Effect of Sessile Drop Volume on the Wetting Anisotropy Observed on

3 Feb 2009 - This study reports experimental measurements of the water contact angle (WCA) measured on surfaces with grooves of different widths using...
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Langmuir 2009, 25, 2567-2571

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Effect of Sessile Drop Volume on the Wetting Anisotropy Observed on Grooved Surfaces Jing Yang,† Felicity R. A. J. Rose,† Nikolaj Gadegaard,‡ and Morgan R. Alexander*,† School of Pharmacy, UniVersity of Nottingham, UniVersity Park, Nottingham NG7 2RD, U.K., and Centre for Cell Engineering, Department of Electronics and Electrical Engineering, Glasgow UniVersity, UniVersity AVenue, Glasgow G12 8LT, U.K. ReceiVed December 2, 2008. ReVised Manuscript ReceiVed January 19, 2009 This study reports experimental measurements of the water contact angle (WCA) measured on surfaces with grooves of different widths using drop volumes ranging from 400 pL to 4.5 µL. These measurements were carried out on both relatively hydrophobic and hydrophilic surface chemistry formed using a conformal plasma polymer coating of topographically embossed poly(methyl methacrylate) (PMMA). Anisotropic wetting of the grooved surfaces was found to be more marked for larger drops on both the hydrophilic and hydrophobic surfaces. Above a certain drop base diameter to groove width ratio, topography had no effect on the measured WCA; this ratio was found to be dependent on the water drop volume. The WCA measured from the direction perpendicular to the grooves using submicroliter water drops is found to be a good indicator of the WCA on the flat surface with equivalent wettabilities. To the best of our knowledge, this is the first study on the phenomenon of anisotropic wetting using picoliter water drops.

Introduction In recent years, the phenomenon of “anisotropic wetting” on micropatterned surfaces has attracted a lot of scientific interest. This is mainly driven by sophisticated technologies involving well-defined chemically and topographically micropatterned surfaces where wettability is an important design parameter.1-3 Such phenomena have practical applications in self-cleaning surfaces and can be applied in microfluidic systems.4-6 On ideal solid surfaces (i.e., flat, chemically homogeneous, rigid, insoluble, and nonreactive), the three-phase contact line of a drop is a circle, and the water contact angle can be predicted by Young’s equation.7 However, for chemically heterogeneous materials or surfaces with topography, the shape of the three-phase contact line and the drop are distorted under the influence of the chemical or topographic patterns. Anisotropic wetting on surfaces with regular chemical patterns has been studied both theoretically and experimentally. Drelich et al. reported contact angles on surfaces consisting of alternating and parallel hydrophilic/hydrophobic strips of 2.5 µm width. The advancing and receding contact angles, when measured with the drop edge normal to the strips, were found to be ∼2-10° lower than those measured when viewing the drop edge parallel to the strips.8 Morita et al. also studied anisotropic wetting on alternating and parallel hydrophilic/hydrophobic strips with four different widths.9 The distortion of drop shape was found to be * Corresponding author. Email: [email protected]. † University of Nottingham. ‡ Glasgow University. (1) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498– 1511. (2) Martines, E.; Seunarine, K.; Morgan, H.; Gadegaard, N.; Wilkinson, C. D. W.; Riehle, M. O. Nano Lett. 2005, 5, 2097–2103. (3) Wouters, D.; Schubert, U. S. Angew. Chem., Int. Ed. 2004, 43, 2480–2495. (4) Blossey, R. Nat. Mater. 2003, 2, 301–306. (5) Grunze, M. Science 1999, 283, 41–42. (6) Zhao, B.; Moore, J. S.; Beebe, D. J. Science 2001, 291, 1023–1026. (7) Young, T. Philos. Trans. R. Soc. London 1805, 95, 65–87. (8) Drelich, J.; Wilbur, J. L.; Miller, J. D.; Whitesides, G. M. Langmuir 1996, 12, 1913–1922. (9) Morita, M.; Koga, T.; Otsuka, H.; Takahara, A. Langmuir 2005, 21, 911– 918.

