Sheet's getting real: dynamics and fragmentation in complex fluid sheets created by impinging jets

We encounter fragmentation in fluid sheets whenever we visit a waterfall or hold our thumb over the end of a hose. Because of their 2D geometry, these sheets present a unique opportunity to explore material instabilities. The vast majority of research into the behavior of fluid sheets has centered around Newtonian fluids; we are working toward an analogous understanding of complex fluid sheets. In complex fluids, the viscosity depends on the applied stress; among them are ketchup, fresh concrete, and mucus. Thus, understanding how and why these fluids fragment is important across fields as diverse as food science, civil engineering, and biology. In our experiments, we generate the sheets via the collision of two liquid jets and film their dynamics using high-speed photography. Our findings indicate that quickly-expanding sheets (created by faster jets) are less stable than slowly flowing sheets, and that higher viscosities generate thicker and more stable sheets. Fragmentation can be categorized into different regimes based on jet velocity, jet diameter, and the fluid's rheological properties. We quantify the observed instabilities by examining the size, shape, and thickness of each sheet, as well as the rate of any hole formation and wavelength of instabilities along the rim.