Horizontal segregation of mono-layer granules coordinated by vertical motion.

We experimentally investigate the segregation of a binary mixture of spherical beads confined between two horizontal vertically vibrating plates. The two kinds of beads are of equal diameter and mass but have different restitution coefficients. Segregation occurs in particular ranges of vibration amplitude and frequency. We find that the collisions between beads at an angle to the horizontal plane induce an effective horizontal repulsive force. When one or both bead types bounce up and down in synchronization, the effective repulsive force between the two types of beads is likely to be larger than that found within a single bead type, resulting in the mixture segregating. Non-horizontal collisions also play a role in stabilizing the segregation state by transferring the horizontal kinetic energy back into vertical motion.