Designing minimally-segregating granular mixtures for gravity-driven surface flows

In dense flowing bidisperse particle mixtures varying in size or density alone, smaller particles sink (driven by percolation) and lighter particles rise (driven by buoyancy). But when the particle species differ from each other in both size and density, percolation and buoyancy can either enhance (large/light and small/heavy) or oppose (large/heavy and small/light) each other. In the latter case, a local equilibrium condition can exist in which the two segregation mechanisms balance and particles remain mixed: this allows the design of minimally-segregating mixtures by specifying particle size ratio, density ratio, and mixture concentration. Using experimentally validated DEM simulations, we show that mixtures specified by the methodology remain relatively well-mixed in the thin rapid surface flows characteristic of heaps and tumblers commonly used in industry. Furthermore, minimally-segregating particle mixtures prepared in a fully segregated state in a tumbler mix over time and eventually reach a state of nearly uniform species concentration.