Strongly Anisotropic Wetting on One-Dimensional Nanopatterned

NANO LETTERS

Strongly Anisotropic Wetting on One-Dimensional Nanopatterned Surfaces

2008 Vol. 8, No. 9 2819-2824

Deying Xia and S. R. J. Brueck* Center for High Technology Materials and Department of Electrical and Computer Engineering, UniVersity of New Mexico, 1313 Goddard, SE, Albuquerque, New Mexico 87106 Received May 14, 2008; Revised Manuscript Received July 11, 2008

ABSTRACT This communication reports strongly anisotropic wetting behavior on one-dimensional nanopatterned surfaces. Contact angles, degree of anisotropy, and droplet distortion are measured on micro- and nanopatterned surfaces fabricated with interference lithography. Both the degree of anisotropy and the droplet distortion are extremely high as compared with previous reports because of the well-defined nanostructural morphology. The surface is manipulated to tune with the wetting from hydrophobic to hydrophilic while retaining the structural wetting anisotropy with a simple silica nanoparticle overcoat. The wetting mechanisms are discussed. Potential applications in microfluidic devices and evaporationinduced pattern formation are demonstrated.

Wetting phenomena on structured solid surfaces are of both fundamental and technological interest.1-3 Engineered surfaces exhibiting superhydrophobicity have been reported.4 The dynamics of drops on water repellant surfaces have been explained in detail.5,6 The enhancement of hydrophobicity by roughness is described by either Wenzel or Cassie models (water or air trapped in the structure below the drop, respectively). A stabilized Cassie regime favors water repellency7 and offers a promising route a droplet-based microfluidic system.8 Anisotropic wetting behavior has been observed on one-dimensional (1D) patterned surfaces achieved either through chemical patterning9 or surface roughness.10,11 Several technological approaches to altering the wetting-todewetting transition have been explored including: chemical mediation,12 laser irradiation,13 and the application of an electric field.14 Here, we report strongly anisotropic wetting behavior on one-dimensional nanopatterned surfaces for a wide range of surface hyrophobicity/hydrophilicity of the underlying materials. We also report the evaporation dynamics of an anisotropic water droplet as well as droplet positioning on predesigned surfaces, important precursors to applications in microfluidic devices and evaporation-driven pattern formation. Surfaces with controlled anisotropic wetting, confining liquid flow to a single direction, have potential applications in microfluidic devices, evaporation-driven formation of patterns, and easy-clean coatings as well as being of fundamental interest.15 Drainage enhancement has been * Corresponding author. E-mail: [email protected]. 10.1021/nl801394w CCC: $40.75 Published on Web 08/05/2008

 2008 American Chemical Society

reported with the aid of wetting anisotropy on etched 1D aluminum surfaces.16 Weak anisotropic wetting characteristics were observed on shallow periodic polymer grooves.17 A relatively strong anisotropic wettability was measured on imprinted hierarchical structures.18 Most of the literature on anisotropic wetting behavior is concerned with relatively low degrees of anisotropy on surfaces with micrometer-scale parallel grooves; there have been relatively few papers reporting strong anisotropic wetting behavior on nanoscale patterns, the variation of the wetting behavior over large material variations, or the evaporation dynamics of anisotropic liquid droplets. Interferometric lithography (IL) is a powerful approach to the fabrication of micro/nanoscale periodic patterns with the advantages of low cost, large-area capability, and flexibility.19,20 Also, with the combination of interferometric lithography and conventional lithography, it is easy to fabricate integrated micro/nanofluidic devices for many practical applications. Figure 1 shows an example of 1D photoresist (PR) patterns (1500 nm pitch) on a Si substrate using IL and the corresponding anisotropic wetting behavior (see experimental methods in Supporting Information). Parallel PR lines with a rectangular cross section (∼500 nm high) were formed over a large area (> square centimeters) with high uniformity (Figure 1a). The spaces between the PR walls were cleared to the bare Si substrate surface which has a thin native oxide layer; the sharp discontinuities and nearly vertical PR sidewalls contribute to the observation of the strong wetting anisotropy detailed below. There is a

Figure 1. 1D, 1500 nm period PR structures and corresponding anisotropic wetting: (a) Large area SEM image (insets: magnified side view in top left and magnified top view in top right); (b) contact angle θy, parallel to the direction of the PR lines with 1.5 µL droplet; (c) contact angle θx, orthogonal to direction of PR lines with 1.5 µL droplet; (d) photograph of top view of 30 µL droplet; (e) photograph of side view of 30 µL droplet (arrows indicate the θx and θy directions).

