Dynamic contact angle measurements of viscoelastic fluids

In this study, the dynamic contact angles of a series of viscoelastic fluids were measured through a modified Wilhelmy plate technique. The advancing and receding contact angles were measured by immersing and withdrawing a PTFE (Teflon) Wilhelmy plate from a reservoir containing a series of different Newtonian and viscoelastic test fluids. The viscoelastic fluids that were tested consisted of either solutions of polyethylene oxide or polyacrylamide in water where the concentration of the polymer was varied to produce solutions with different amounts of fluid elasticity with and without shear thinning. The advancing contact angles of all the viscoelastic fluids tested were found to increase with increasing plate velocity. Conversely, the receding contact angles in each case were found to decrease with increasing contact line velocity. A number of previous measurements have been performed for shear thinning fluids. The measurements presented here are the first to probe the response of the dynamic contact angle at large Weissenberg numbers where the elasticity of the liquid becomes important. For fluids with increased fluid elasticity, the onset of the variation of receding contact angles was found to be delayed to higher contact line velocities and capillary numbers. It was also found that, unlike the case of Newtonian fluids, the cube of advancing and the cube of the receding contact angles were both found to be proportional to the square of the capillary number for highly elastic fluids. Finally, a simple model was proposed to account for the role of elasticity and shear thinning of the viscosity on the dynamic contact angle.