Extensional Rheology of Colloidal Particles and Polymer Mixtures

Colloid-polymer mixtures are ubiquitous in many different applications ranging from cosmetic products to energy materials. During manufacturing or application, these mixtures often undergo extensional deformation. Understanding how colloid-polymer mixtures flow under elongational stress is important to control and optimize the processability of these materials. We study the extensional flow properties by characterizing the capillarity-driven pinching dynamics of colloid-polymer mixtures using a dripping-onto-substrate (DoS) protocol. Methacrylate copolymer particles with acrylamide copolymer brushes are suspended in a refractive-index- and density-matched mixture of glycerol-water with NaCl added to screen the electrostatic repulsions. A non-adsorbing polymer, polyacrylamide, is added at varying molecular weight and dispersity. Addition of polyacrylamide induces depletion attractions between the colloids. The extensional properties is characterized by calculating the extensional relaxation time and the filament lifespan from the pinching dynamics. In polyacrylamide solutions without particles, the concentration dependence of extensional relaxation time is controlled by coil-stretch hysteresis. The scaling exponent of the extensional relaxation time with concentration increases with polymer size, which is attributed to the screening of excluded volume interactions by the presence of coil-stretch hysteresis under extensional flow. In colloid-polymer mixtures, the presence of particles does not affect the extensional relaxation times. The concentration-dependent scaling of extensional relaxation times collapses when they are scaled with free volume polymer concentration, suggesting that the polymer elastic properties control the extensional time scale. The filament lifespans of polymer solutions and colloid-polymer mixtures, when normalized by the filament lifespan of the corresponding fluid without polymer, follow a master curve as a function of free volume concentration. These results provide insight into the role of polymer in the extensional rheology of colloid-polymer mixtures.