Pinching Dynamics, Extensional Rheology and Dispensing Behavior of Formulations Containing Polymer-Surfactant or Polymer-Particle Mixtures

Understanding and control over rheological behavior of polymer-surfactant and polymer-particle mixtures is critically important for applications ranging from enhanced oil recovery, pharmaceutical and biological fluids, cosmetics, food, paints and coatings. Addition of an ionic surfactant to an aqueous solution of neutral polymer leads to formation of association complexes with manifest effect on both dynamic and equilibrium surface tension. For a fixed polymer concentration, increasing surfactant concentration, results in a non-monotonic concentration-dependent increase in shear viscosity. Most processing flows, especially those involving drop formation or liquid transfer, involve strong extensional flows and capillary-driven thinning and break-up pinch-off dynamics. However, capillary-break-up dynamics and extensional rheology response of polymer-surfactant mixtures is not well-characterized, primarily due to the well-known challenges in characterization of low viscosity, low-elasticity fluids. Using dripping-onto-substrate protocols we recently introduced, we characterize both pinch-off dynamics and extensional rheology response of association complexes. Finally, we examine the capillary break-up dynamics and rheological response of model, but multicomponent 'paints and coatings' or 'cosmetic' formulations, to investigate how addition to particles changes the behavior compared to solutions containing only polymers or surfactants or their mixtures.