Sharkskin texturing instabilities in the flow of polymer melts

The capillary flow of molten polymers has received much attention because at high flow rates the extrusion of the polymer melt is commonly accompanied by instabilities. These manifest themselves as surface distortions, called melt fracture, in the final plastic product. We have modeled this flow, incorporating a stick-slip boundary condition at the capillary wall. The boundary condition is enforced by a model for the local state of the polymer, which undergoes a phase transition controlled by the local shear. We numerically solve the model, coupled to the hydrodynamic flow and to a Maxwell model for viscoelasticity. In various regimes, the model exhibits steady flow, periodic oscillations, and more complicated spatiotemporal structures, which explain the sharkskin texturing observed in melt fracture