Fine particle transport in granular beds*

Predicting segregation and mixing of size-bidisperse granular material is challenging in many industrial applications and natural phenomena. Existing models accurately describe segregation of mixtures with large-to-small particle diameter ratios, R<3, but do not work nearly as well for larger R where segregation behavior changes significantly due to free sifting (i.e., small particles percolate downward even in the absence of shear). Here we study segregation in mixtures with large size ratios, 4<R<10, by analyzing the diffusion and transport mechanisms of "fine" particles in a bed of large particles using DEM simulations. Additionally, we explore the influence of gravitational acceleration, particle inelasticity, and granular temperature in terms of constrictions in the large particle bed. While for R>7, the primary mode of transport is free sifting, we observe that an increase in the fluctuation velocity of fine particles significantly reduces their percolation velocity. For mixtures with intermediate size ratios, the flow behavior not only depends on fluctuation velocity but also on the constriction size distribution in the bed. This investigation illuminates the influence of relevant variables on percolation velocity and offers insights into potential strategies for its control.