Langmuir 1998, 14, 7047-7051
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Dynamic Wetting and Dewetting of a Low-Energy Surface by Pure Liquids Matthew Schneemilch, Robert A. Hayes,* Jordan G. Petrov,† and John Ralston Ian Wark Research Institute, University of South Australia, The Levels, SA 5095, Australia Received April 24, 1998. In Final Form: August 13, 1998 A high resolution optical method was used to acquire contact angle-velocity data for hexadecane and octamethylcyclotetrasiloxane (OMCTS) advancing and receding on a smooth, virtually homogeneous fluorinated surface. The predictive abilities of the hydrodynamic, molecular-kinetic and combined molecular-hydrodynamic models of wetting dynamics were compared. In this context the behavior of hexadecane and OMCTS were distinctly different. Where application of the molecular-kinetic model is most appropriate (hexadecane advancing) the motion of the contact line does agree well with the molecular length. However when hydrodynamics play a role and the combined molecular-hydrodynamic model is required, then the characteristic lengths obtained greatly exceed molecular dimensions.
Introduction The wetting and dewetting of solid surfaces, a commonly observed phenomenon, is also central to a number of biological and industrial systems. Systematic studies of wetting processes have predominantly involved equilibrium, or static, measurements. However in most cases it is the dynamic wetting and dewetting behavior that is of practical relevance.1 In either case the wetting behavior is generally characterized by the “contact angle”, measured through the denser phase, that the tangent to the liquid interface makes with the solid surface at the contact line. In the dynamic case the contact angle varies with the speed and direction of movement of the contact line.2 Models have been developed which relate the velocity dependence of the contact angle to readily measured properties such as liquid viscosity, surface or interfacial tension, and the static contact angle. The models may be broadly classified as either hydrodynamic3,4 or molecularkinetic.5,6 In the former, essentially macroscopic, case the role of the solid surface is discounted while in the latter the microscopic properties of the solid surface are explicitly accommodated. Intuitively one might expect that at high and low velocities of the contact line hydrodynamics and surface chemistry, respectively, predominate.1 An alternative view is that both surface and hydrodynamic factors are simultaneously important because they relate to the behavior of the wetting front near to and far from, respectively, the contact line. Indeed a model that combines the hydrodynamic and surface chemical contributions has recently been proposed.7 While in principle the nature of the dynamic wetting models allows their general validity to be tested using a set of contact angle-velocity data, in practice there are a number of reasons why the reconciliation of theory and
experiment is somewhat fraught. First, the predicted contact angle-velocity dependence of the models is surprisingly similar over the limited velocity ranges typically studied.2 It is therefore vital that contact angles of high accuracy and precision are measured over an extended velocity range. The solid surfaces studied are usually heterogeneous, which is confirmed by the large hysteresis in the advancing and receding static contact angles, and attempts to explicitly incorporate heterogeneity in the models are in their infancy.8 The liquids used have often been mixtures, enabling the viscosity to be varied, which also complicates the application of the models. The objective of the work reported here was first to measure the contact angle-velocity dependence for the advancing and receding movement of pure liquids, octamethylcyclotetrasiloxane (OMCTS) and hexadecane on a homogeneous low energy surface. These liquids have previously been the focus of fundamental surface forces studies9 and allow the effect of liquid properties, specifically size and shape (whether spherical or rod) on the wetting dynamics to be assessed. The solid surface used is a fluorinated polymer that is receiving growing attention as a model substrate in wetting and surface force studies.10-12 The second and major task undertaken was the reconciliation of experimental data with the hydrodynamic, molecular-kinetic and combined molecularhydrodynamic models of wetting dynamics. Models of Dynamic Wetting Hydrodynamic. The commonly used model of CoxVoinov3,4 was employed. For the contact angles and viscosities prevailing in this study7 the dynamic contact angle (θd) is related to velocity (V) by
(θd)3) (θo)3 + 9
()
ηV L ln γlv Ls
(1)
* Corresponding author. E-mail:
[email protected]. † Permanent address: Institute of Biophysics, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Block 21, 1113 Sofia, Bulgaria.
where θo is the static contact angle, η the liquid viscosity,
(1) de Gennes, P. G. Rev. Mod. Phys. 1985, 57, 827. (2) Hayes, R. A.; Ralston, J. J. Colloid Interface Sci. 1993, 159, 429. (3) Cox, R. G. J. of Fluid Mech. 1986, 168, 169. (4) Voinov, O. V. J. Fluid Dyn. 1976, 11, 714. (5) Blake, T. D.; Haynes, J. M. J. Colloid Interface Sci. 1969, 30, 421. (6) Blake, T. D. In Wettability; Berg, J. C., Ed.; Marcel Dekker: New York 1993. p 251. (7) Petrov, P. G.; Petrov, J. G. Langmuir 1992, 8, 1762.
