm 109( 8 117( 3 8 113( 1 119( 2 62 µm ∠ 2 µm 90( 1 125( 1 35

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resulted in anisotropic wettabilities from 8° to 32°. The variationsin the CA and the degree of wetting anisotropy can also be seenin Figure 5.

The wetting behaviors of PMMA with different surfacestructures are shown in Figure 6. The measured CA for barePMMA is consistent with the literature reported values.39-40

Single gratings, both 2µm and 250 nm, resulted in a high degreeof wetting anisotropy on PMMA, which is characterized byθy

values larger andθx values smaller than the intrinsic CA. The“2 µm ⊥ 250 nm” and “2µm ⊥ 2 µm” hierarchical structuresrendered the film more hydrophobic, with CAs of 121° and 119°,respectively, in theY-direction. These values are very close tothe highest literature reported CA, which was∼120° and wasobtained by surface patterning and chemical treatment withperfluorodecyltriethoxysilane.41 The “2 µm ∠ 250 nm” and “2µm // 250 nm” structures, however, resulted in lowerθy valuesas compared to the samples where the secondary imprint was inperpendicular direction to the primary imprint. As can be seenfrom Table 2 and Figure 6, the various hierarchical structuresresulted in anisotropic wettabilities from 6° to 54°.

Analyses on Anisotropic Wetting Behaviors.There areseveral 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 Wenzelequation:42 cosθ ) r cosθ0, wherer is the roughness ratio, orthe 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 thehydrophilicity or hydrophobicity, depending on the chemistry ofthe material itself. If the droplet sits only on the peaks of theroughened surface and leaves the air trapped below (heteroge-neous wetting), its contact angle follows the Cassie-Baxter (CB)equation:43 cosθ ) fs(cosθ0 + 1) - 1, wherefs is the fractionof the liquid droplet surface in contact with the solid andθ0 isthe intrinsic CA of the material on its flat surface. In the casesof 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., A2001, 19, 1490.

(40) Briggs, D.; Chan, H.; Hearn, M. J.; McBriar, D. I.; Munro, H. S.Langmuir1990, 6, 420.

(41) Jung, Y. C.; Bhushan, B.Nanotechnology2006, 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.Nature2004, 432, 36.

Figure 4. Schematic for the directional measurement of contactangles on grating structures.X and Y represent the directions inwhich the contact angles are measured.

Figure 5. DI water contact angles on differently imprinted PSsurfaces.

Figure 6. DI water contact angles on differently imprinted PMMAsurfaces.

Figure 7. Diameters of water droplets sitting on (A) PS and (B)PMMA surfaces with different surface structures, observed in thedirection orthogonal to the primary grating.

7796 Langmuir, Vol. 23, No. 14, 2007 Zhang and Low

structures result in a rough surface where the apparent contactangles are explained by the CB equation. In the present work,we use both equations to study the wetting behaviors of PS andPMMA with different surface structures. The solid fractionfsand the roughness factorr for four representative structures werecalculated from the structural dimensions obtained from the SEMimages and are listed in Table 3. The evaluation details areavailable in the Supporting Information. Withfs and r beingknown, the CB equation and Wenzel equation were solved forthe apparent contact angles, which are shown also in Table 3.

It can be seen from Table 3 that the experimental contactangles of various PS surface structures do not agree with theWenzel equation; the calculatedθW value does not varysignificantly with the varying structures, and it shows noappreciable enhancement of hydrophobicity compared with thatof bare PS (CA) 91°). The above results are best explained byrecognizing that the cosine of the intrinsic CA of bare PS hasa value very close to zero. As a result, the roughness factor willexpectedly have an insignificant effect on the cosine value andthus the apparent CA. Similar results were obtained for PMMAwhere the CA values calculated from the Wenzel equation, thoughthey vary substantially with different surface structures, are notin agreement with the experimentalθx or θy values. On the otherhand, the CA values calculated using the CB equation (θCB) arewithin 10% of the experimentally measuredθy values for bothPS and PMMA. Morita et al. reported similar results: the apparentCA calculated from the CB equation well agreed with themeasured value in one direction on a chemically line-patternedsurface.25

Table 3 also shows that the surfaces with hierarchical structureshave lower solid fractions and consequently higher calculatedCAs than the surface with a pure 2µm grating. In addition,different dimensions and alignments of the secondary gratingsresult in different solid-air fractions, which further determinedthe wettability of the roughened surfaces. The above findings areexactly where the tuning effect from the hierarchical structurescomes into play. The measuredθy value on PS well demonstratedthis tuning effect and followed the order “2µm ⊥ 250 nm” >“2 µm // 250 nm”> “2 µm ⊥ 2 µm” > “2 µm”, which is exactlythe opposite order of their solid-air fractions. In the case ofPMMA, the tuning effect was demonstrated on the “2µm ⊥ 250nm” and “2µm ⊥ 2 µm” structures; the measuredθy results onthe “2µm ∠ 250 nm” and “2µm // 250 nm” structures, however,are lower than that on the pure 2µm grating structure, which isnot consistent with the calculated results. At this point, we donot have a good explanation for this inconsistency, although itmay have been caused by the changes in the dimensions of theimprinted structures.

