Pinch-off Dynamics, Extensional Rheology and Dripping-onto-Substrate (DoS) Rheometry of Polymer Solutions
2019-02-01T00:00:00Z (GMT) by
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 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 polymers in good solvent exhibit a 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 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. Finally, we elucidate how macromolecular stretching and orientation in response to strong extensional flows modifies excluded volume and hydrodynamic interactions, affecting terminal extensional viscosity response as well as polymer relaxation dynamics, and consequently, determines the filament lifespan, the processing timescale, and processability for printing, coating, dispensing, and spraying applications.