increased by increasing the drop size from 0.2 to 5 µL. Zhao et al. reported anisotropic wetting on submicrometer-scale (groove widths of 318, 396, and 513 nm with varying nanometer-scale depth) periodic topographic structures consisting of parallel grooves. It was found that the water contact angle measured using sessile drops of 3 µL from the direction perpendicular to the grooves increased with increasing groove depth, which was assigned to the increasing energy barrier to drop spreading caused by pinning of the groove edge.10 Long et al. used a thermodynamic model to predict the water contact angle on rough surfaces with different topographic geometries. The contact angle with the three-phase contact line parallel to the groove was found to be in a metastable state. The predicted apparent contact angle was in a relative lower energy state and was trapped by higher neighboring energy states.11 Chen et al. numerically quantified the apparent contact angles on a hydrophobic surface with parallel grooves.12 It was found that there are multiple equilibrium shapes for a drop on rough surfaces with parallel grooves. A particular equilibrium shape was obtained by fixing the number of grooves on which the drop resided. Thus far, all experimental studies on anisotropic wetting have been conducted using microliter volume drops; consequently, the diameter of the drop base relative to the striped chemical or geometric feature size was large. With the increasing use of microarrays, gradient samples, and microelectromechanical system devices for biological studies,13-15 there has been a drive to reduce the dimensions on which surface wettability may be characterized to improve spatial resolution, which has been met by developing systems utilizing smaller water drops. Using a piezo dispenser, a commercial system capable of routinely dosing (10) Zhao, Y.; Lu, Q. H.; Li, M.; Li, X. Langmuir 2007, 23, 6212–6217. (11) Long, J.; Hyder, M. N.; Huang, R. Y. M.; Chen, P. AdV. Colloid Interface Sci. 2005, 118, 173–190. (12) Chen, Y.; He, B.; Lee, J. H.; Patankar, N. A. J. Colloid Interface Sci. 2005, 281, 458–464. (13) Gallant, N. D.; Lavery, K. A.; Amis, E. J.; Becker, M. L. AdV. Mater. 2007, 19, 965–969. (14) Anderson, D. G.; Levenberg, S.; Langer, R. Nat. Biotechnol. 2004, 22, 863–866. (15) Urquhart, A. J.; Anderson, D. G.; Taylor, M.; Alexander, M. R.; Langer, R.; Davies, M. C. AdV. Mater. 2007, 19, 2486–2491.

10.1021/la803942h CCC: $40.75  2009 American Chemical Society Published on Web 02/03/2009

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and measuring contact angles of picoliter volume water drops has been developed. The contact angle measured using picoliter drops on flat homogeneous polymer surfaces has been found to agree well with that measured by using microliter drops.16 However, the effect of drop size on the contact angle measured on surfaces with anisotropic topography using submicroliter drops has not yet been studied. Here, we report water contact angles measured using drop size ranging from picoliters to microliters on surfaces with parallel grooves. PMMA surfaces embossed with grooves were coated with plasma polymers to achieve a relatively hydrophobic surface and a more hydrophilic surface. Micrometer-width grooves of five different dimensions were fabricated on PMMA substrates using hot embossing. The relationships between the water contact angle and the drop volume is investigated, with comparison made to predictions by Wenzel and Cassie equations.

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Figure 1. Water contact angle measured on a flat surface coated with ppAAm and ppHex. Values are averages of five measurements. Error bars are the measured standard deviation. The average roughness (Ra) values are 2.8 ( 0.2 nm (mean ( standard deviation) and 3.3 ( 0.8 nm for ppAAm- and ppHex-coated surfaces, respectively.