significant difference in the contact angles for isotropic surfaces of PR (76°) and Si (38°). Strongly anisotropic wetting behavior was observed on this structure. The contact angle measured from the direction orthogonal to the PR lines is defined as θx () 130°) and from the direction parallel to the PR lines is θy () 51°) [Figures 1b,c]. ∆θ () θx - θy), a measure of the wetting anisotropy,18 is 79°, much higher than in previous reports.17,18 The wetting is ultrahydrophobic (θx > 90°) in the direction perpendicular to the PR lines. The strongly anisotropic wetting is clearly evident from the oblong shape observed in the top and 45° views of a large water droplet (30 µL; Figure 1d,e). This anisotropic wetting is due to discontinuities in the three-phase contact line and pinning of the droplet at the edges of the 1D PR structures, resulting in preferential spreading parallel to the PR lines.16 The influence of the substrate surface wetting properties on the wetting anisotropy was investigated by manipulating the surfaces from hydrophobic to hydrophilic. First, we compared the contact angles in two directions for PR patterns with a 1500 nm period on Si substrate and similar Si substrate with a SiO2 film. An ∼200 nm thick SiO2 layer was formed by thermal oxidation. There were only minor differences in the wetting characteristics of these two samples (for Si substrate: θx ) 130°, θy ) 51°; for thermal SiO2 film substrate: θx ) 125°, θy ) 43°). Additionally, we tested 1D PR patterns formed atop silica nanoparticle films deposited on a Si substrate. Silica nanoparticles are good candidates to generate hierarchical structures and to tune surface wetting properties.21,22 Modified substrates were formed by spin coating uniform thickness layers of 50 nm diameter silica nanoparticles onto a Si substrate.23 Blanket silica nanoparticle films exhibited hydrophilic wetting with a contact angle of 2820

∼5°. Spin coating of a 5 wt %, 50 nm diameter silica nanoparticle suspension (OL from Nissan Chemicals American), produced roughly two-particle thick films on a Si substrate. In order to provide a complete coverage of the silica nanoparticles, we deposited the silica nanoparticle films with two (OL2) or three (OL3) cycles of the spin-coating. A one-dimensional PR pattern was fabricated atop the blanket silica nanoparticle films using IL as in the previous Si substrate case. As shown in Figure 2 for samples with 1000 nm period, the anisotropic wetting behavior of the composite structure was very similar for patterns on a Si substrate and on a silica nanoparticle film. The results for all three samples (Si substrate, OL2, and OL3 silica nanoparticle film) are within the green circle in Figure 2 exhibiting strongly anisotropic hydrophobic wetting. Similar results (not shown) were observed on the thermally grown SiO2 surfaces. The anisotropic wetting on 1D patterned PR samples is only weakly dependent on the wetting properties of the substrate. This suggests that a Cassie (composite) model is appropriate. The water droplet is suspended on the PR structures and does not directly contact the substrate. The as-prepared structures have rectangular rather than triangular or sinusoidal profiles and are relatively deep, which is favorable to forming a water droplet with air trapped in the underlying roughness. The morphology of 1D nanopatterned surface is the key parameter for generating such a large droplet distortion. In previous research, the observed weakly anisotropic wetting resulted from chemical surface modification9 or shallow or sinusoidal structures which lacked well-defined structural discontinuities.15,17 There has been limited work on modifying the wetting behavior of isotropic surfaces;4,12,13 it remains a challenge Nano Lett., Vol. 8, No. 9, 2008

Figure 2. Anisotropic wetting on 1000 nm period samples. On OL2: 1D PR patterns on silica nanoparticle film formed with two 5 wt % OL (50 nm silica suspension) spin coats. On OL3: 1D PR patterns on silica nanoparticle film formed with three spin coats of 5 wt % OL. PR: PR layer without patterns. Si: Si substrate without PR and patterns. 1% OL: modified with 1 wt % OL for 1D PR patterns. On Si: Si substrate with PR patterns. 2.5% OL: modified with 2.5 wt % OL for 1D PR patterns. 5% OL: 5 wt % OL nanoparticle film. OL2, 1%: modified with 1 wt % OL two times for 1D PR patterns. OL3, 1%: modified with 1 wt % OL three times for 1D PR patterns.

