The Interfacial Physics Group

Abstract

Our studies on a series of non-Newtonian fluids suggest looking at dynamic wetting from a new perspective. Flow in the wedge-like region near the moving contact line demands that shear rates increase to very high values as the contact line is approached. Thus, somewhere in the flow field, molecular relaxations in the fluid will be unable to relax, leading to non-Newtonian behavior on some small scale near the contact line. This is crucial because in dynamic wetting, behavior at a very small, micron scale has impact on wetting behavior at the submillimeter and millimeter scale where fluid behavior must be precisely controlled in many applications. We find that non-Newtonian fluids dominated by shear thinning reduce viscous bending near the contact line while fluids dominated by elasticity increase the bending, compared to a Newtonian fluid with the same zero shear viscosity. These changes in the viscous bending can, in turn, impact the dependence of the dynamic contact angle on capillary number (Ca), changing it from the classic ωo ~ Ca1/3 exhibited by Newtonian fluids. We find that even fluids with such short relaxation times and undetectable non-Newtonian behavior in their rheological characterization, do not show dynamic wetting described by models that assume Newtonian behavior. Our work suggests a modified flow field with a region of non-Newtonian behavior intervening between the region at shortest length scales where the classical contact line singularity is alleviated and the region far from the contact line where viscous forces just begin to become significant.