ARTICLE pubs.acs.org/JPCC
Dependence of Wetting Behavior on the Thickness of Highly Viscoelastic Films 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:
Both static and dynamic contact angle measurements were used to examine the impact of thickness on the wetting of highly viscoelastic films. The films are composed of a thermally stripped, solvent-borne acrylic polymer. Results show that wetting behavior is strongly influenced for polymer coatings < 10 μm thick and plateaus above this. This dependency is attributed to the extent to which surface tension forces can induce deformation. For thinner coatings, surface deformation is limited, which allows propagation of the three-phase line, resulting in low contact angles for sessile drop measurements and little or no discernible pinning in dynamic wetting measurements. With increasing film thickness, residual deformations due to the pinning of the wetting line start to appear and become quite distinct at thicknesses corresponding to the plateau wetting behavior. The onset of pinning is accompanied by nonwetting contact angles for sessile drops and stick-and-break propagations in dynamic measurements. The observed relationship between wetting behavior and decreased coating weight is similar to that reported previously between (static and dynamic) contact angle and increased wetting rate. It appears both relationships result from an increase in the elastic response of the film. The findings reviewed demonstrate that both film thickness and wetting rate can be used to control drop movement on viscoelastic surfaces.
’ INTRODUCTION Recently, wetting of soft materials has received considerable attention, due, in large part, to the increasing interest in microfluidic devices.13 An important consideration with soft surfaces is the potential influence of the vertical surface tension component, which can induce the formation of a ridge structure along the contact line, that is, a wetting ridge.48 Such deformations can have a significant impact on wetting behavior often as important as surface chemistry or topology. Shanahan and Carre proposed the well-known “viscoelastic braking” mechanism that slows the three-phase line as it moves toward its equilibrium position.810 This is believed to result from the dissipation of capillarity energy through surface deformation cycles, which occur at the contact line as it propagates. Published research related to this mechanism focuses on dynamic expansion of liquid droplets driven by capillarity forces (e.g., radiustime relationships). Little attention has been paid thus far to the mechanistic detail for the movement of the wetting line. Previously, the authors published studies showing that propagation of wetting lines for various liquids on r 2011 American Chemical Society
highly viscoelastic acrylic surfaces produced complex patterns during advancing and receding motions.11,12 These patterns were found to be dependent on properties of both the liquids and the surface and indicate that wetting processes can be controlled by adjusting surface mechanical properties. For thin, soft coatings, thickness is an important variable in determining mechanical behavior. Given that the influence of a viscoelastic film on wetting appears to be governed primarily by its mechanical response, film thickness is likely a key parameter. However, there appears to be few published studies on this issue. Pericet-Camara et al. studied the effect of film thickness on deformation induced by static sessile droplets from both surface tension (deformations along the contact line) and capillarity pressure (indentation of a quasi-spherical dimple underneath the drop).13 When the length scale of surface deformation is Received: June 16, 2011 Revised: August 8, 2011 Published: August 15, 2011 18729
dx.doi.org/10.1021/jp205662v | J. Phys. Chem. C 2011, 115, 18729–18735
The Journal of Physical Chemistry C comparable with the film thickness, such surface deformations became dependent on the film thickness. Below a certain critical film thickness, the ridge at the drop rim was less elevated than for the bulk. For dynamic wetting, Voue et al. reported a transition from viscous dissipation inside the liquid drop to viscoelastic dissipation through surface deformation as the thickness of the film increased.14 Similar transitions were observed as dynamic wetting rates for an advancing liquid were slowed.15 These results for both static and dynamic wetting indicate that the ability to deform a surface is critical in determining the extent to which the deformation impacts the wetting. In this paper, results are reviewed for a study on the relationship between the wetting behaviors on a highly viscoelastic polymeric film, gauged via contact angle using sessile drop and dynamic Wilhelmy plate measurements, and the thickness of the film. Contact angles for sessile drops deposited on the polymer are shown to increase and plateau with film thickness. For dynamic wetting, typical stick-and-break behavior is observed for water on sufficiently thick film surfaces. However, when coating weight is decreased, patterns become irregular, eventually disappearing completely. The absence of wetting ridges is what is commonly found, given that most polymeric surfaces are rigid enough to resist deformation. The results again demonstrate that, like physical (Wenzel model) and chemical (CassieBaxter model) heterogeneity, under certain circumstances, mechanical properties of the wetting surface can play a critical role in controlling behavior. These results are particularly important for microfluidic devices involving thin polymeric layers or wall structures and provide possible means for controlling drop movement.
