Water Evaporation on Highly Viscoelastic Polymer Surfaces

May 30, 2012 - As a result, the drop area is unable to shrink and actually increases to a small extent due to drop gauge pressure and viscous characte...
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Water Evaporation on Highly Viscoelastic Polymer Surfaces Gang Pu and Steven J. Severtson* Department of Bioproducts and Biosystems Engineering, University of Minnesota, 2004 Folwell Avenue, St. Paul, Minnesota 55108, United States ABSTRACT: Results are reported for a study on the evaporation of water droplets from a highly viscoelastic acrylic polymer surface. These are contrasted with those collected for the same measurements carried out on polydimethylsiloxane (PDMS). For PDMS, the evaporation process involves the expected multistep process including constant drop area, constant contact angle, and finally a combination of these steps until the liquid is gone. In contrast, water evaporation from the acrylic polymer shows a constant drop area mode throughout. Furthermore, during the evaporation process, the drop area actually expands on the acrylic polymer. The single mode evaporation process is consistent with formation of wetting structures, which cannot be propagated by the capillary forces. Expansion of the drop area is attributed to the influence of the drop capillary pressure. Furthermore, the rate of drop area expansion is shown to be dependent on the thickness of the polymer film.

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INTRODUCTION The evaporation of water drops from a solid surface is a familiar occurrence, which is part of many industrial, climate, and biological processes. As such, it has received considerable attention from scientists and engineers over the years. In principle, drop evaporation is much like the steady dissolution of a spherical particle into a continuous liquid phase for which the dissolution rate is inversely proportional to particle radius.1 In fact, similar relationships have been reported for the evaporation of liquid drops.2−5 However in the case of a sessile drop, the geometry is roughly that of a spherical cap with a radius that is dependent on the contact angle, which in theory is determined by Young’s equation.6 Furthermore, as the liquid evaporates, the drop shape often changes in a variety of ways as gauged by profile characteristics such as base radius, height, and contact angle. Relationships between characteristics of drop size and shape and evaporation time for water have been studied on a variety of smooth and textured surfaces for which superhydrophobic and -hydrophilic and a combination of wetting behaviors are induced.2−5,7−11 From these studies, two distinct mechanisms have been identified: one in which the three-phase line as well as its interfacial area with a surface remain constant during the process while the contact angle decreases with drop volume, and a second for which the contact angle is constant while the drop area decreases with volume.2,3,12,13 For water, the first mode is more commonly observed for hydrophobic surfaces and the second on those that are hydrophilic. However, in practice, both mechanisms are usually demonstrated over the span of the entire evaporation process. For example, evaporation of water from most polymeric surfaces initially occurs about a pinned three-phase line but later progresses to a © 2012 American Chemical Society

mixed-mode process involving a decreasing angle and a reduction in drop area that lags behind the volume loss.13−17 In the evaporation processes reviewed here, motion of the three-phase line is completely arrested due to the development of deformation ridges on the polymer surface, a situation not previously examined.18−20 The formed wetting ridge structures are induced by the vertical component of the surface tension acting on the three-phase line. The capillary energy that normally impels wetting lines toward equilibrium positions is in this case insufficient to do so because of the added requirement of also propagating the highly viscoelastic ridge structure. As a result, the drop area is unable to shrink and actually increases to a small extent due to drop gauge pressure and viscous character of the polymer surface. A closely related phenomenon is that of coffee-ring formation in which colloidal particles deposit along the contact line to form a ring-shaped ridge during evaporation. Although the mechanism involved is still not entirely understood, it appears to result from the pinning of the contact line and suppression of Marangoni flow.21,22 That is, the high evaporation rate near the contact line coupled with the presence of materials that reduce surface tension inhibits the natural flow of liquid back into the liquid drop resulting in a build-up of colloidal particles. These effects result in accumulation of particles at the outer edge of the liquid, the extent of which is determined in large part by the size and concentration of colloidal particles. A near complete pinning of the contact line for the entire evaporation process occurs for polystyrene bead suspensions when concentrations are above Received: April 30, 2012 Revised: May 30, 2012 Published: May 30, 2012 10007

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Figure 1. Water droplet mass versus time during evaporation from (a) PDMS and (b) the acrylic polymer films. The average evaporation rate (initial mass divided by the time required for complete evaporation) and initial droplet mass relationship is shown in plot (c). The evaporation time versus initial water drop mass is shown in plot (d). Note for plots c and d, solid and open circles represent PDMS and acrylic polymer films, respectively.

