Onset of avalanches in granular media

Marginal stability and avalanches at angles ΘΘaval are associated with granular media (GM) subject to special conditions. We study the Newtonian dynamics of random size-mismatched hard-core-like disks in a two-dimensional box which confirm claims that avalanches in GM are restricted to a few topmost boundary layers. We find that the velocity profile of the top layer grains, which obey -tγ, typically with 3.5≤γ, signal the onset of an avalanche in GM. Our studies suggest that the dynamics of a single particle in a cosine potential in the presence of a linear field well describes the onset of motion of a top layer grain.