(306g) Extensional Relaxation Times of Dilute and Semi-Dilute Polymer Solutions

Stream-wise velocity-gradients associated with extensional flows arise in thinning liquid necks spontaneously formed during capillary-driven thinning and break-up of liquid sheets, jets or stretched liquid bridges. Though shear flows only weakly perturb the chain dimensions, extensional flows can strongly stretch and orient the chains, leading to coil-stretch transition and even full extension. Understanding, characterizing and controlling the extensional viscosity that quantifies the resistance to stream-wise gradients and extensional relaxation time of polymer solutions that determines the pinch-off time are of critical importance for varied applications that rely on drop formation and liquid transfer. Fortunately, visualization and analysis of capillary-thinning dynamics allows for quantitative comparison of transient as well as steady terminal extensional viscosity, extensional relaxation time, and pinch-off time for applications involving jetting, printing, coating, spraying, atomization and microfluidic-based drop/ particle/ emulsion formation. In this contribution, we employ the recently developed dripping-onto-substrate (DoS) rheometry protocols for visualizing and analyzing capillary-driven thinning and pinch-off dynamics of a columnar neck formed between a nozzle and a sessile drop. We examine the effect of increasing polymer concentration on pinch-off dynamics and extensional relaxation times of polymer solutions considered semi-dilute even in quiescent state as the concentration exceeds the critical overlap concentration computed using the criteria based on close packing of unperturbed coils. Despite the relative simplicity and overall price, DoS rheometry overcomes longstanding challenges associated with characterization of low viscosity fluids, including aqueous polymer solutions. We find that the magnitude and concentration-dependence displayed by extensional relaxation time and extensional viscosity are quite distinct from the values obtained in response to shear flow, as the progressive screening of excluded volume and hydrodynamic interactions depends on polymer concentration as well as on degree of stretching and conformational anisotropy.