CBE Seminar: Vivek Sharma

“Stretched Polymer Physics, Pinch-off Dynamics and Printability of Polymeric Complex Fluids”

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 pinchoff. 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 relaxation time of polymeric complex fluids. We show that DoS rheometry enables 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 semi-dilute, unentangled, uncharged polymers in good solvent exhibit a stronger concentration than observed in shear rheology, or anticipated by blob models developed for relaxation of weakly perturbed chains in a good solvent. We show that the interplay of electrostatic and hydrodynamic stretching leads to unexpected and unexplored concentration-dependent response for polyelectrolyte solutions. Finally we elucidate how polymer composition, flexibility, extensibility (molecular weight), and charge influence hydrodynamics and interactions of strongly stretched chains, and consequently, determine processability and processing timescale for printing, coating, dispensing, and spraying applications, especially in cases where fine time pinch-off is highly desirable.

Seminar Flyer_Vivek Sharma