’ MATERIALS AND METHODS The acrylic thermoplastic polymer used to produce films was synthesized and processed at Franklin International Co. (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. The synthesis product is subjected to additional unit operations to remove the solvent, residual monomers, and other additives. The polymer has a weight-average molecular weight of approximately 150 000 g/mol and a glass transition temperature of less than 50 °C. At room temperature, it possesses a Young’s modulus in the range of 10100 kPa, similar to that of hydrogels and biological tissues. A master curve of the polymer dynamic mechanical properties shows that, at room temperature and low deformation rates believed typical of wetting processes, viscous behavior dominates with both its storage modulus (G0 ) and its loss modulus (G00 ) being below 10 kPa.15 Coatings for wetting experiments were generated with the spin-coater KW-4A (Chemat Technology, Northridge, CA). The polymer was dissolved in tetrahydrofuran solvent and spincoated on glass plates (22 mm × 22 mm × 1.5 mm) (Thermo Fisher, Waltham, MA). The film thickness was varied through changes in spin-coater rotation speeds and polymer solution concentrations. The film thickness was gauged using an alpha 300R confocal Raman microscope equipped with a UHTS200 spectrometer and a DV401 CCD detector from WITec (Ulm, Germany). A Nikon 100× objective was used for all measurements. An Ar-ion laser with the wavelength of 514.5 nm and maximum power of 50 mW was used for excitation. The lateral resolution of the confocal Raman microscope according to the
ARTICLE
theory of light diffraction is about 250 nm, and the vertical resolution is about 500 nm. The scan area of the sample was about 10 μm × 10 μm in the XZ plane with a resolution of 60 × 50 points. The integration time was 0.1 s for each point. Raman imaging was performed by raster scanning a sample under the microscope objective. An array of spectra was collected with the same integration time at each pixel location. Raman images were generated by integrating the intensity of the characteristic peak of the CH bond in the polymer and rendering the peak intensity as brightness for each pixel location.16 The static contact angle was measured under ambient conditions by the sessile drop method with the Kruss Drop Shape Analysis System DSA10 (Hamburg, Germany). Deionized water drops of 5 μL in volume were gently lowered and attached to polymer coatings from an acid-cleaned, 250 μL glass syringe. The water was purified to 18 MΩ 3 cm with a Type II Laboratory SpectraPure Water System (Tempe, AZ). Static contact angles were obtained from a fit of drop profiles using a horizontal baseline. Analyses were carried out within seconds of drop placement. A minimum of 10 drops were analyzed and averaged per film sample. Wetting ridge shape profiles were obtained by placing and drying water drops on polymer films coated with 50 Å of platinum. The optical profilometry was then performed using a Veeco Wyko NT1100 Optical Profiler (Edina, MN). Dynamic contact angle measurements were performed at room temperature using a Kruss model K100 tensiometer (Hamburg, Germany) according to the Wilhelmy plate method. Polymercoated plates were immersed in test probe liquids at a constant velocity. The formamide, ethylene glycol, and glycerol were purchased from Mallinckrodt Chemicals (Philipsburg, NJ) with 99% purity. The paraffin oil was purchased from Fisher Scientific (Fair Lawn, NJ). Force data were collected for every 0.02 mm of plate displacement. Tack tests were performed with an Instron model 5542 (Norwood, MA) equipped with a stainless steel spherical-tip probe of diameter ∼11 mm. An initial application force of 0.5 N and 20 s of dwell time were utilized. The maximum force necessary to separate the probe from the polymer surface (max tack force) was recorded during its retreat at a rate of 0.15 mm/s.
’ RESULTS AND DISCUSSION Because of the highly viscoelastic and transparent nature of the polymer used in this study, it was not possible to accurately obtain film thickness values using techniques, such as atomic force microscopy, optical or contact profilometry, and cross-sectional imaging using scanning electron microscopy. Instead, confocal Raman microcopy (CRM) was used for this purpose. CRM is a noncontact technique that provides depth profiles based on chemical composition. This technique has been applied to characterize film polymorphs and map component distributions in the out-of-plane direction of thin films.16,17 Although it does not appear that CRM has been utilized to gauge film thickness in the past, the application would appear to be well within its realm capabilities. Figure 1a shows Raman spectra for the acrylic film. The intense peak located near 2800 cm1 is associated with saturated-hydrocarbon, CH stretching.18 Figure 1b shows the depth profile generated according to the distribution of intensity for this peak over the entire XZ scanning area. It can be seen that the polymeric film is easily distinguishable from its surroundings and glass substrate on which it is coated. Both of these phases 18730
dx.doi.org/10.1021/jp205662v |J. Phys. Chem. C 2011, 115, 18729–18735
The Journal of Physical Chemistry C
ARTICLE
Figure 2. Relationship between the static contact angle and thickness of the acrylic polymer film.
Figure 1. (a) Raman spectrum of the acrylic polymer and (b) thickness mapping based on the intensity of scattering associated with saturated CH bond stretching.
