Explaining axial band formation for bidisperse particles in rotating tumblers

A long-standing question in granular flows is the origin of segregated axial bands of small and large particles in long rotating tumblers. Using DEM simulations, we show that initial radial segregation of the small and large particles in the surface flowing layer results in a layer of mixed particles with a higher bulk density due to more efficient packing over a layer of monodisperse small particles with a lower bulk density. This leads to a granular Rayleigh-Taylor-like instability that continues as radial-segregation-driven recirculation cells that reinforce band formation. The granular Rayleigh-Taylor instability only occurs when the rotation speed, particle size ratio, and flowing layer thickness are sufficient to support an initial layer of mixed particles over a layer of monodisperse small particles, thereby explaining why axially segregated bands appear only under certain conditions.