Granular collapse as a percolation transition

Inelastic collapse is found in a two-dimensional system of inelastic hard disks confined between two walls which act as an energy source. As the coefficient of restitution is lowered, there is a transition between a state containing small collapsed clusters and a state dominated by a large collapsed cluster. The transition is analogous to that of a percolation transition. At the transition the number of clusters $n_s$ of size s scales as $n_s\sim s^{-\tau }$ with $\tau \approx \mathrm{2.7.}$