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8%.23 The transition between Cassie−Baxter and Wenzel wetting is another example where fixed contact area evaporation is distinctly identified, at least temporarily.24−27 As water is no longer suspended across a naturally rough or textured surface and flows into it, the receding angle typically decreases to a lower threshold value. Drop height and radius then decrease with a constant contact angle up to the final stage of evaporation, which usually involves a complex mixed-mode evaporation processes. The experimental monitoring of evaporation processes is challenging, and obtaining reliable data on contributions from the different mechanisms involved at various stages in the process is difficult. The polymer used in the study reviewed here is a solvent-based acrylate produced primarily from 2ethylhexyl acrylate. The polymer is thermally stripped in multiple stages eliminating components that would interfere with accurate contact angle measurements. The polymer is highly viscoelastic, which is necessary for its primary application as a pressure-sensitive adhesive. It tends to be nonpolar with an intrinsic contact angle for water of approximately 113°. It was shown previously that this polymer provides for the complete pinning of the wetting line for sessile drops allowing for angles of greater than 150° and down to 0° to be achieved for most liquids simply by changing drop volume.18 This effect allows for the monitoring of drop characteristics during the entire evaporation process with the three-phase line pinned in its initial position, something which has not previously been done. It also allows the use of a variety of drop sizes for a fixed evaporation process. Another unexpected finding discussed here is the impact of capillary pressure, which appears to expand the base of the drop. The results reviewed are believed to be of value in understanding phenomena such as the coffeering effect, which is utilized in DNA microarray technologies.28

MATERIALS AND METHODS

The acrylic thermoplastic was synthesized and processed at Franklin International Company (Columbus, OH). The main application for the polymer is as a pressure-sensitive adhesive (PSA). It is generated via solvent polymerization primarily from the monomer 2-ethylhexyl acrylate with much lesser amounts of amide and vinyl aromatic comonomers. Its glass transition temperature is less than −50 °C, and at room temperature, it possesses a Young’s modulus in the range of 10− 100 kPa, similar to that of hydrogels and biological tissues. The water used was purified to a resistivity of at least 18 MΩ·cm using a Type II Laboratory SpectraPure Water System (Tempe, AZ). Control experiments were carried out on films cast from polydimethylsiloxane or PDMS (Sylgard 184 Silicone Elastomer, monomer/cross-link = 10:1 in mass) obtained from Dow Corning Corporation (Midland, MI). Films were cured in a 65 °C oven for 2 h prior to use. The droplet evaporation experiments were performed in a sample conditioning room maintained at a constant temperature and relative humidity of 25 °C and 60%, respectively. Water droplets of various volumes were deposited on polymer films with a 10 μL adjustable glass syringe. Polymer films were stored in the condition room for >3 days before use. Drop evaporation images were obtained by the Kruss Droplet Shape Analysis System DSA10 (Hamburg, Germany). Drop mass data were collected with a Mettler AT250 analytical/semimicro balance (Columbus, OH). Collected digital images were analyzed using the UTHSCSA Image Tool. Images of wetting ridge structures were collected using a Veeco Wyko NT1100 Optical Profiler (Edina, MN) for polymer-coated plates. The plates were coated with 50 Å of platinum prior to testing. Several 5-μL water drops were deposited on surfaces and removed after certain times. The heights of formed ridges were determined immediately following drop removal. Ridge relaxation times were determined by removing the drop and monitoring changes in ridge height over time. 10008

