Drag forces in granular segragtion

Granular materials that contain particles of a range of sizes frequently segregate when sheared. The rate at this segregation occurs is strongly dependent on flow parameters such as the shear rate and pressure. Here we show that this dependence is not due to changes in the forces which drive segregation, but instead occurs primarily due to changes in the strength of the inter-species drag force, which opposes the driving segregation forces and acts to slow segregation. We study the drag force in detail, using a novel configuration of discrete element simulations to study drag in a uniform environment. Measuring the speed at which two species of grains percolate through each other then allows us to accurately measure the drag force, which we show follows a power law in the granular inertial number with exponent -7/4. We explain this scaling with a simple model, and show that it explains much of the previously observed dependence of segregation rate on shear rate and pressure in free-surface flows.