The water entry of cups: how burping alters cavities and splashes

When a rigid body with a convex nose impacts water it wets progressively, and the splash separation is a function of geometry and surface properties. However, when an inverted thin-walled cup impacts the water, the splash and cavity evolve quite differently. Here we show experimentally that air initially trapped inside the cup "burps" out as the body descends, creating a toroidal cavity emanating from the cup bottom. The timescale associated with this air leakage depends on the impact velocity and cup depth, and is rationalized by an analytical model. Depending on these experimental conditions, the leaked gas may reconnect to the free-surface, allowing for the cavity to be inflated. Under other conditions, the toroidal cavity seals on the body and prevents inflation through the free-surface, yielding a small air donut that descends with the body. These different scenarios directly couple to the splash behavior above the surface, influencing the motion of the splash and ultimately the seal time on the body. In this talk, we classify these different regimes, which are revealed through high speed imaging.