Department of Chemical Engineering,
University of Illinois at Chicago
Abstract: Stretched Polymer Physics, Extensional Rheology and Free Surface Flows
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 neck thinning dynamics. In rheologically complex fluids, extra elastic stresses as well as non-Newtonian shear and extensional viscosities dramatically alter the pinchoff 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 extensional viscosity, extensional relaxation time and finite extensibility effects, as well as macromolecular properties that determine pinch-off dynamics are beyond capabilities of conventional shear and extensional rheology techniques in which free surface flows are absent. Here we show that dripping-onto-substrate (DoS) rheometry protocols we developed recently can be used for measuring extensional viscosity and extensional relaxation time of polymeric complex fluids, including low viscosity printing inks and polymer solutions that are beyond the measurable range of commercially-available capillary break-up extensional rheometer (CaBER). Using 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, we elucidate the stretched polymer hydrodynamics underlying observed rheological response and processing behavior of polymeric complex fluids. We discover that our pursuit involves myriad, intertwined quests and insights into conformation-dependent hydrodynamic and excluded volume interactions, Pincus’ tension blobs, finite extensibility effects, as well as coil-stretch transition and hysteresis. We find that the extensional relaxation times exhibit concentration dependence distinct from shear rheology response or anticipated by blob models developed for relaxation of weakly perturbed chains. We show that the influence of molecular weight and chemistry can be evaluated a priori, using three macromolecular parameters: flexibility, extensibility and segmental dissymmetry for neutral polymers. Finally, we characterize andanalyze the pinch-off dynamics of charged polymers and ionic surfactant–neutral polymer complexes in the presence of varying salt concentration, to elucidate the influence of complex interplay of electrostatic and hydrodynamic stretching of macromolecules on processability.
Dr. Vivek Sharma is an Associate Professor of Chemical Engineering at the University of Illinois Chicago. Dr. Sharma’s Soft Matter ODES-lab (optics, dynamics, elasticity and self-assembly laboratory) focuses on molecular science and engineering of flow behavior, stability, and processability of complex fluids. His group develops distinctive experiments and theory for investigating interfacial flows, surface forces, optics, and nonlinear viscoelasticity of industrial and biological soft matter. Before joining UIC in November 2012, Dr. Sharma worked as a post-doctoral researcher 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 was selected as the Distinguished Young Rheologist by TA Instruments in 2015, won the 2017 College of Engineering Teaching Award at UIC, and was awarded the 3M Non-Tenured Faculty Award in 2019.