(8) Petrov, J. G.; Ralston, J.; Hayes, R. A. Langmuir, submitted for publication. (9) Chan, D. Y. C.; Horn, R. G. J. Chem. Phys. 1985, 83, 5311. (10) Drummond, C. J.; Georgaklis, G.; Chan, D. Y. C. Langmuir 1996, 12, 2617. (11) Drummond, C. J.; Chan, D. Y. C. Langmuir 1996, 12, 3356. (12) Drummond, C. J.; Chan, D. Y. C. Langmuir 1997, 13, 3890.
10.1021/la980476k CCC: $15.00 © 1998 American Chemical Society Published on Web 10/30/1998
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γlv the liquid-vapor surface tension, L a characterizic capillary length and Ls the slip length. The normal convention, adopted here, is that the positive velocities correspond to advancing liquid motion. Ls corresponds to the thickness of the meniscus immediately adjacent to the solid wall over which the “no slip” boundary condition of classical hydrodynamics is relaxed. A cubic dependence of the dynamic contact angle on velocity is therefore predicted by the hydrodynamic model. When the predictive ability of the equation is examined, it is usually used with two free parameters, the static contact angle and the slip length. Another issue relates to the characteristic value assigned for the outer region of the meniscus, L. For this purpose the maximum capillary height, corresponding to zero contact angle, is commonly used. For a meniscus adjacent to a flat wall the Laplace equation has an analytical solution and L ) x(2γlv/Fg). In this work however the solid is a fiber of diameter 0.12 mm and numerical solution of the Laplace equation was required to obtain L. Molecular-Kinetic. These models5,13,14 are based on the application of Eyring’s15 activated rate theory to transport processes. The approach proposed by Blake,5,6 which is most commonly applied to dynamic wetting, was employed here. In this case the dynamic contact angle, θd, is related to velocity by
[
θd ) arccos cos θo -
( )]
2kT V arcsinh 2Kwλ γlvλ2
(2)
where k is the Boltzmann constant, T the temperature, and λ and Kw are parameters corresponding to the length and characteristic rate constant for molecular displacements of the contact line. From curve fitting experimental data, one typically obtains values of λ and Kw of approximately a nanometer and 106 s-1, respectively. It is often concluded that these values are physically reasonable in the context of atomic and molecular dimensions and bulk diffusion constants. It is usually proposed that λ relates to the properties of the solid surface such as the distance between adsorption sites, or centers. An assumption inherent in the use of eq 2 is that the distribution of these adsorption sites is isotropic. The fluorinated surface used in this work is amorphous and close to homogeneous and the liquids vary in size and shape; thus, the system is particularly appropriate for exploring the effect of liquid properties on λ. Combined Molecular-Hydrodynamic. Petrov and Petrov7 showed that the recession of glycerol/water solutions on poly(ethylene terephthalate) (PET) could not be predicted over the entire velocity range by either molecular-kinetic or hydrodynamic models. While the theories were adequate at intermediate velocities the molecular-kinetic and hydrodynamic theories were clearly inadequate at high and low velocities, respectively. A combination of the two models was, however, capable of fitting the experimental data over the entire velocity range. An identical procedure, which involves a combination of eqs 1 and 2, was also adopted here. This combination assumes that the “microscopic” contact angle, θo, in the hydrodynamic expression is velocity dependent and that an appropriate value can be obtained from the molecular(13) Cherry, B. W.; Holmes, C. M. J. Colloid Interface Sci. 1969, 29, 174. (14) Ruckenstein, E.; Dunn, C. S. J. Colloid Interface Sci. 1977, 59, 135. (15) Glasstone, S.; Laidler, K. J.; Eyring, H. J. The Theory of Rate Processes; McGraw-Hill: New York, 1941.