appear as dark regions due to the absence of CH bonds. The color yellow is used to indicate regions where scattering by the CH bonds is present with the intensity being related to bond concentration. The distance between the intense colors was then used to determine film thickness. As a check on the use of CRM for this purpose, the film thickness determined via CRM was plotted against coating weight subsequent to spin-coating. This produced a straight line (r2 = 0.94). It appears that CRM provides an accurate as well as novel method for measuring film thickness. Figure 2 shows the contact angles for water on glass coated with the polymer at various thicknesses. For the uncoated glass, cleaned thoroughly with tetrahydrofuran and dried, the measured contact angle for water was 64.3 ( 2.3°. The contact angle for water on the polymer-coated plate increased with film thickness until it reached a plateau value of 113.0 ( 1.8° for film thicknesses greater than approximately 10 μm. There was a clear difference in the way that sessile water drops reached their apparent angles for the different film thicknesses. For the thinner films on which water demonstrated partial wetting, contact lines of placed drops slowly expanded to decrease contact angles until stable apparent values were reached, which required only a few seconds. When these drops were removed, there was no visible wetting ridge. For the thicker films on which water demonstrated contact angles greater than 100°, wetting lines did not expand and appeared to be pinned immediately upon coming into contact with the polymer coating. When these drops were removed, a distinct wetting ridge along the initial drop perimeter remained on the polymer coating. Figures 3a and 3b show wetting ridge profiles measured via optical profilometry for polymer films 3 and 40 μm thick, respectively. Both wetting ridge profiles possess characteristics of those previously described, for example, symmetric sloped structures. However, the width and height of the ridge on the thicker film are substantially greater than those found on the thinner film. The height and width of the thin film ridge are about
0.1 and 6 μm, respectively, whereas the height and width of the ridge for the thicker film are about 13.4 and 300 μm, respectively. The magnitude of the ridge formed on the thinner film is similar to that reported for soft elastomers, which could be carried with contact line motion.8 Figure 4 shows forcedisplacement curves obtained for polymercoated, glass plates when advanced into deionized water at the rate of 0.5 mm/min. It can be seen that, when the film thickness is less than 1 μm, the force displacement curve is smooth, indicating an absence of the stick-and-break mechanism and, in general, inhibition to the movement of the wetting line. With increasing film thickness, the forcedisplacement curves first start to show an irregular pattern. Such patterns turn quasi-periodic with a period similar to the separation distance between wetting ridges, as described previously. With a further increase in film thickness, pseudoperiodic structures characteristic of the stick-and-break mechanism appear. This mechanism begins with the establishment of an initial contact line, where the vertical surface tension component acts on the polymer surface inducing deformation. Because of the highly viscoelastic nature of the polymer coating, large deformations cannot be propagated and remain at their initial position. The result is an increasing contact angle and force on the plate as the plate continues to advance into the water. Eventually, as the pinned wetting line is pulled to greater depths beneath the global liquid level, the meniscus breaks and the restrained liquid fills the void region to establish a new wetting line. This accounts for the sharp, nearly vertical, decrease in the magnitude of the measured force. As the plate is advanced into the liquid, the process is repeated, which explains the pseudoperiodic nature of the patterns in the forcedisplacement curves. The maximum force obtained during the pinning process and the distance between where the contact lines are pinned are defined as the pinning force and pinning distance, respectively. These quantities are marked on the figure. Figure 5a shows the average pinning distance between each stick-and-break cycle as a function of film thickness for water at various advancing rates. Figure 5b shows the thickness-pinning distance data for glycerol, formamide, ethylene glycol, and paraffin oil measured at a constant advancing rate of 0.5 mm/ min. When the rates are varied, similar results are observed for film thicknesses below about 810 μm, but significant differences are observed above this as curves diverge. For a particular film thickness, the pinning distance decreases with an increasing advancing rate for the thicker films. Thus, it appears that 18731
dx.doi.org/10.1021/jp205662v |J. Phys. Chem. C 2011, 115, 18729–18735
The Journal of Physical Chemistry C
ARTICLE
Figure 3. Residual deformation left after the evaporation of a water droplet placed on (a) a thin (3 μm) film and (b) a thick (40 μm) film.
increasing film thickness has an influence on wetting that is at least qualitatively similar to that induced by slowing down the wetting rate.15 When different liquids are used to wet the polymer surface at a given rate, plotted pinning distance versus film thickness curves initially climb then level. The precise film thickness or thickness range where leveling occurs is difficult to determine, but it does appear to occur near 10 μm for all of the liquids tested, which is consistent with the contact angle results. The fact that different plateau values are obtained with the different liquids is expected. It was previously shown that the magnitude of the wetting ridges formed on the plate can be connected to the spacing between the ridges. For a fixed advancing rate, the magnitude of the ridge will be dependent on the magnitude of the vertical surface tension force induced during the stick-and-break cycle, which is dependent on the wetting properties for the particular liquid. Here, it can be seen that, for the same advancing rate, water provides the largest pinning distance, whereas paraffin oil provides the smallest.
For changes in wetting rate, the connection to mechanical properties appears clear. Higher wetting rates result in more rapid deformation, which, for viscoelastic materials, produces a more elastic response, as discussed previously: an argument that was bolstered with the measured master curve for the dynamic mechanical properties of the polymer. (The height of the wetting ridge, depending on the rate/contact time, could range from around 100 nm to 30 μm.) It is also evident from previous work that different liquids generate different magnitudes of force on the coating surface. However, the connection between film thickness and mechanical properties, a result that appears to be independent of the liquid involved, is less intuitive and can be better described through the example of a probe-tack test. Tack is a measure of the ability of a film to rapidly wet a surface and form an adhesive bond.19 In practice, tack measurements gauge the force or energy necessary to separate two surfaces that are brought together at low pressure (e.g.,
