Computer simulation of the mechanical sorting of grains

Abstract A two-dimensional system of inelastic frictional disks all of equal diameter save one was studied by computer simulation. A single large disk was placed on the bottom of a container and covered by 30 smaller disks. When the container was agitated to induce a shear motion in the disk assembly, the large particle showed a tendency to rise toward the surface. This sorting process was mediated by shear-driven rotational motion, the large grain rolling up on top of neighboring small grains. The grain-grain friction coefficient μ is a critical parameter in this kind of sorting process, since if μ is too small, the large grain cannot get sufficient purchase to roll without slipping.