Experimental Section Topography Preparation. Topography with equally sized alternating grooves and ridges where the grooves are 3 µm deep with five different widths (5, 10, 25, 50, and 100 µm) was fabricated using hot embossing. Silicon masters with designed topography were made by photolithography and reactive ion etching. PMMA sheets with a diameter of 10 cm were compressed against the silicon master under a pressure of 0.48 MPa at 180 °C for 3 min. The pressure was allowed to decrease to 0.24 MPa over a period of approximately 5 min, at the same time as the temperature decreased to 70 °C. The master was then released from the PMMA replica, which was allowed to cool to room temperature. Plasma Polymerization Coating. Plasma polymerization was conducted in a T-shaped borosilicate glass chamber. Plasma was initiated by a 13.56 MHz radio frequency power source (Coaxial Power System Ltd.). Plasma polymerization was carried out at a power of 20 W under a working pressure of 300 mTorr. The amounts of deposited plasma-polymerized allylamine (ppAAm) and hexane (ppHex) were controlled by reading a quartz crystal microbalance above the sample in the plasma chamber. The plasma power source was switched off when the reading of the thickness of plasmapolymerized polymer reached a thickness of 50 nm. Allylamine and hexane were used as monomers and were obtained from SigmaAldrich. AFM and Water Contact Angle Measurements. A Nanoscope IIIa AFM (Digital Instruments) operating in tapping mode was used to examine the surfaces of ppAAm- and ppHex-coated PMMA. The roughness mean value was measured at five different places on the sample. Water droplets with four different sizes, namely, 0.4, 4, 12, and 40 nL, were dispensed by a piezo doser onto each polymer sample using a DSA100 contact angle measurement machine (Kru¨ss, Germany). Measurements were taken over five areas of 10 × 10 µm2 for each polymer sample, from which average and standard deviation values were calculated. A CAM200 instrument (KSV Instruments, Ltd.) was used to dispense ∼1.3 and ∼4.5 µL volume water droplets onto each polymer sample. Again, five WCA measurements were taken for each polymer sample over different areas. Ultrapure water was used for all WCA measurements (18.2 MΩ resistivity at 25 °C). The WCAs were measured using a circle and a Young-Laplace fitting function for water drops with volumes of less than and more than 1 µL, respectively. The camera for recorded images of drops worked at a speed of 107 frames/s. Because picoliter drops evaporate much faster than microliter drops, to minimize the effect of evaporation on the accuracy of WCA measurements, all WCAs were measured from the first image of the drop profile on the surface. It has been previously reported that when measured from the first image of the drop profile on flat surfaces, the WCAs measured using picoliter drops were close to those measured using microliter drops.16 No oscillation of drops (16) Taylor, M.; Urquhart, A. J.; Zelzer, M.; Davies, M. C.; Alexander, M. R. Langmuir 2007, 23, 6875–6878.

was observed after being dosed by the DSA100 small drop dispensing unit. This is supported by the video in the Supporting Information. The oscillation-free behavior of picoliter drops might be attributed to their small volumes and correspondingly small momentum.

Results and Discussion The WCAs measured on planar ppAAm- and ppHex-coated surfaces using drops varying from 0.4 nL to 4.5 µL are presented in Figure 1 as the cosine of the WCA (θ) versus the drop size represented by the base diameter of the drop. The WCAs were observed to increase for larger drops on the ppAAm surface. In contrast, WCAs on ppHex-coated surfaces were approximately unchanged within the range of drop volumes used (0.4 nL to 4.5 µL). The systematic increase in the WCA with increasing drop volume may be caused by the line tension of the three-phase contact line.17-19 The line tension was predicted by Gibbs to be a 1D analog of the surface tension.20 A modified Young equation was proposed to account for the effect of line tension on the contact angle21

cos θ ) cos θ∞ -

γSLV γLVr

(1)

where θ ) θ∞ for r f ∞. Equation 1 indicates that there is a linear relationship between the cosine of the contact angle (cos θ) and the reciprocal of the drop base radius (1/r). The line tension can be calculated from the slope of a plot of cos θ versus 1/r as shown in Figure 1. The linetensionsmeasuredforppAAm-water-airandppHex-water-air systems are 3 × 10-5 and -4 × 10-7 J m-1, respectively. Theoretical analysis of the three-phase contact line indicates that the line tension value ranges from 1 × 10-12 to 1 × 10-10 J m-1.18 However, experimental line tension values are much larger because of the limit of resolution of instrumental techniques and the limit of accuracy of measurements. In the case of measuring the line tension involving a solid phase, difficulties arise from imperfections in the solid surface with respect to the heterogeneity, roughness, elasticity, and reactivity with the contacting liquid.22 Because the average roughness values of ppAAm and ppHex are (17) Amirfazli, A.; Kwok, D. Y.; Gaydos, J.; Neumann, A. W. J. Colloid Interface Sci. 1998, 205, 1–11. (18) Drelich, J. Colloid Surf., A 1996, 116, 43–54. (19) Drelich, J.; Miller, J. D.; Good, R. J. J. Colloid Interface Sci. 1996, 179, 37–50. (20) Gibbs, J. W. The Collected Works of J. Willard Gibbs, Yale University Press: London, 1957; p 288. (21) Boruvka, L.; Neumann, A. W. J. Chem. Phys. 1977, 66, 5464–5476. (22) Drelich, J.; Miller, J. D. J. Colloid Interface Sci. 1994, 164, 252–259.

