(78c) Flow-Microstructure Coupling: Instabilities and Shear Banding in Polymeric Fluids

Complex fluids such as polymers, self-assembled micellar solutions, and colloidal suspensions can have slow timescales for microstructural relaxations. When shear rates exceed these timescales the rheological coupling between flow and microstructure can lead to numerous instabilities, such as shear banding, melt fracture, and sharkskin. For example, entangled polymers have been intensely studied since at least the 1940s, and wormlike micelles since the 1970s. In the last decade particle velocimetry and other imaging methods, combined with rheology, have shown that some dramatic instabilities to inhomogeneous flows such as shear banding and apparent “fracture” can occur in strongly sheared well-entangled polymeric systems. I will discuss how many of these observations can be understood based on their molecular constitutive behavior, such as the Doi-Edwards theory for entangled polymers. I will highlight some of the current controversies specifically in the area of entangled polymers, and hopefully also provide a perspective for other materials such as yield stress fluids, self-assembling materials, and dense colloidal suspensions.