Extensional Rheology of Yield Stress Fluids

Many commercial formulations that appear to flow only beyond a critical stress value are classified as yield stress fluids. Despite the wide availability and applications of such fluids, characterization of their response to extensional stresses and flows. We investigate the flow characteristics of extruded filaments and by analyzing the transition from solid to liquid in a neck and subsequent pinching dynamics, we characterize the extensional yield stress and response after yielding. The neck shapes, radius evolution as a function of time, and shape and size of dispensed drops are characterized and analyzed for prototypical yield stress fluids formulated by using densely packed microgels, particles, drops or macromolecules. We seek a deeper appreciation of the diversity in response as influenced by microstructure.