Table 1. Liquid Properties density, F viscosity, η (cP) surface tension, γlv (mN/m) maximum capillary height, L (cm) (g/cm3)
hexadecane
OMCTS
0.774 3.3a 26.9c 0.0258d
0.955 2.30b 17.7c 0.0239d
a Reference 21. b Reference 22. c Drop weight method at 23 °C, the temperature at which the dynamic contact angles were also measured. d Calculated by numerical solution of the Laplace equation.
kinetic equation. The possibility of a nonhydrodynamic velocity dependence of θo has been acknowledged by a number of workers.3,4 Materials and Methods The solid surface was prepared by dipcoating 0.12 mm diameter silica optical fibers in a 1% w/v solution of AF1600 (Dupont) dissolved in the fluorocarbon solvent FC75 (3M). The fiber was immersed in the solvent at a speed of 5 mm/s to a depth of 4 cm, and withdrawn at the same speed after 1 min. The fiber was stored for 1 h in a laminar flow cabinet to allow the solvent to evaporate, leaving a smooth, stable coating. AFM measurements of AF1600 surfaces exhibit rms roughnesses of less than 0.4 nm.10 The smoothness of surfaces can be attributed to the amorphous, rather than crystalline, nature of the fluoropolymer. OMCTS was obtained from Fluka (purum, >99%) and hexadecane from Merck (>99%) in 100 cm3 volumes which were freshly opened for each series of measurements. The liquid properties are summarized in Table 1. The solid was suspended vertically from a motorized stage (Time and Precision, Basingstoke, U.K.) with a screw of pitch 2 mm. Wetting velocities from 2 µm/s up to 20 mm/s were then obtainable with a maximum acceleration of 50 mm/s2. When using the higher velocities, the initial acceleration was limited to 5 mm/s2 to avoid inertial entrainment. As the contact angles were always less than 90°, the measurements were performed in slightly overfilled beakers. The entire apparatus was suspended on a concrete slab with elasticated rope in an enclosed cabinet to isolate the experiment from vibration and air currents. Backlit Ko¨hler illumination created a silhouette of the meniscus profile. Imaging of the interface profile was conducted with a long working distance 5× objective lens and a 6.7× field lens. Video images were captured with a Sony Iris CCD camera and stored on VHS tape before digitization of appropriate frames. At each velocity, 20 frames, on average, were selected for analysis from footage of the interface after the steady state had been reached. As the acceleration was limited, the advancing images were taken as the wetting line was close to the top of the fiber, and the receding images were taken at the bottom of the fiber. The frames were converted to 256 Gy level images with a pixel resolution of approximately 0.9 µm. A graticule with 20 µm gradations was used to calibrate images in both the x and y directions. The location of the interface was determined from the positions of the outermost image pixels with gray level values below 80. The position of the interface determined using this threshold value coincided with the location of pixels with maximum gray level gradient calculated from a five by five matrix around each pixel. The pixel coordinates were fitted to numerical solutions of the Laplace equation, generated using an algorithm proposed by Huh and Scriven.16 The contact angle was determined from the fit to the Laplace equation. It is in fact the extrapolated17 contact angle. Due to the low capillary numbers prevailing (Ca < 10-3), the interface profile obeys the Laplace equation, and thus the actual and extrapolated contact angles coincide. A more detailed discussion of the methodology is presented elsewhere.18
Results and Discussion The experimental data for OMCTS and hexadecane advancing or receding on AF1600 are reproduced in (16) Huh, C.; Scriven, L. E. J. Colloid Interface Sci. 1969, 30, 323. (17) Petrov, P. G.; Petrov, J. G. Langmuir 1995, 11, 3261. (18) Schneemilch, M. Ph.D. Thesis, University of South Australia, 1998.
