(573a) Pinch-Off Dynamics, Extensional Rheology, Foamability and Processability of Aqueous Solutions of Surfactant-Polymer Mixtures

Addition of polymer to a surfactant solution affects the dynamic adsorption and the rheological response due to the formation of association complexes. Liquid transfer and drop formation/deposition processes associated with printing, spraying and coating flows as well as bubble and foam formation involve complex free-surface flows including the formation of columnar necks that undergo spontaneous capillary-driven instability, thinning and pinch-off. For
simple Newtonian fluids, the interplay of capillary, inertial and viscous stresses determines the self-similar thinning and pinch-off dynamics. In rheologically complex fluids, extra elastic stresses alter the pinch-off dynamics. Here we utilized that dripping-onto-substrate (DoS) rheometry protocols (that we developed) to visualize and analyze capillary-driven thinning of a columnar neck and measure extensional viscosity and extensional relaxation time. In this study, we examine how pinch-off dynamics and rheological properties of polymer solutions are modified in the presence of added surfactants by including a critical examination of concentration-dependent shear and extensional rheological responses. It is well known that dynamic adsorption of a freshly created interface is intimately linked with the rate of mass transfer of surfactant from liquid sub-phase to the interface, and this adsorption-limited kinetics impacts the stability of the newly formed interface. Dynamic surface tension refers to the time dependent variation in surface tension, which is related with the rate of mass transfer of a surfactant from liquid sub-phase to the interface, and often credited with influencing foamability. Dynamic surface tension measurements carried out with conventional methods like pendant drop analysis, Wilhelmy plate, etc. are limited in their temporal resolution. In this study, we apply the method of maximum bubble pressure tensiometry for the measurement of dynamic surface tension effects at extremely short (1-50 ms) timescales. We discuss the overall adsorption kinetics of charged surfactants and the influence of added polymer on dynamic surface tension, and utilize our experimental and theoretical analysis to determine the influence of adsorption kinetics on the pinch-off dynamics and processability of polymer-surfactant mixtures.