Impact of gel balls beyond the Hertzian regime.

We study experimentally the impact of spherical gel balls on a rigid substrate, where the balls largely deform like a pancake at high impact velocities. In our previous study (Y. Tanaka, Y. Yamazaki, K. Okumura, Europhys. Lett. 63, 149 (2003)), we measured the contact time tau(f) and maximally deformed size versus impact velocity and explained the behaviors at the scaling level. In this study, we further measure tau(m), the time required to reach the maximum deformation (from the initial contact), and the restitution coefficient e. We also make a static experiment where we obtain the force-deformation curve of the gel balls up to fairly large deformations to explain the data on the impact. We propose two phenomenological treatments going beyond the scaling argument, one for intermediate impact velocities and the other for large velocities; the former is based on the static experiment while the latter on a Lagrangian constructed from appropriate constraints. Results from these treatments reproduce the experimental behavior of tau(m).