Gelatin cavity dynamics in the wake of high-speed solid sphere impact*

We investigate the impact and penetration of a small solid sphere onto gelatin at speeds greater than 100 m/s. High-speed videography allows us to capture the cavity dynamics in the wake of the sphere. Varying the gelatin concentrations affects the elastic response of the medium. The high-speed videography reveals several unique features of the cavity dynamics in gelatin when compared to water (e.g., the appearance of the texture on the wall of gelatin cavity, the attenuation of the vertical jet upon the pinch-off of gelatin cavity). We present a phase diagram that classifies the cavity pinch-off type using the elastic Froude (inertia vs. elasticity) and the elastic Grashof (gravity vs. elasticity) numbers (Akers {\&} Belmonte, J. Non-Newtonian Fluid Mech., 2006), similar to the Weber and Bond numbers used for water entry (Aristoff {\&} Bush, J. Fluid Mech, 2009). We also discuss the detailed dynamics of each cavity type based on high-speed images.