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RESULTS AND DISCUSSION

Figure 1, panels a and b, show plots of water drop mass versus time for evaporation from the surfaces of the PDMS and acrylic polymer, respectively. The plots cover an initial water drop mass range of 1 to 15 mg. For both polymeric surfaces, plots are initially linear, demonstrating a nearly constant evaporation rate, but when the drops become small, i.e., are nearly gone, rates slow and the relationship deviates from linear. The average evaporation rates are plotted against initial water drop mass in Figure 1c. These data were extracted from the same drops highlighted in Figure 1, panels a and b, and are fit with a linear model. Figure 1d plots the total evaporation time for these drops versus their initial mass. When initial drops are small, they have similar evaporation times on the different polymer surfaces, but the differences in evaporation times increase with larger drop sizes. For the maximum drop mass used, around 15 mg, the drop evaporation time is about 15% longer on the PDMS surface than on the surface of the acrylic polymer. In Figure 2, images of the water drops during the evaporation process are shown. The PDMS (left) and acrylic polymer (right) have similar initial contact angles of 113.0 ± 1.8° and 112.7 ± 1.9°, respectively. Our previous optical profilometry results show that the acrylic polymer surface appears smooth with rms roughness (Rq) of about 60 nm.29 The fresh-cured PDMS on optical glass is also smooth with rms roughness (Rq) of about 1 nm.30 These results emphasize that surface morphology is not a key factor in determining the observed behavior. Although their evaporation curves (Figure 1, panels a and b) appeared to show similar behavior, the drop images indicate that different mechanisms are at work for the 2 polymers. For PDMS, a 3-stage drop evaporation process can be resolved. During the first stage the wetting line is pinned and the contact area is constant, which results in a decreasing contact angle. This is shown in Figure 2, panels a and b, on the left-hand side. During the second stage just the opposite occurs, that is, the contact angle remains constant and the contact area decreases (Figure 2, panels c and d, left). During the final stage of evaporation from polymer surfaces, both contact angle and contact area decreased until the drop is completely evaporated (Figure 2e, left). These observations are consistent with those reported previously for the evaporation of liquids from PDMS surfaces, which has been studied extensively due to its importance to the field of microfluidics. The initial pinning of the contact line reportedly lasts for several minutes under ambient conditions, with the time required for depinning being influenced by various factors including surface roughness. The influence of roughness is evident from Sefiane’s studies in which the evaporation of water/methanol sessile drops from ultrasmooth (Ra < 0.5 nm) PDMS layers was monitored.31 Under these circumstances, the initial pinning of the wetting lasted for only about 10 s. This was followed by a monotonic decrease in drop radius at a constant contact angle for most of the remaining drop lifetime, about 40 min in their experiments. In recent studies, the evaporation of water from patterned PDMS surfaces was studied.26,27 An array of various pillar structures of micrometer and submicrometer size was patterned to provide for superhydrophobicity. Results for evaporation from these surfaces indicate that the initial pinning of the contact line may not be a requirement. In fact, a constant contact angle and a decreasing contact area can be associated with the Cassie−

Figure 2. Various stages involved in the evaporation of water droplets from PDMS (left) and acrylic (right) polymer surfaces. Although PDMS demonstrates three-distinct stages of evaporation including constant drop area (a and b) and contact angle (c and d) stages followed by a mixed mode behavior (e), for the acrylic polymer, the contact line is pinned during the entire process resulting in a continuously decreasing contact angle.

Baxter state, whereas penetration of the liquid into the surface and Wenzel wetting, which can occur seemingly spontaneously, produces the opposite combination. These results indicate that the same factors that influence contact angle hysteresis, also affect the movement of the wetting line during the evaporation process. Thus it should be of no surprise that the evaporation of water from the surface of the acrylic polymer involves a relatively constant contact area and a continuously decreasing contact angle (Figure 2, panels a−e, right). (The term “relatively” is used here because the contact area actually expands slightly, which will be addressed later.) Our previous studies demonstrated a strong pinning of the wetting line for water and other liquids due to the formation of a ridge deformation induced by the vertical surface tension component. The wetting ridge structure is visible in the last image of the acrylic polymer surface. Formed wetting ridges are large and the material is highly viscoelastic, preventing propagation via capillary forces, and thus the wetting line remains fixed for a sessile drop throughout the evaporation 10009

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Figure 3. Drop height ratio (a), contact angle (b), and drop radius ratio (c) versus time for evaporation from PDMS (left) and the acrylic polymer (right) films.

