Elastic yielding in entangled polymeric liquids: Exploring origin of flow inhomogeneity

We will present an overview of the latest developments in the area of nonlinear flow behavior of entangled polymer solutions and melts, on both experimental and theoretical fronts. From visualization-based experiments, we have derived a theoretical understanding of a host of striking flow phenomena ranging from elastic breakdown after a step strain, to emergence of shear inhomogeneity in startup and large amplitude oscillatory shear to universal scaling behavior associated with the yield point identified to be the force maximum during start flow. In this presentation, we address basic questions such as (a) where cohesion comes from in polymeric liquids, (b) how cohesive failure occurs during startup flow and after step flow respectively, (c) whether and how well entangled polymers as transient solids break up inhomogeneously first before forced to undergo subsequent continual deformation. Clearly, effects of chain entanglement are not only dynamical as recognized in the past but also mechanical, and we must be concerned about the cohesive strength of such "solids". The new picture has allowed us to depict and analyze both simple shear and uniaxial extension behaviors in a unified manner.