Pinching Dynamics of Dense Colloidal Suspensions with Depletion Attractions

The macromolecular properties of polymers have a significant effect on their rheological response under extensional flow. Polymers are often added to concentrated particulate suspensions in applications such as 3-D printing, but how they affect the extensional rheology of the mixtures is still incompletely understood. We study the extensional flow properties via the pinch-off dynamics of both polymer solutions and concentrated colloidal suspensions (volume fraction Φ ≥ 0.40) with induced depletion interactions using a dripping-onto-substrate (DoS) protocol. We use a model colloidal system of methacrylate copolymer particles with acrylamide copolymer brushes suspended in a refractive index and density matched mixture of glycerol and water. Depletion attractions between the colloids are introduced by adding polyacrylamide polymers of various molecular weight and dispersity. We characterize the extensional properties by calculating the extensional relaxation time λE from the filament-thinning dynamics. The dependence of λE on polymer concentration becomes stronger with increasing polymer size. Addition of polymers to concentrated particle suspensions delays and modifies the pinch-off dynamics accordingly. Our results suggest that we can tune the processability of concentrated suspensions by varying the size and dispersity of polymer additives.