ONSET OF AVALANCHES IN GRANULAR MEDIA

Marginal stability and avalanches at angles THETA greater-than-or-equa l-to THETA(aval) are associated with granular media (GM) subject to sp ecial initial conditions. We study the Newtonian dynamics of random si ze-mismatched hard-core-like disks in a two-dimensional box which conf irm claims that avalanches in GM are restricted to a few topmost bound ary layers. We find that the velocity profile of the top layer grains, which obey Absolute value of v is-proportional-to t(gamma), typically with 3.5 less-than-or-equal-to gamma, signal the onset of ail avalanc he in GM. Our studies suggest that the dynamics of a single particle i n a cosine potential in the presence of a linear field well describes the onset of motion of a top layer grain.