Table 1. Measured Contacted Angles and Droplet Distortion Dd samples

θy, parallel θx, orthogonal Dd (L/w) direction of direction of droplet line line distortion

Figure 3. SEM images (a,b) and contact angles (c,d) on 1500 nm period sample modified by spin coating with a 5 wt % silica nanoparticle suspension.

420 nm period on Si 1000 nm period on Si 1500 nm period on Si 1500 nm period modified with silica nanoparticles 1500 nm period on thermal SiO2 layer

to adjust anisotropic wetting properties while retaining the structural anisotropy with simple, effective, and low cost techniques. A simple spin-coating deposition of a silica nanoparticle suspension atop the PR/substrate structures was used to modify the anisotropic wetting from hydrophobic to hydrophilic. The variation with different concentrations of silica nanoparticle suspensions and the number of spincoating cycles on 1D PR samples with 1000 nm period is shown in Figure 2. The trend from hydrophobic to hydrophilic is indicated by the brown arrow. As the concentrations of the silica suspension and the thicknesses of the overlayer are increased, the behavior moves toward anisotropic hydrophilic wetting as indicated in the orange ellipse. The wetting became almost isotropic and strongly hydrophilic for a 5 wt % concentration, which almost completely filled the grooves and covered the PR tops. Therefore, we can tune the surface wetting from hydrophobic to hydrophilic while maintaining the anisotropy. All of the points in Figure 2 are below the isotropic (green) line indicating that the structural anisotropy dominates the wetting characteristics independently of the material properties. Figure 3 shows the results of applying the silica nanoparticle coating (5 wt %) to a sample with a larger 1500 nm period. The silica nanoparticles fully cover the trenches between PR walls and partially cover the top surfaces of the

PR (Figures 3a,b). Table 1 shows additional data. The surface wetting is converted from a strongly anisotropic hydrophobicity (θx, 130°; θy, 51°; ∆θ ) 79°) to strongly anisotropic hydrophilicity (θx, 38°; θy, 8°; ∆θ ) 30°) for a 1500 nm period sample. The droplet distortion, Dd ) L/W where the L is the length of the major axis (along the y direction in Figure 1e) divided by the width of the minor axis W (along the x direction), is an alternate parameter to describe the wetting anisotropy.9 Qualitatively, the droplet distortion is only weakly dependent on the droplet volume. Figure 1d,e is for a 30 µL droplet and shows an anisotropy of Dd ∼ 4.5; with 0.3 µL water droplets, we measured Dd ∼ 3.5 to 5 for 1D PR patterns and 8 to 10 for modified (silica nanoparticle overcoat) 1D samples. The droplet distortion is obvious from top-down images and is much larger than in previous reports (Dd 2, droplet volume much larger than 0.3 µL, around several microliters).9 Especially with the silica particle modification, a dramatic stretching of the droplet is observed. Even though the degree of wetting anisotropy ∆θ is smaller, only ∼30°, for the modified sample, the droplet distortion is as large as 10 as a result of the hydrophilic wetting in both directions. This droplet distortion is attributed to the difference in the wetting energy barrier in the two directions. Because of the hydrophilic silica wetting, we expect the wetting to be described by a Wenzel model

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Figure 4. Anisotropic wetting behavior on a macroscale defined sample (positive PR 510, pattern with 1000 nm period, sample size 2 × 2 cm2, 10 µL water drop) on the sample (a) U-shape with vertical 1D PR patterns; (b) cross shape of horizontal and vertical 1D PR patterns; bottom row indicates corresponding structure schematics.

(water filling the grooves and contacting the bottom surface). In addition, we can see that the periodicity has a modest effect on the strongly anisotropic wetting of the as-prepared PR patterns as the period varies from 400 to 1500 nm. We further verified the fact that the wetting properties of the lower surface only weakly impact the anisotropic wetting of 1D PR patterns. Integrating these anisotropic patterns into macrostructures provides possible applications in microfluidic control as