Dynamic Wetting and Dewetting
Figures 1-4. At each velocity, the contact angle presented is the average obtained from analysis of at least 10 images. The variation in the contact angle values at a particular velocity is typically less than half a degree. Images of the dynamic meniscii were obtained at velocities ranging from 0.1 up to 20 mm/s. Upon cessation of forced movement, the contact angle was not found to be measurably different from the value measured at the lowest velocity of 0.1 mm/ s. For OMCTS and hexadecane the static contact angle hystereses (the difference between advancing and receding angles) are 6 and 7° respectively. For water on AF1600 the values of the static advancing and receding contact angles are 117 and 110°, respectively. The small degree of contact angle hysteresis, even for high energy liquids such as water, confirms the homogeneity of the solid surfaces used. Furthermore the measured contact angles are found to be unaffected by the time of liquid contact, a common problem for partially wetting surfaces prepared by silanation or from less inert polymers. As the magnitude of the contact line velocity is increased, then the contact angles depart from their static values. In the advancing case, the contact angle increases while receding liquid movement causes a decrease in the contact angle. The reconciliation of the experimental data with the prevailing models was a major focus in this work. The best fits of the respective models to the experimental data are also presented in Figures 1-4. The corresponding values of the fitting parameters are stated in the respective figure captions. The use of the models is discussed in turn below. Hydrodynamic. Overall this proved to be the least appropriate model. Only at low contact angles, as for OMCTS receding at high speed, was the hydrodynamic model clearly superior to the molecular-kinetic model. At low velocities the deviation between experiment and theory was particularly evident which resulted in the low velocity values being excluded from the fitting procedure. This also meant that the value of θo obtained differed by 2-3° from the measured value. Hexadecane advancing was the only exception in this regard due to the absence of low velocity contact angle dependence. The values of the slip length, Ls, obtained were less than a molecular dimension for hexadecane in the receding and, particularly, advancing cases. For OMCTS however Ls was approximately 2 and 10 nm in the advancing and receding cases, respectively. It has been commonly observed that application of the hydrodynamic model yields values of Ls that are significantly less than atomic or molecular dimensions,2,19 which has naturally led to the physical significance of Ls being questioned. Molecular-Kinetic. The molecular-kinetic model is particularly useful for describing the contact anglevelocity dependence at low and intermediate velocities. Its main deficiency is obvious at receding contact angles close to zero which lead to a dramatic overestimation of the maximum velocity of dewetting (Figure 2). The values of θo obtained agree well with the experimental values. For hexadecane the values of λ and Kw are approximately 2 nm and 106 s-1, respectively, with the difference between the advancing and receding parameters being significant. For OMCTS the parameter values obtained are similar in magnitude to those obtained for hexadecane with the difference between the advancing and receding parameters being even greater: a factor of almost 2 and 6 for λ and Kw respectively. Combined Molecular-Hydrodynamic. The combined model is clearly superior in terms of its ability to (19) Petrov, J. G.; Petrov, P. G. Colloids Surf. 1992, 64, 143.
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Figure 1. Contact angle-velocity data for OMCTS advancing on AF1600. The symbols correspond to the experimental data and the lines to the “best fit” of the respective dynamic wetting models. The fits were weighted to accommodate the variation in uncertainty (see error bars) in the experimental data. As a result the χ2 values can be used to compare the goodness of fit of the various models. The fitting parameters are listed as follows. Hydrodynamic model: θo ) 45.1 ( 0.3°, ln(L/Ls) ) 11.8 ( 0.6, and Ls ) 1.8 nm. The two low velocity points were excluded from this fit. χ2 ) 5.91 (χ2 ) 40.4 for the fit to the full data set (not shown)). Molecular-kinetic model: θo ) 42.5 ( 0.4°, λ ) 2.93 ( 0.11 nm, Kw ) (7.0 ( 1.3) × 105 s-1, and χ2 ) 15.21. combined molecular-hydrodynamic model as follows: θo ) 41.9 ( 0.6°, λ ) 5.2 ( 1.0 nm, Kw ) (6.1 ( 5.8) × 104 s-1, ln(L/Ls) ) 8.1 ( 1.8, Ls ) 73 nm, and χ2 ) 6.93.
describe the contact angle-velocity dependence over the entire velocity range. This conclusion is well supported by the respective values of the “goodness-of-fit” parameter, χ2. The most clear example of this is OMCTS receding where the experimental data is “S” shaped for which only the combined model is capable of predicting the contact angle-velocity dependence. For OMCTS the parameters obtained in the receding and advancing cases are not statistically different. In other words the advancing and receding behaviors are effectively symmetrical in the context of the model. This issue is explored in more detail below. The values of λ, Kw, and ln(L/Ls) are approximately 6 nm, 104 s-1, and 8, respectively. The values of both λ and Ls (∼50 nm) are significantly larger, and the Kw values significantly smaller, than those obtained using the hydrodynamic and molecular-kinetic models independently. A similar trend is observed for hexadecane receding which indicates that both hydrodynamic and molecular-kinetic factors affect the wetting dynamics. In the case of hexadecane advancing however the combined model returns parameter values that are essentially identical to the molecular-kinetic parameters. This is hardly surprising given the overlap of the combined and molecular-kinetic fits (Figure 4) and almost identical values of χ2. One concludes that hydrodynamics do not play a significant role, which may simply be because the contact angles involved are largest in this case (70-80°). This question may be resolved in future work involving the wetting and dewetting of AF1600 by liquids of higher energy. The Effect of Molecular Size and Shape. The molecular diameter of OMCTS is approximately 0.8 nm.
