Extensional Rheology and Stringiness of Yield Stress Fluids.

Many commercial formulations that appear to flow only beyond a critical stress value are classified as yield stress fluids. The apparent yield stress can be computed from a large variety of measurements that rely on transient, oscillatory, or steady shear response. Despite the wide availability and applications of such fluids, there is a considerable lack of studies that characterize their response to extensional stress and flows, motivating this study. Here we investigate the flow characteristics of extruded filaments using dripping and dripping-onto-substrate. We analyze the transition from solid to liquid in a neck to estimate the extensional yield stress and after yielding, characterize the neck pinching dynamics. The yielding transition, 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 using densely packed microgels, particles, drops, or macromolecules. We seek a deeper appreciation of the diversity in extensional response of formulations as influenced by their microstructure, and challenges involved in the characterization using methods based on free-surface flows.