Dynamics of Partial Wetting and Dewetting in Well-Defined Systems

By analyzing the assumptions of the hydrodynamic, molecular-kinetic and molecular-hydrodynamic theories of wetting dynamics, we formulated qualitative criteria for their applicability on the basis of the wetting−dewetting asymmetry of the dynamic contact angle/velocity dependence ϑ/V presented on characteristic theoretical scales. The quantitative test concerns the physical plausibility of the values of the microscopic dynamic parameters obtained from the fit of the theoretical equations to experimental data. New ϑ/V data were obtained and used to compare theory and experiment under well-defined conditions close to the assumptions of the theoretical models. The advancing and receding of methanol, ethanol, 1-butanol, and 1-octanol on the amorphous fluoropolymer AF 1600 were studied. The small AFM roughness (0.3−0.4 nm/μm2), static angle hysteresis between 3 and 7°, and reproducibility within ±0.2° of the dynamic contact angles at different contact line positions demonstrate the good quality of the solid substrates. The use of pure liquids avoids complications with adsorption kinetics, and the low surface energy of the solid and the surface tension of the liquids minimize contamination effects. We found that the molecular-kinetic theory predicting a symmetry of advancing and receding branches of the cos ϑ/Ca dependence works well at Ca = μV/γ < ±5 × 10-4 but cannot explain the asymmetry at larger Ca values. The plot on the characteristic ϑ3/Ca scale shows that two-region hydrodynamic solutions work above Ca = ±2 × 10-3 but do not describe the observed wetting−dewetting asymmetry. The same holds for the three-region solutions introducing the slip law or/and geometry of the system as numerical constants. The molecular-hydrodynamic theory fits the data over the whole Ca range studied and represents the wetting−dewetting asymmetry of the ϑ/Ca plots. However, it gives inadequately small values of the molecular oscillation frequency K0 unless the viscous friction in the contact line vicinity is not taken into account. Unreasonably small slip lengths LS,max and distances λ between the potential wells of the liquid molecules that are too large were obtained with all of the theories. They indicate shortcuts of the theoretical models for systems with partial wetting. At the same time, literature data for complete wetting confirm the ϑ3/Ca dependence and fluid interface shape predicted by the hydrodynamic theory.