time. In contrast, no transitions could be resolved for the evaporation of water droplets from the surface of the acrylic polymer. The drop height ratio and contact angle both decrease in an approximately linear fashion. Furthermore, the drop radius ratio actually increases and does so again in a regular and nearly linear fashion. By monitoring drop height, contact angle and radius, an even clearer distinction in evaporation behavior is drawn between the 2 polymers surfaces. What remains perplexing is the expansion of the drop base observed during the evaporation process for the acrylic polymer. Figure 4 shows optical profilometry images of the wetting ridge for an evaporating sessile drop after 30 s and 10 min. Both wetting ridge profiles possess characteristics of those previously described, i.e., symmetric structures consisting of 2 upward sloping sides. However, the size of the ridge formed after 10 min is substantially larger in both height (15.7 μm) and base width (250 μm) compared with the ridge formed after 30 s, which has a height and width of 1.7 and 100 μm, respectively. This difference is clearly demonstrated in Figure 4c, which compares the profiles for both wetting ridges. The result indicates that the growth of the ridge structure is time-dependent and appears to grow continuously under the load applied by the surface tension of the liquid. In fact, if the load is maintained continuously over a longer time period, the ridge continues to

process. Contrast this with the wetting of PDMS. Although ridge structures have been shown to form on PDMS polymers similar to that used here, these are orders of magnitude smaller and because PDMS is stiffer and demonstrates considerably greater elasticity compared with the acrylic, the ridge can be propagated by the wetting line. It could be said that PDMS provides an intermediate behavior between the complete pinning of the wetting line demonstrated by the acrylic and the nearly constant contact angle shown by more elastic and smooth surfaces discussed above. Figure 3 shows typical drop characteristics often used to monitor evaporation (height ratio, contact angle and radius ratio) as a function of time, Figure 3, panels a−c, respectively. Again the results for PDMS are on the left and those for the acrylic polymer are on the right. The height ratio changes roughly in a linear fashion with time for both polymers, although slight inflection points can be detected at the middle of the curve for the PDMS consistent with a three-stage process. These stages are much more evident in plots of contact angle and drop radius ratio versus time for PDMS (Figure 3b,c, left). In general, the evaporation of water from PDMS behaved as expected, with three clear stages including a constant contact area stage, followed by a constant contact angle stage and finally a more complex mixed-mode stage in the last 10 min in which the contact angle and radius greatly decreased in a very short 10010

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H = H0(1 − exp[−(t/τc)β c ])

(1A)

and H = H0 exp[− (t /τr)β r ]

(1B)

was used to fit the deformation (creep) recovery data. Fitting parameters include the maximum ridge height (H0) characteristic lifetime (τ) and shape parameter (β) with the subscripts c and r indicating fits for the creep straining and recovery stages. Values for these parameters from the fit of data are listed in Table 1. This model is based on the assumption that the Table 1. Parameters for the Fit of Data Using eqs 1A and 1B stage

H0 (μm)

τ (min)

β

creep recovery

20.73 18.86

7.93 57.4

0.97 0.75

viscoelastic changes occur through incremental jumps in which the segments of polymers move between positions of relative stability.32,33 The viscoelastic behavior is represented mechanically as the activation and deactivation of time-dependent latch elements. It was found that polymeric creep and recovery could be accurately represented by equations based on the Weibull distribution function. As can be seen from Figure 5, the model provides a good overall fit of changes in wetting ridge height. Figure 6 shows images of the polymer surface prior and subsequent to water removal during droplet expansion. The left Figure 4. Optical profilometry images of the wetting ridge for an evaporating sessile drop after (a) 30 s and (b) 10 min, and (c) the height profiles for these respective times.

grow. For example, when ethylene glycol, which was selected for its slow evaporation rate, was used instead of water, the ridge climbed to over 130 μm in height after 24 h. Data collected for changes in wetting ridge height due to surface tension forces and deformation relaxation subsequent to drop removal are shown in Figure 5. The solid curve is the theoretical fit of data using a stretched exponential model to fit the height-time data. Two equations were used here. Ridge height (H) vs time (t) data for ridge formation under the surface tension load were fit with

Figure 6. Images of the relative position between the contact line and the top of the wetting ridge before and immediately after the water droplet removal.