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Figure 2. Optical images (top view) of the PMMA substrate with alternating grooves and ridges of equal sizes. The scale bar is 100 µm. (a) PMMA substrate with 5 µm grooves. The inset is the top view of a ∼1.3 µL water drop on a ppAAm-coated surface with 5-µm-wide grooves. Profile view of a sessile drop (1.3 µL) on a ppAAm-coated surface (b) parallel and (c) perpendicular to the grooves.

similar, the differences in the sign and magnitude of the line tension may be attributed to surface factors such as elasticity and reactivity. The optical images of a water drop placed on a grooved surface are shown in Figure 2 from above (a) and in profile parallel (b) and perpendicular (c) to the grooves. The WCAs measured from the parallel direction increased significantly with larger drops (0.4 nL to 1.3 µL) as shown in Figure 3a,b. In contrast, WCA measured from the perpendicular direction only increased slightly for most drop sizes (Figure 3c,d). The increase in WCAs measured from the parallel direction with increasing water drop volume on the ppHex-coated surface was much less than those measured on the ppAAm-coated surfaces. WCAs measured from the parallel direction on 5 and 10 µm grooves were higher than those measured on the other three grooves when 0.4 and 4 nL water drops were used (Figure 3b). There was a notable increase in WCAs measured on 25, 50, and 100 µm surfaces when the drop volume was increased from 4 to 40 nL (Figure 3b). Viewed in the plane perpendicular to the grooves, WCAs on all grooved surfaces were similar to those on flat surfaces except for the largest drop volume of 4.5 µL (Figure 3d). The upper and lower dashed lines in Figure 3 represent the predictions of the Cassie and Wenzel models, respectively. The Wenzel and Cassie equations are used to calculate WCAs on rough surfaces. The Wenzel equation is recommended when the drop wets the whole surface, and the Cassie equation applies when the drop/solid surface is partially wet (i.e., the drop/solid surface is an air/solid composite). These two models are usually used to calculate WCAs of drops with circular three-phase contact lines on isotropic surfaces and are used here to provide a theoretical comparison with the experimental data. The equations for the two models are shown below:

Cassie equation: cos θ ) f1 cos θ1 + (1 - f1)cos θ2 (2) Wenzel equation: cos θ ) r cos θi

(3)

θ is the apparent contact angle, and f1 is the area fraction of the solid surface of a composite wetting stage (solid/air). In this case, f1 ) 0.5 because the grooves and ridges are equal in size. θ1 and θ2 are the intrinsic contact angles on surface 1 (solid surface) and surface 2 (air, θ2 ) 180°), respectively, and r is the ratio of the real contact area to the projected area:

r)

w+d w

w and d are the width and depth of the groove, respectively. The value of WCA predicted by the Wenzel model in Figure 3 is

