Extensional Relaxation Times of Dilute and Semi-Dilute Polymer Solutions

Liquid transfer and drop formation in jetting, printing, coating, spraying and atomization applications involve complex free-surface flows, including the formation of columnar necks that undergo spontaneous capillary-driven thinning and pinch-off. The progressive self-thinning of neck is often characterized by self-similar profiles and scaling laws that depend on the relative magnitude of capillary, inertial and viscous stresses for simple, Newtonian fluids. Stream-wise velocity gradients that arise within the thinning columnar neck create an extensional flow field that can orient and stretch macromolecules, contributing extra elastic stresses and extensional viscosity that change the pinch-off dynamics for polymeric complex fluids. Here, we employ the dripping-onto-substrate (DoS) rheometry protocols for visualizing and analyzing the pinch-off dynamics of a columnar neck formed between a nozzle and a sessile drop. We show that the magnitude and concentration-dependence displayed by extensional viscosity and extensional relaxation time are quite distinct from the values obtained in response to shear flow, as the progressive screening of excluded volume and hydrodynamic interactions depends on polymer concentration as well as on degree of stretching and conformational anisotropy.