image has a contact angle of 102.3°, whereas the right one has a value of 85.6°. The arrows point to the three-phase lines before and after the water removal, which are located on the top of the wetting ridges in both systems. The results suggest that positions of the contact lines relative to the top of their wetting ridges are unchanged even after the center of the ridges have expanded outward. It was previously established that the capillary energy due to the difference between the time dynamic and equilibrium contact angles is insufficient to pull the drop ridge outward.18 The results above indicate that the relative position of the wetting line and ridge peak near the three-phase line is undisturbed by drop expansion. A sequence of images showing the evaporation of a 5-mg pendant droplet of water is shown in Figure 7. The water was initially placed as a sessile drop and then the plate face was slowly rotated upside down. The initial contact angle was 98.4° measured just after the plate was flipped. No differences in the evaporation process for the pendant drop versus the sessile drop could be detected. The contact angle and drop height continuously decrease while the contact line slowly expands outward increasing the droplet perimeter. The base radius

Figure 5. Wetting ridge height versus time for a sessile drop including ridge relaxation upon drop removal. The solid line is from the fit of data using eqs 1A and 1B. 10011

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tension component of the liquid pulls the wetted surface vertically upward at the three-phase line. Growth and recession of the ridge height subsequent to water removal is fit well by a viscoelastic model as shown above. This indicates that the deformation of the polymer surface to applied loads is timedependent and can be substantial even under the relatively minor loads such as those applied by surface tension forces. Experimental results reviewed here also show that the entire wetting ridge shifts outward during drop evaporation. The propagation of the three-phase line requires that this ridge also be propagated, which can inhibit or, in our case, halt the process. It has been shown that the relative position of the contact line and the ridge peak is unchanged. Thus, forces responsible for the vertical deformation are likely not the same forces inducing the outward shift. The pendant drop data would appear to rule out the influence hydrostatic pressure might have on ridge expansion. Given the discussion above and the observation that the drop area expands in a symmetric fashion, which indicates a horizontal load is being applied to the entire formed ridge structure, the most likely cause is capillary pressure. The ability of capillary pressure to induce a dimple deformation under a liquid drop has been theoretical described and characterized experimentally.34,35 The shape of the dimple formed by the capillary pressure of a sessile drop depends on film thickness. For thick films, beyond a threshold value, which depends on the properties of the material, the dimple formed is parabolic. When the thickness falls below this apparent threshold value, the formed dimple is no longer parabolic and develops a flat base or bottom, which can actually be convex, rising near the center of the drop area. For purely elastic materials, the drop boundaries are retained and the deformation is recovered once the liquid and thus the capillary pressure are removed. However, for highly viscoelastic materials, formed dimples remain upon removal of the liquid and the tension forces induced in the walls of the drop boundaries will result in an expansion of the three-phase line and an increase in the drop circumference and radius. Figure 9a plots the drop radius ratio as a function of time for the various film thicknesses. (These films were cast to represent the thin portion of the film-thickness spectrum.) Figure 9b shows the average drop expansion rate versus film thickness. The authors showed previously that the size of the deformation ridge is dependent on the thickness of the polymer film.20 It was observed that the extent of wetting deformation increased sharply with film thickness for thickness values below about 10 μm and more gradually after this. It can be seen that the drop radius ratio also climbs and plateaus with film thickness. The radius ratio is independent of contact angle. It is unclear if the ratio increase results from the increased ridge structure or other associated effects such as the increased tendency of the films to demonstrate viscoelastic behavior for the greater thicknesses.