calculated using w ) 5 µm, d ) 3 µm, and θi ) 59.1 (WCA measured using 1.3 µL water droplets on the flat ppAAm surface). The WCA predicted by the Cassie model in Figure 3 is calculated using θi ) 94.4° (WCA measured using 4.5 µL water droplets on the flat ppHex surface). Interestingly, the WCA measured from the perpendicular direction using 4.5 µL water droplets on ppHex surfaces approached the prediction by the Cassie model with decreasing groove width (Figure 3d). We postulate that as grooves become smaller it is more difficult for the microliter droplet to intrude into the recessed areas, and the contact area becomes an air/solid composite state on the hydrophobic surface (i.e., the wetting mode of drops on hydrophobic surfaces changed from that described by the Wenzel model to the Cassie description while the grooves become smaller).23,24 However, except the data from the hydrophobic grooved substrate viewed from the perpendicular plane (Figure 3d), a large deviation of measured WCAs compared with predictions by the Wenzel and Cassie equations was found. This indicates that the Wenzel and Cassie models are not suitable for predicting WCA on anisotropic surfaces because they do not consider the pinning effect of the surface patterns. One particularly interesting aspect of these findings is that WCAs measured from the perpendicular direction were similar to those on the flat surface when submicroliter drops were used (Figure 3c,d). In addition, WCAs using subnanoliter drops (0.4 nL) showed good agreement with those on flat surfaces except for that measured from the parallel direction on the hydrophobic ppHex surface with 5 µm grooves. Further interrogation of Figure 3a,b indicated that the WCAs measured on ppHex and ppAAm parallel to the grooves reached a plateau with increasing drop size. This suggested that the effect of topographic feature size on WCA may be dependent on the relative water drop and topographic feature size. To take this into account, we investigate the relationship between the WCA difference (between flat and topographically patterned surface) and the ratio of the water drop base diameter to the groove width. Because the contact line is not circular in this case, the diameter refers to either the long or the short axis of the distorted drop depending on which is viewed. The difference between the WCA measured on the grooved surface and that measured on the flat surface is plotted against the ratio of drop base diameter/groove width in Figure 4. Only small WCA differences were found when the drops were viewed parallel to the grooves with 0.4 nL droplets on the ppAAm surface (Figure 4a). For water drops larger than 0.4 nL, the WCA increased prominently before reaching a plateau, indicating a threshold ratio (approximately 10) of drop base diameter to groove size beyond which the influence of topographic on WCA became marginal. In contrast, WCAs measured from the perpendicular direction on the ppAAm surface are much less dependent on the ratio (Figure 4c). A similar threshold ratio was also observed for the ppHex surface whereas the water drop was smaller than 40 nL (Figure 4b). WCAs measured from the perpendicular direction on ppHex surfaces were similar to that on the flat surface, with the exception of the largest 4.5 µL drop, for which there was a sharp increase in WCA when the ratio increased from ∼20 to ∼80 (Figure 4d). The marked dependence of WCA on drop size for grooved surfaces may be attributed to the pinning effect of the groove edge. During the process of reaching the equilibrium state after contacting the surface, the drop that is initially present in the air as a sphere has to overcome a series of energy barriers caused (23) Cassie, A. B. D. Discuss. Faraday Soc. 1948, 3, 11–16. (24) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988–944.

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Figure 3. Water contact angle vs water drop volume on grooves of different sizes. The upper and lower dashed lines in each graph represent the predicted WCA using the Cassie and Wenzel models, respectively.

Figure 4. Water contact angle difference vs ratio of drop base diameter to groove width. The difference is calculated by subtracting the WCA on the rough surface from that on the flat surface. θi and θ1 used in both Wenzel and Cassie equations are WCAs measured on flat surfaces using microliter droplets.

by the groove edge. The final state of the drop will stay at a metastable state at which its free surface energy is lower than its neighboring states.11 We postulate that during the process of reaching a metastable state, differently sized drops reach different metastable states that correspond to different WCAs. A mild increase in the apparent contact angle (140.4 to 150.7°) has been observed with increasing drop volume (0.59 to 5.68 mL) on a surface with parallel grooves (groove width and depth, 25.6 and 30 µm, respectively). The experimental results agreed well with predicted WCAs based on minimizing the free energy of the system to obtain the equilibrium drop shape using a algorithm developed by Brakke.12 The increase in WCA could be more as drop volume increases from picoliters to microliters. However,

a numeric simulation for predicting WCA on surfaces with microgrooves is beyond the scope of this study.

Conclusions Water contact angles measured using picoliter to microliter water drops on model hydrophilic and hydrophobic grooved surfaces have been reported for the first time. A greater WCA is measured when the drop is viewed from the direction parallel to the grooves compared with that measured in the perpendicular direction, which is close to that on the flat surface in most cases. Interestingly, even on the more hydrophilic ppAAm-coated surface, WCAs measured from the parallel direction exhibit an apparently hydrophobic WCA of approximately 120°. The

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significant change in WCAs measured when varying the volume of the drop from picoliters to microliters indicates that great attention needs to be paid when measuring contact angles on rough surfaces using submicroliter drops. The authors recommend showing drop size alongside contact angle results in future studies on anisotropic wetting because of the impinging effect of anisotropic surfaces on drops. The WCAs measured from the perpendicular direction with submicroliter water drops on all grooved surfaces tested were found to be a good indicator of the WCA measured on flat surfaces.

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Acknowledgment. This work was financially supported by the BBSRC (grant no. BB/E012256/1). We thank M. Robertson for her help with the fabrication of topographically patterned substrates. Supporting Information Available: Video of a picoliter water drop profile after contacting the surface. This material is available free of charge via the Internet at http://pubs.acs.org. LA803942H