illustrated in Figure 4 which shows two extended structures using anisotropic hydrophobicity (PR patterns) to control the water droplet structure. Five 10 µL water droplets were applied on different positions on the U-shaped structure (Figure 4a). Water droplets a and b were positioned on the horizontal bar (with vertical PR lines), while water droplets c, d, and f were put on the vertical arms (with vertical PR lines). The anisotropy of the water droplets is well-displayed by droplets c and the merged d and f. Because of the hydrophobic character of the surrounding PR films, droplets a and b exhibit further shape deformation arising from macroscopic pattern boundary effects. Droplets d and f were initially the same distance apart as a and b. However, d and f were separated parallel to the grooves and readily merge while a and b were separated in the direction perpendicular to the grooves and remain distinct. These simple demonstrations clearly show the potential for using anisotropic wetting to mix and control fluid transport in microfluidic devices.24 For example, with different positioning of the droplets, we can mix the fluids or maintain the separation as demonstrated in the above configurations. Moreover, by taking advantage of the associated anistropic sliding angles in the two directions, we can easily control fluid transport in microfluidic devices. The structure of water droplets was also investigated on crossshape samples (Supporting Information Figure S1). All of the droplets are stretched out and isolated from the pattern boundaries on a symmetric cross sample (Figure 4b). After pulling these samples from a water bath, the blanket hydrophobic PR surfaces are dry, and water is only retained

Figure 5. 5× microscope images of evaporation of a water droplet on 1D 1500 nm period PR pattern surface and a modified surface; (a) evaporation process of 0.25 µL water droplet on an anisotropic hydrophobic (PR/Si) surface; (b) photograph of initial contact line of 0.1 µL water droplet; (c) photograph of residual mark after drying of (a); (d) photograph and SEM image of drying patterns of water droplet containing 0.2 wt % 4.61 µm PS spheres on anisotropic hyrophobic surface; (e) dynamics of drying of a 0.10 µL droplet on an anisotropic hydrophilic surface. 2822

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in the nanopatterned regions (Supporting Information Figure S2). It is straightforward to envision forming complex networks to define water patterns in microfluidic devices or guiding reactants in microreactors using macro-patterned anisotropic wetting structures.3 While there have been a few recent reports detailing the characterization analysis of the evaporation of droplets on isotropic superhydrophobic surfaces,25,26 only very limited observations of droplet evaporation phenomena on anisotropic wetting surfaces are available. Figure 5a shows some characteristics of the dynamics of the evaporation of a water droplet on an unmodified hydrophobic (PR on a Si substrate) wetting surface. The total evaporation time at ambient atmosphere (∼36% relative humidity and 19 °C) was 168 s for a 0.25 µL water droplet. The contact line of the water droplet initially has the shape of an irregular oval (Figure 5a) in excellent agreement with a composite contact (Cassie) model numerical simulation (see Supporting Information Figure S3).27 For a smaller 0.1 µL drop, the entire drop is within the microscope field of view (Figure 5b). As the evaporation proceeds, the θy contact angle changes dramatically as the height of the droplet decreases while the contact line is initially unchanged. Once the droplet reaches a critical thickness, the evaporation is completed very rapidly. The fluid moves along the PR pattern direction in the final evaporation stage, shrinking from both ends, and the drying mark has an oblong shape (Figure 5c). The decrease of contact angle as the droplet evaporates and the receding of contact line provide clues to the anisotropic wetting mechanism. 28 We observe thick, dark edges at the two circular ends connected by straight line uniform marks. There have been reports of the application of isotropic water droplets and cylindrical droplets to evaporation-induced pattern formation of anisotropic nanomaterials such as DNA29 and CNT.30,31 Drying phenomena on anisotropic structures provide additional opportunities for forming aligned arrays of 1D nanomaterials. We demonstrate the potential of anisotropic wetting to induce patterns in Figure 5d using a liquid drop containing colloidal polystyrene (PS) microspheres. Wide bands of PS microspheres are deposited at the two circular ends and narrow bands of microspheres are found along the straight lines (Figure 5d). For this noncircular droplet, the deposition rates are different along contact lines similar to the phenomena in coffee stains caused by capillary flow where the deposition is denser in more convex regions of the starting drop.1 In straight line regions, the microspheres are roughly organized along the initial contact line of the water droplet (see Supporting Information Figure S4). We also investigated the evaporation of water droplets on silica nanoparticle modified samples. The water droplet on this anisotropic hydrophilic surface initially spreads out widely with a low vertical height and an enlarged contact line. The evaporation is very fast due to the large exposed surface area; the total evaporation time is 60 s for a 0.25 µL water droplet and only 25 s for a 0.10 µL water droplet for the same environmental condition as above (Figure 5e). In contrast to the PR pattern case where Nano Lett., Vol. 8, No. 9, 2008

