Description:

Vivek Sharma of UIC.

Host: Wes Burghardt

Title:
Stretched Polymer Physics, Pinch-off Dynamics, Rheology and Printability of Polymeric Complex Fluids

Abstract:
Liquid transfer and drop formation/deposition processes associated with printing, spraying, atomization 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 and inelastic) fluids, a complex interplay of capillary, inertial and viscous stresses determines the nonlinear dynamics underlying finite-time singularity as well as self- similar capillary thinning and pinch-off dynamics. In rheologically complex fluids, extra elastic stresses as well as non-Newtonian shear and extensional viscosities dramatically alter the pinch- off dynamics. Stream-wise velocity gradients that arise within the thinning columnar neck create an extensional flow field, and many complex fluids exhibit a much larger resistance to elongational flows than Newtonian fluids with similar shear viscosity. Characterization of the response to both shear and extensional flows that influence dispensing and liquid transfer applications requires bespoke instrumentation not available, or easily replicated, in most laboratories. Here we show that dripping-onto-substrate (DoS) rheometry protocols that involve visualization and analysis of capillary-driven thinning and pinch-off dynamics of a columnar neck formed between a nozzle and a sessile drop can be used for measuring extensional viscosity and extensional relaxation time of polymeric complex fluids. We show that the DoS rheometry protocols we have developed enable the characterization of low viscosity printing inks and polymer solutions that are beyond the measurable range of commercially-available capillary break-up extensional rheometer (CaBER). We find that the extensional relaxation times of dilute and semi-dilute, unentangled polymers in good solvent exhibit much stronger concentration dependence than observed in shear rheology response or anticipated by blob models developed for relaxation of weakly perturbed chains in a good solvent. We investigate the role of charge by contrasting the pinch-off dynamics and the rheological response of weak and strong polyelectrolytes and characterizing the influence of the electrolyte concentration. We elucidate the influence of chemical structure on stretched polymer physics by contrasting the behavior of aqueous solutions of flexible polyethylene oxide (PEO) with solutions of semi-flexible hydroxyethyl cellulose (HEC). We show that flexibility, extensibility and charge dramatically influence the extensional rheology response and the macromolecular relaxation dynamics. Finally, we elucidate how macromolecular stretching and orientation in response to strong extensional flows modifies the excluded volume and hydrodynamic interactions, affecting the terminal extensional viscosity response as well as polymer relaxation dynamics, and consequently, determine the filament lifespan, the processing timescale, and processability for printing, coating, dispensing, and spraying applications.

Biography:
Dr. Vivek Sharma is an Assistant Professor of Chemical Engineering at the University of Illinois Chicago. Before joining UIC in November 2012, he worked as a post-doctoral research associate in Mechanical Engineering at Massachusetts Institute of Technology. He received his Ph. D. (Polymers/MSE, 2008) and M. S. (Chemical Engineering, 2006) from Georgia Tech., an M. S. (Polymer Science, 2003) from the University of Akron, and a bachelor's degree from IIT Delhi. Dr. Sharma's research interests broadly lie in optics, dynamics, elasticity, and self-assembly (ODES) of complex fluids and soft materials. At UIC, Dr. Sharma's Soft Matter ODES-lab combines experiments and theory to pursue the understanding of, and control over interfacial and nonlinear flows, focused on the interplay of (a) viscoelasticity and capillarity for printing applications and extensional rheometry, and (b) interfacial thermodynamics and hydrodynamics in fizzics (the science of bubbles, drops, thin films, jets, fibers, emulsions and foams). Dr. Sharma was selected as the Distinguished Young Rheologist by TA Instruments in 2015, and won the 2017 College of Engineering Teaching Award at UIC.