Figure 7. Pendant drop evaporation on the acrylic polymer surface.

reached its maximum value, 8% larger than its initial size, when the contact angle decreased to about 44°. The average percent of base expansion for the sessile drops was 12.8 ± 3.8%, whereas the maximum value measured was 17.8%. The average percent of base expansion for the pendant drops was 11.6 ± 4.3%, and the maximum value was 16.3%. The percent of base expansion was found to be independent of the initial droplet mass. These similarities between sessile and pendant drop expansions indicate that gravity is not a significant influencing factor on the process. It is clear from the data presented here that the wetting ridge that forms at the edge of the drop during the evaporation expands in both the vertical and horizontal directions (Figure 8). As is well established, the unbalanced vertical surface

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CONCLUSIONS Evaporation from the surfaces of PDMS and an acrylic polymer demonstrate differences in the evaporation mechanisms for increasing degrees of viscoelasticity. For the more elastic PDMS, evaporation demonstrates the expected three stages including constant drop area, constant contact angle, and mixed mode. For the acrylic polymer, only a constant drop area mode was observed. Interestingly, during this process, the drop area actually expanded. This was shown to be consistent with an expansion of the drop circumference resulting from the liquid

Figure 8. Schematic showing the impact of capillary pressure on dimple formation and possibly deformation ridge expansion. 10012

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(4) Peiss, N. C. Evaporation of Small Water Drops Maintained at Constant Volume. J. Appl. Phys. 1989, 65, 5235−5237. (5) Rowan, S. M.; Newton, M. I.; McHale, G. Evaporation of Microdroplets and the Wetting of Solid Surfaces. J. Phys. Chem. 1995, 99, 13268−13271. (6) Israelachvili, J. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, U.K., 1992. (7) Gelderblom, H.; Marin., G. A.; Nair, H.; Van Houselt, A.; Lefferts, L.; Snoeijer, H. J.; Lohse, D. How Water Droplets Evaporate on a Superhydrophobic Substrate. Phys. Rev. E 2011, 83, 026306. (8) Zhang, X. Y.; Tan, S. X.; Zhao, N.; Guo, X. L.; Zhang, X. L.; Zhang, Y. J.; Xu, J. Evaporation of Sessile Water Droplets on Superhydrophobic Natural Lotus and Biomimetic Polymer Surfaces. ChemPhysChem 2006, 7, 2067−2070. (9) Furuta, T.; Sakai, M.; Isobe, T.; Nakajima, A. Evaporation and Sliding of Water Droplets on Fluoroalkylsilane Coatings with Nanoscale Roughness. Langmuir 2009, 25, 11998−12001. (10) Kulinich, S. A.; Farzaneh, K. Effect of Contact Angle Hysteresis on Water Droplet Evaporation from Super-hydrophobic Surfaces. Appl. Surf. Sci. 2009, 255, 4056−4060. (11) Poulard, C.; Benichou, O.; Cazabat, A. M. Freely Receding Evaporating Droplets. Langmuir 2003, 19, 8828−8834. (12) Erbil, H. Y.; McHale, G.; Newton, M. I. Drop Evaporation on Solid Surfaces: Constant Contact Angle Mode. Langmuir 2002, 18, 2636−2641. (13) Ourgen-Monnier, C.; Shanahan, M. E. R. Influence of Evaporation on Contact Angle. Langmuir 1995, 11, 2820−2829. (14) Girara, F.; Antoni, M.; Faure, S.; Steinchen, A. Influence of heating temperature and relative humidity in the evaporation of pinned droplets. Colloids Surf. A 2008, 323, 36−49. (15) Kim, J. H.; Ahn, S. I.; Kim, J. H.; Zin, W. C. Evaporation of Water Droplets on Polymer Surfaces. Langmuir 2007, 23, 6163−6169. (16) Rowan, S. M.; Newton, M. I.; McHale, G. Evaporation of Microdroplets and the Wetting of Solid Surfaces. J. Phys. Chem. 1995, 99, 13268−13271. (17) Fang, X. H.; Pimentel, M.; Solov, J.; Rafailovich, M. Dewetting of the Three-Phase Contact Line on Solids. Langmuir 2010, 26, 7682− 7685. (18) Pu., G.; Guo, J. H.; Gwin, E. L.; Severtson., J. S. Mechanical Pinning of Liquids through Inelastic Wetting Ridge Formation on Thermally Stripped Acrylic Polymers. Langmuir 2007, 23, 12142− 12146. (19) Pu., G.; Severtson, J. S. Characterization of Dynamic Stick-andBreak Wetting Behavior for Various Liquids on the Surface of a Highly Viscoelastic Polymer. Langmuir 2008, 24, 4685−4692. (20) Pu., G.; Severtson, J. S. Dependence of Wetting Behavior on the Thickness of Highly Viscoelastic Films. J. Phys. Chem. C 2011, 115, 18729−18735. (21) Deegan, R. D.; Bakajin, O.; Dupont, F. T.; Huber, G.; Nagel, R. S.; Witten, A. T. Capillary flow as the cause of ring stains from dried liquid drops. Nature 1997, 389, 827−829. (22) Hu, H.; Larson, G. R. Marangoni Effect Reverses Coffee-Ring Depositions. J. Phys. Chem. B 2006, 110, 7090−7094. (23) Conway, J.; Korns, H.; Fisch, R. M. Evaporation Kinematics of Polystyrene Bead Suspensions. Langmuir 1997, 13, 426−431. (24) McHale, G.; Aqil, S.; Shirtcliffe, N. J.; Newton, M. I.; Erbil, H. Y. Analysis of Droplet Evaporation on a Superhydrophobic Surface. Langmuir 2005, 21, 11053−11060. (25) Dmytro, S.; Golovko, S. D.; Butt, H. J.; Bonaccurso, E. Transition in the Evaporation Kinetics of Water Microdrops on Hydrophilic Surfaces. Langmuir 2009, 25, 75−78. (26) Tsai, P.; Lammeritink, G. H. R.; Wessling, M.; Lohse., D. Evaporation-Triggered Wetting Transition for Water Droplets upon Hydrophobic Microstructures. Phys. Rev. Lett. 2010, 104, 116102. (27) He, B.; Patankar, A. N.; Lee., J. Multiple Equilibrium Droplet Shapes and Design Criterion for Rough Hydrophobic Surfaces. Langmuir 2003, 19, 4999−5003.