The wetting anisotropy on line or groove structures has beenexplained in the literature by the line-tension effect using amodified Cassie model,46 by the squeezing effect of a groove ona liquid droplet,32 and by the energy barrier effect.25,47 In the

current work, since PS and PMMA are both more hydrophilicthan air, a water droplet tends to flow or spread along the grating;when it spread orthogonal to the longitudinal direction of thegrating, the three-phase contact line had to overcome the energybarrier exerted by the air in the trenches of the macro- or nanoscalestructures until the trenches were bridged by the droplet. As aresult, the droplet was elongated with the three-phase contactline pinned alongside the grating when reaching an equilibriumstate and thus anisotropic wetting behavior was observed. Onhierarchical structures, the above-described elongation event wasdisturbed by the secondary gratings, which serve as redirectingchannels for the wetting liquid. The primary grating and secondarygrating compete with each other to elongate the droplet alongtheir own longitudinal directions, resulting in different dimensionsof droplets. For instance, on the “2µm ⊥ 250 nm” structure, theperpendicular 250 nm grating tends to elongate the droplet in thedirection orthogonal to the longitudinal direction of the 2µmgrating; as a result, the diameter of the droplet was reducedrelative to that on the pure 2µm grating when observed in theX-direction. The secondary grating induced changes in thedimensions of water droplets sitting on various imprinted surfacesare shown in Figure 7. For PS, the diameter of the water dropletvaried between 2% and 25% compared to the diameter of thewater droplet on the bare PS surface, while, for PMMA, thechanges in the water droplet diameter were 4-39% of the barePMMA. In addition to tuning the directional wettability onpolymer surfaces, the tuning of the liquid droplet diameter couldpotentially generate interest in applications where liquids areused as a focusing lens.48

We noted here that the mechanism of anisotropic wettabilityis still not well understood. The use of the CB equation to supportthe experimental observation is still a simplification of a complexwetting behavior. As pointed out by Gao et al., the interactionof the water with the solid at the three-phase contact linedetermined the wetting behavior of a liquid on a solid, but notthe actual contact area between the liquid and the solid phaseunderneath the liquid droplet.49 While a detailed study is stillongoing to understand anisotropic wettability, the current studyshows that the use of two-level hierarchical structures obtainedby nanoimprint lithography is a relatively easy method to tunethe anisotropic wettability of polymer films.

Conclusions

Imprint lithography is a highly versatile technique for thefabrication of 2D and 3D surface structures. In this work, wehave reported a series of two-level hierarchical structuresimprinted on PS and PMMA films. Learning from the hierarchicalstructures shown by biological systems, we have demonstratedthat two-level hierarchical structures consisting of grating patternscan be an effective and potentially low cost method to modifythe wettability of polymeric films. By using different formats

(46) Drelich, J.; Wilbur, J. L.; Miller, J. D.; Whitesides, G. M.Langmuir1996,12, 1913.

(47) Youngblood, J. P.; McCarthy, T. J.Macromolecules1999, 32, 6800.

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(49) Gao, L.; McCarthy, J.Langmuir2007, 23, 3762.

Table 3. Comparison between Calculated and Measured Contact Angles

PS PMMA

sample fs θCB (°) measθy (°) r θW (°) measθx (°) fs θCB (°) measθy (°) r θW (°) measθx (°)2 µm 0.50 120 115( 5 2 92 77( 4 0.50 109 112( 3 2 44 61( 22 µm ⊥ 250 nm 0.34 131 135( 2 5 96 108( 5 0.34 128 121( 2 5 noa 67 ( 22 µm // 250 nm 0.40 127 127( 1 2.4 93 100( 2 0.40 117 109( 2 2.4 31 62( 12 µm ∠ 2 µm 0.44 124 117( 3 1.8 92 109( 8 0.44 114 119( 2 1.8 48 113( 1

a Not applicable sinceθ0 ) 69° and cosθW ) 5cosθ0 > 1.

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and alignments of patterns in the secondary imprints, varioustwo-level hierarchical structures were formed, which resulted intunable anisotropic wettabilities. The wetting behaviors of thetwo polymers carrying various surface structures were bettermodeled by the CB equation than the Wenzel equation. Theanisotropic wettability tuning effect by hierarchical structuresmay result from the redirecting effect from the secondary gratings,as suggested by the different dimensions of a droplet on those

structures. The technique may find potential applications in fieldssuch as antifouling, microfluidics, micro- or nano-optics, and soforth.

Supporting Information Available: Calculations offs and rin the Cassie-Baxtar and Wenzel equations. This material is availablefree of charge via the Internet at http:// pubs.acs.org.

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7798 Langmuir, Vol. 23, No. 14, 2007 Zhang and Low