Reversed, suppressed and layered granular segregation at large particle size ratios

Particle size segregation is a common occurrence in sheared granular flows under gravity. Segregation of size-bidisperse grain mixtures at size ratios of three or less has been extensively studied, but comparatively little is known about segregation of grains with more widely varying sizes, despite their relevance to natural and industrial flows. At larger size ratios the segregation behaviour of bidisperse mixtures may change drastically, including reversal of the direction of segregation, which no existing continuum model accounts for. This paper investigates the segregation behaviour of bidisperse granular mixtures up to a size ratio of seven and formulates a new continuum model for size segregation that captures the observed suppression and reversal of segregation. Discrete element method (DEM) simulations of flows on an inclined plane show a reversal of behaviour as the volume fraction of small particles increases, from states where the large particles rise to the free surface to states where they sink. At intermediate small-particle volume fractions, segregation is significantly reduced or even entirely absent, leading to well-mixed flows. In addition, a striking layering effect is observed at large size ratios, where large particles organise into distinct layers one particle thick, separated by thin bands of small particles. This layering is demonstrated both in simulations and, for the first time, in laboratory experiments. The continuum segregation model introduces a new bidirectional segregation flux that accounts for the reversal in segregation. The model is in good quantitative agreement with DEM simulations across a range of small-particle volume fractions.