Impact dynamics of a solid sphere falling into a viscoelastic micellar fluid

We present an experimental study of the impact of a solid sphere on the free surface of a viscoelastic wormlike micellar fluid. Spheres of various densities and diameters are dropped from different heights above the fluid surface, reaching it with a nonzero velocity which determines the subsequent dynamics. Measurements of the initial sphere penetration are found to scale with the ratio of the kinetic energy of the sphere at impact to the elastic modulus of the fluid. The cavity formed in the wake of the sphere, observed with high-speed video imaging, also undergoes transitions from a smooth to fractured surface texture, dependent on both the Deborah number and the ratio of the gravitational force to elasticity.