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Figure 2. Contact angle-velocity data for OMCTS receding on AF1600. The symbols correspond to the experimental data and the lines to the “best fit” of the respective dynamic wetting models. The fitting parameters are listed as follows. Hydrodynamic model: θo ) 33.9 ( 0.2°, ln(L/Ls) ) 10.1 ( 0.3, and Ls ) 9.8 nm. The low velocity point was excluded from this fit. χ2 ) 8.83 (χ2 ) 48.9 for the fit to the full data set (not shown)). Molecular-kinetic model. θo ) 36.2 ( 0.3°, λ ) 1.7 ( 0.2 nm, Kw ) (4.0 ( 1.6) × 106 s-1, and χ2 ) 12.38. Combined molecularhydrodynamic model: θo ) 37.3 ( 1.1°, λ ) 6.8 ( 1.0 nm, Kw ) (1.2 ( 1.4) × 104 s-1, ln(L/Ls) ) 8.4 ( 0.7, Ls ) 54 nm, χ2 ) 5.21.
Figure 3. Contact angle-velocity data for hexadecane advancing on AF1600. The symbols correspond to the experimental data and the lines to the “best fit” of the respective dynamic wetting models. The fitting parameters are listed below. Hydrodynamic Model: θo ) 69.1 ( 0.1°, ln(L/Ls) ) 43 ( 1, Ls ) 3.6 × 10-23 m, and χ2 ) 4.68. Molecular-kinetic model: θo ) 69.0 ( 0.1°, λ ) 1.40 ( 0.12 nm, Kw ) (5.0 ( 1.5) × 106 s-1, and χ2 ) 1.89. Combined molecular-hydrodynamic model: θo ) 69.0 ( 0.1°, λ ) 1.5 ( 0.3 nm, Kw ) (4.5 ( 1.8) × 106 s-1, ln(L/Ls) ) 5.8 ( 14.9, Ls ) 780 nm, and χ2 ) 1.84.
This is less than the values of Ls and λ obtained from any of the models. In the context of the combined model, which clearly gave the best fit to the experimental data, the
Schneemilch et al.
Figure 4. Contact angle-velocity data for hexadecane receding on AF1600. The symbols correspond to the experimental data and the lines to the “best fit” of the respective dynamic wetting models. The fitting parameters are listed below. Hydrodynamic model: θo ) 60.0 ( 0.1°, ln(L/Ls) ) 15.2 ( 0.7, and Ls ) 0.065 nm. The low velocity point was excluded from this fit. χ2 ) 1.85 (χ2 ) 49.1 for the fit to the full data set (not shown)). Molecularkinetic model: θo ) 61.1 ( 0.2°, λ ) 2.18 ( 0.15 nm, Kw ) (1.8 ( 0.5) × 106 s-1, and χ2 ) 25.7. Combined molecularhydrodynamic model: θo ) 62.3 ( 0.6°, λ ) 6.6 ( 0.5 nm, Kw ) (2.0 ( 1.8) × 103 s-1, ln(L/Ls) ) 12.3 ( 1.0, Ls ) 1.2 nm, and χ2 ) 4.78.
