(90f) Dynamic Wetting by Non-Newtonian Fluids

The impact of fluid elasticity and shear thinning on the dynamic wetting of polymer solutions is important because many fluids, even those that are normally considered Newtonian, exhibit non-Newtonian behaviors in the high shear environment of the wedge-like geometry near a moving contact line. Even though this behavior is on the microscopic scale, it has significant impact on wetting on the millimeter scale. Our experimental work shows that shear thinning reduces the curvature of the free surface near a moving contact line and the dependence of the dynamic contact angle on speed as compared to a Newtonian fluid having the same zero-shear viscosity. The origins of the reduced curvature have been successfully elucidated in lubrication models using a constitutive relation with power law shear thinning. However, we have also observed that elasticity increases the curvature of the free surface and the dependence of the dynamic contact angle with speed. Both experimental and theoretical results will be discussed. The fluids we use are dilute solutions of high molecular weight polyisobutylene (PIB) which exhibit elasticity dominated rheology with minimal shear thinning. Their wetting behaviors are compared to both a Newtonian fluid having the same viscosity and their oligomer ?solvent?. We will discuss the results of a lubrication analysis of the hydrodynamics using an Oldroyd B constitutive relation to probe the origins of the observed enhanced curvature in the case of an advancing contact line.