Radial granular segregation under chaotic flow in two-dimensional tumblers

An initially well mixed granular material composed of two distinct subclasses of particles, small and large or light and heavy, segregates radially into stable lobed patterns when rotated in various quasi-two-dimensional, regular polygonal tumblers. The patterns are highly sensitive to the time-periodic flow, which in turn depends critically on the fill fraction and container shape. Simulations of a simple model reproduce the segregation patterns observed in experiment. Kolmogorov-Arnol'd-Moser (KAM) regions in Poincare plots of the velocity field used to model the flow attract smaller (denser) particles and their spatial symmetries mirror those of the segregation patterns, suggesting that competition between the driving forces for radial segregation (percolation and buoyancy) and those for chaotic mixing plays a key role.