The Interfacial Physics Group

Abstract

Dynamic wetting is an intrinsically complex problem because of the unique hydrodynamics in the microscopic region near the contact line. While the region is small in extent, it essentially controls macroscopic wetting phenomena. Both the fluid motion and the material properties of the liquid and the solid surface influence the dynamic wetting behavior. We have expanded the regime treated by previous research in wetting to new regimes: unsteady wetting and wetting by a specific class of non-Newtonian fluids, shear-thinning fluids. Our basic experimental technique is the precise measurement of the liquid/vapor interface shape using video microscopy. Our main analysis tool is comparison to hydrodynamic models based on Stokes flow. We have also developed a quasi-steady model to describe the contact angle relaxation during the unsteady spreading process and a two-region shear-thinning model to predict the shear thinning effects on steady state interface shapes. Deviations from these models help us understand the influence of unsteady flow and non-Newtonian behavior on wetting. In our unsteady wetting studies, we observe both unsteady and quasi-steady behaviors. The unsteadiness in our moderately high viscosity system arises from temporal relaxation of the contact angle rather than inertial effects or momentum diffusion. In our study of non-Newtonian fluids, the shear-thinning properties reduce the viscous force in the fluid flow and lead to a less curved dynamic interface than that of a Newtonian fluid at the same contact line speed and zero-shear viscosity.