difference between the molecular dimension and either Ls or λ was most obvious. λ and Ls values, exceeded the molecular dimension by 1 and 2 orders of magnitude, respectively. Given that these results have been obtained on an amorphous and virtually homogeneous solid, where in principle only the liquid molecule dimensions have the potential to affect the λ or Ls values, this strongly suggests that the parameter values do not correspond to the molecular size. There are several possible explanations for this difference. The parameters obtained may simply not relate to either liquid and solid molecular properties, or the relationship may be indirect and as yet undetermined, or alternatively, they may relate directly to some process that occurs on a scale larger than atomic or molecular dimensions. In future work we aim to study a broad range of pure liquids that vary more dramatically in their size, shape, and other properties that may give greater insight into which of the possibilities above is valid. Hexadecane has a width of 0.4 nm and a length of 1.9 nm. For hexadecane the molecular-kinetic and combined models did yield values of λ and Ls that are similar to molecular dimensions. The values of λ correspond to the molecular length except when the combined model was applied to the receding data. Results from surface force measurements, involving the drainage of hexadecane from between hydrophilic mica sheets, indicated that the hexadecane molecule was found to lay flat on the surface, which is consistent with the current analysis of the wetting data. Of course the agreement could be purely coincidental and result from implementation of the molecular-kinetic theory. The latter conclusion is supported by the OMCTS results and also by the parameter values obtained by fitting the receding hexadecane data to the combined model. In this case λ is 31/2 times greater than the
Dynamic Wetting and Dewetting
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low degree of contact angle hysteresis for the fluorinated surfaces in the current study, they are much more amenable to the simultaneous fitting of both advancing and receding contact angle data. The symmetry of the parameters obtained from the application of the combined model to the OMCTS data (Figures 2 and 3) suggest that this model is the most appropriate for this task. In Figure 5 the combined molecular-kinetic hydrodynamic model is fitted to the complete set of advancing and receding data for OMCTS on AF1600. The fit is uniformly good over the entire velocity range, including the hysteresis region. Only the value of Kw (1400 s-1) changes significantly from the values obtained from independent fitting of the advancing and receding data. However because the static contact angle hysteresis is minimal the change in this value is simply an order of magnitude. The success of this procedure supports the contention that the partitioning of advancing and receding data for the purposes of fitting is an artificial construction forced on experimentalists by the effects of physical and chemical heterogeneity on their solid surfaces. Figure 5. Simultaneous fitting of the combined model to the entire set of OMCTS data (advancing and receding). The parameters used were θo ) 40.0°, λ ) 6.4 ( 0.2 nm, Kw ) (1.4 ( 0.6) × 103 s-1, ln(L/Ls) ) 8.25, and χ2 ) 25.4.
molecular dimension, and it is the slip length, Ls, which more closely equates with the molecular dimension. The Simultaneous Fitting of Advancing and Receding Data. In the analysis of the data so far and in work elsewhere, the advancing and receding data have been separately fitted to the respective wetting models. In the absence of a complete set of advancing and receding contact angle-velocity data this is unavoidable. Contact angle hysteresis also usually demands such an approach. However if the correspondence between the parameters obtained from fitting advancing and receding data is good then the partitioning of data is somewhat arbitrary. Indeed it has been shown that advancing and receding data can be simultaneously fitted by the molecular-kinetic equation for the case of water and glycerol/water on PET.20 This procedure involved letting θo be a free parameter with the “best fit” value usually bisecting those of the measured static advancing and receding contact angles. While the data were well fitted by Equation 2 over the entire velocity range, the value of Kw obtained was unreasonably small as a consequence of the large degree of contact angle hysteresis (typically 30°). Due to the very (20) Hayes, R. A.; Ralston, J. Langmuir 1994, 10, 340. (21) Lide, D. R.; Handbook of Chemistry and Physics, 78th ed.; CRC Press: Boca Raton, FL, 1997. (22) Hunter, M. J.; Hyde, J. F.; Warrick, E. L.; Fletcher, H. J. J. Chem. Phys. 1946, 68, 667.
Summary and Conclusions In this work a number of models have been applied to the wetting and dewetting of an amorphous, low energy surface by OMCTS and hexadecane, liquids of differing size and shape. The behavior of the two liquids is distinctly different and in the case of hexadecane depends on the direction of liquid movement. Advancing movement of hexadecane is independent of hydrodynamics and is well predicted by the molecular-kinetic model. The microscopic displacements of the contact line (λ) correspond well with the molecular length (1.9 nm). Hydrodynamics does however affect the receding movement of hexadecane, and the combined molecular-hydrodynamic model best describes the measured contact angle-velocity dependence. In this case it is the slip length (Ls) that is very similar to the molecular length. In contrast, for OMCTS, the wetting behavior is essentially symmetric with respect to advancing and receding liquid motion and depends on both molecular-kinetic and hydrodynamic factors. As a result the experimental data is best predicted by the combined molecular-hydrodynamic model. The values of λ and Ls are much greater than the molecular dimension (0.8 nm). These results show that when hydrodynamics affect the dynamics of wetting and dewetting the characteristic process dimension does not correspond to typical molecular dimensions, but indeed it dramatically exceeds them. Acknowledgment. We are most grateful for the support received by the Australian Research Council and the Department of Industry, Science and Technology. LA980476K
