CCFE - Center for Complex Fluid Engineering - Carnegie Mellon University

The aim of this study is to systematically characterize the effects of fluid elasticity on the motion of liquid-vapor interfaces near contact lines. Dynamic wetting, defined as the displacement of one fluid by another immiscible fluid on a solid surface, occurs both in nature and in industrial applications where the processes are all dependent on the wetting characteristics of the materials involved. This is an important step in modeling and understanding spreading of non-Newtonian fluids. The fluid mechanics near the three phase (solid-liquid-vapor) boundary are complex and the impact of elasticity on this inherently confined geometry is poorly understood. We study dynamic wetting by observing the liquid-vapor interface formed on the outside of a silica surface forced into a bath of the test fluid at controlled rates. A typical image of the liquid vapor interface obtained in our experiments is shown to the right.
We find that two polymer melts, polyisobutylene (PIB) and polystyrene (PS) exhibit dynamic wetting characteristics of a weakly elastic fluid. The results show that the interface shapes of these fluids deviate from the prediction of models that include only Newtonian flow behavior. A series of interface shapes at increasing capillary numbers along with the deviations from the Newtonian model are shown in the figure below.

Using standard rotational rheometry, we examine the shear rates needed to cause non-Newtonian behavior in these fluids and ask where such shear rates arise near the contact line. Experiments on xanthan gum solutions have shown that an increase in shear thinning in the high shear region near the contact line decreases the curvature of the interface near a moving contact line. However, fluids with some level of elasticity and minimal shear thinning show an increase in curvature near the contact line.
Further experiments on a series of polystyrene-based Boger fluids of increasing elasticity also show deviations from the model developed for Newtonian fluids. These viscous fluids exhibit the lack of shear-thinning and large first normal stress differences characteristic of Boger fluids.
Our current work is focuses on quantifying the effect of elasticity on the deviations seen from Newtonian behavior for these polymer melts and Boger fluids. From this we hope to understand the extent to which the interface deviates from the Newtonian model for a range of applied disturbances. This deviation from Newtonian behavior near the contact line has ramifications for macroscopic wetting phenomena properties of the spreading meniscus such its dynamic contact angle and its spontaneous spreading rate.