The Interfacial Physics Group

Carnegie Mellon University, Department of Physics

The Dynamic Wetting of Polymer Melts: an Investigation of the Role of Material Properties and the Inner Scale Hydrodynamics

by Keith R. Willson



Dynamic Wetting is the process wherein a fluid body spreads across a solid surface while displacing a second immiscible fluid. Such phenomena are ubiquitous in both natural and industrial settings. The work presented here has been done in the pursuit of two goals. The first goal has been to produce the first systematic study of the relation between material properties and the dynamic wetting of a material system. This stands in analogy to the work of Zisman, who examined the static wetting of a variety of systems to understand what materials characteristics controlled static wetting phenomena. The second goal has been to assess the applicability of theoretical models of the unique hydrodynamics which occur near the contact line. This is analogous to later attempts to test models of the chemical interactions at interfaces which could account for the static wetting behavior.

We characterize the dynamic wetting of a viscous bulk polymer liquid by the shape of the liquid-vapor interface near the moving contact line as a function of the contact line speed. We show that the dynamic wetting behavior is strongly affected by several properties of the material system. These properties are the surface energy of the solid substrate, the chain length and end termination of the polymer, and the presence or absence of a multi-molecular, mobile water layer at the solid surface. The dynamic wetting is not a universal function of the capillary number and we show that the parameters describing the hydrodynamics near the contact line must be functions of the contact line speed. Our measurements are more consistent with slip models of the hydrodynamics in the region near the contact line than with models which depend on precursing thin films.