Effects of scale and inertia on granular banding segregation

We report that the formation of much reported axial segregation bands in rotating cylinders loaded with different sized particles depends critically on scale and inertia. Specifically, when the ratio, δ, of the diameter of the cylinder to the average diameter of the particles is large, axial bands invariably appear, when δ is small, bands never appear, and between these extremes lies a reversible state where the presence or absence of bands depends on container rotation speed. Our results indicate that banding is associated with a Rayleigh-like instability of a granular core of fine particles, and that this instability is controlled by the inertia of the larger species – and consequently on scale.