Macromolecular Dynamics, Extensional Rheology, Pinch-off Dynamics, and Printability of Aqueous Solutions of Flexible and Semi-Flexible Polymers

Liquid transfer and drop formation/deposition processes associated with printing, spraying and coating flows involve complex free-surface flows including the formation of
columnar necks that undergo spontaneous capillary-driven instability, thinning and pinch-off. For
simple Newtonian fluids, the interplay of capillary, inertial and viscous
stresses determines the self-similar thinning and pinch-off dynamics. In rheologically complex fluids, extra elastic stresses alter the pinch-off dynamics. Here we show that dripping-onto-substrate (DoS) rheometry protocols that involve visualization and analysis of capillary-driven thinning of a columnar neck can be used for measuring extensional viscosity and extensional relaxation time of polymeric fluids. We investigate the role of chemical structure by contrasting behavior of aqueous solutions of flexible polyethylene oxide (PEO) with solutions of semi-flexible hydroxyethyl cellulose (HEC) and show that both flexibility and extensibility of chains dramatically influence the extensional rheology response and the macromolecular relaxation dynamics. We elucidate how macromolecular stretching and orientation modify excluded volume and hydrodynamic interactions, affecting extensional viscosity response as well as polymer relaxation dynamics.