the water volume first decreased without a major change in the 2D contact line, the initial receding of the contact line for these modified structures is obvious. We expected that evaporation-induced pattern formation will be much different from that described above; additional investigations are underway. In conclusion, strongly anisotropic wetting on 1D PR patterned surfaces is observed as the surface wetting is tuned from hydrophobic to hydrophilic using silica nanoparticles while retaining the structural anisotropy. Both the degree of anisotropy and the droplet distortion are extremely high as compared with previous reports. The wetting in the hydrophobic range fits a Cassie model, but the mechanism of anisotropic wettability is complex and not well-understood. The applications in microfluidic devices and microreactors for strong anisotropic wetting are demonstrated through simple configurations of macroscopic patterned surfaces. Acknowledgment. This work was supported by the National Science Foundation under Grant 0515684 and by the ARO under a subcontract from Redondo Optics, Inc. The facilities of the NSF-sponsored NNIN mode at UNM were used for the fabrication. Supporting Information Available: Experimental methods and additional SEM images and optical micrographs. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Nature 1997, 389, 827. (2) Yoshimitsu, Z.; Nakajima, A.; Watanabe, T.; Hashimoto, K. Langmuir 2002, 18, 5818. (3) Gau, H.; Herminghaus, S.; Lenz, P.; Lipowsky, R. Science 1999, 283, 46. (4) Sun, T.; Feng, L.; Gao, X.; Jiang, L. Acc. Chem. Res. 2005, 38, 644. (5) Callies, M.; Quere, D. Soft Matter 2005, 1, 55. (6) Quere, D. Rep. Prog. Phys. 2005, 68, 2495. (7) Lafuma, A.; Quere, D. Nat. Mater. 2003, 2, 457. (8) Blossey, R. Nat. Mater. 2003, 2, 301. (9) Morita, M.; Koga, T.; Otsuka, H.; Takahara, A. Langmuir 2005, 21, 911. (10) Gleiche, M.; Chi, L. F.; Fuchs, H. Nature 2000, 403, 173. (11) Chung, J. Y.; Youngblood, J. P.; Stafford, C. M. Soft Matter 2007, 3, 1163. (12) Martines, E.; Seunarine, K.; Morgan, H.; Gadegaard, N.; Wilkinson, C. D.; Riehle, M. O. Nano Lett. 2005, 5, 2097. (13) Han, J. T.; Kim, S.; Karim, A. Langmuir 2007, 23, 2608. (14) Krupenkin, T. N.; Taylor, J. A.; Wang, E. N.; Kolodner, P.; Hodes, M.; Salamon, T. R. Langmuir 2007, 23, 9128. (15) Feng, L.; Li, S.; Li, Y.; Li, H.; Zhang, L.; Zhai, J.; Song, Y.; Liu, B.; Jiang, L.; Zhu, D. AdV. Mater. 2002, 14, 1857. (16) Sommers, A. D.; Jacobi, A. M. J. Micromech. Microeng. 2006, 16, 1571. (17) Zhao, Y.; Lu, Q.; Li, M.; Li, X. Langmuir 2007, 23, 6212. (18) Zhang, F.; Low, H. Y. Langmuir 2007, 23, 7793. (19) Xia, D.; Brueck, S. R. J. Nano Lett. 2004, 4, 1295. (20) Xia, D.; Li, D.; Luo, Y.; Brueck, S. R. J. AdV. Mater. 2006, 18 930. (21) Zhai, L.; Cebeci, F. C.; Cohen, R. E.; Rubner, M. F. Nano Lett. 2004, 4, 1349. (22) Bravo, J.; Zhai, L.; Wu, Z.; Cohen, R. E; Rubner, M. F. Langmuir 2007, 23, 7293. (23) Xia, D.; Li, D.; Ku, Z.; Luo, Y.; Brueck, S. R. J. Langmuir 2007, 23, 5377. (24) Czolkos, L.; Erkan, Y.; Dommersnes, P.; Jesorka, A.; Orwar, O. Nano Lett. 2007, 7, 1980. 2823

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Nano Lett., Vol. 8, No. 9, 2008