Axial segregation of a settling suspension in a rotating cylinder

A rotating suspension of nonbuoyant particles can develop striking inhomogeneities in particle concentration, with regular bands of high and low concentration along the symmetry axis. We report Stokes-flow simulations showing that the formation of axial bands is correlated with an inhomogeneous particle distribution in the radial plane. An order parameter, based on the average angular velocity of the particles, characterizes two distinct phases: a low-frequency segregated phase and a high-frequency dispersed phase. The axial band structure develops during the transition between these two phases.