Shooting the sheet: high-speed photography of complex fluid sheets reveals material mesostructure

Spoons encountering a high-pressure faucet can cause drama while doing the dishes, but the expanding fluid sheet created by this mishap presents a wonderful opportunity to explore material instabilities. The sheet cannot expand indefinitely, and fragmentation is well-understood for Newtonian fluids like water. We are working toward an analogous understanding of complex fluid sheets, where the fluid's viscosity depends on applied stress. Such materials include ketchup, concrete, mucus, and hair gel, so this understanding is important across diverse fields. We generate our fluid sheets via the collision of two liquid jets and film their dynamics using high-speed photography. Fragmentation patterns in polymeric fluid sheets in particular demonstrate the distinct effect of mesoscale structure on material stability. Our experiments focus on Carbopol 940, which self-assembles into microgels that give rise to unique fragmentation patterns. We observe that when the gels are concentrated enough to jam, a nontrivial length scale emerges which is independent of bulk flow properties or jet parameters. Rather, it appears to be coupled to the size of microgel aggregates. Additionally, we investigate how the size of the microgels (controlled by the solution's pH) affect this length scale, as well as hole nucleation and hole expansion rates.