Figure 9. Increase in radius ratio versus time for various acrylic film thicknesses (a) and the average expansion rate versus time (b).

capillary pressure, which is also responsible for the drop dimple that forms underneath the liquid. The rate and extent of drop area expansion on the acrylic polymer was shown to be dependent on the film thickness, which determines the extent of viscoelatic behavior as well as the size of formed wetting deformations.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (612) 625-5265. Fax: (612) 625-6286. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Mr. Andrew Sipple of Boston Scientific Corporation for his generous assistance in collecting optical profilometry data. This research is financially supported in part by a grant from the U.S. Postal Service, Stamp Acquisition and Distribution and the U.S. Department of Energy Project No. DE-FC36-04GO14309.

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(28) Schena, M.; Shalon, D.; Davis, W. R.; Brown, O. P. Quantitative monitoring of gene expression patterns with a complementary DNA microarray. Science 1995, 270, 467−470. (29) Pu, G.; Severtson, S. J. Characterization of Dynamic Stick-andBreak Wetting Behavior for Various Liquids on the Surface of a Highly Viscoelastic Polymer. Langmuir 2008, 24, 4685−4692. (30) Cordeiro, A. L.; Nitschke, M.; Janke, A.; Helbig, R.; Souza, F. D; Donnelly, G. T.; Willemsen, P. R.; Werner, C. Fluorination of Poly(dimethylsiloxane) Surfaces by Low Pressure CF4 Plasma − Physicochemical and Antifouling Properties. Express Polym. Lett. 2009, 3, 70−83. (31) Sefiane, K.; David, S.; Shanahan, M. R. R. Wetting and Evaporation of Binary Mixture Drops. J. Phys. Chem. B 2008, 112, 11317−11323. (32) Fancey, S. K. A Latch-based Weibull Model for Polymeric Creep and Recovery. J. Polym. Eng. 2001, 21, 489−509. (33) Fancey, S. K. A Mechanical Model for Creep, Recovery and Stress Relaxation in Polymeric Materials. J. Mater. Sci. 2005, 40, 4827− 4831. (34) Pericet-Camara, R.; Auernhammer, K. G.; Koynov, K.; Lorenzoni, S.; Raiteri, R.; Bonaccurso., E. Solid-supported Thin Elastomer Films Deformed by Microdrops. Soft Matter 2009, 5, 3611− 3617. (35) Rusanov, A. I. Effect of Contact Line Roughness on Contact Angle. Colloid J. USSR 1975, 37, 614−622.

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