Large particle segregation in two-dimensional sheared granular flows

We studied the segregation of single large intruder particles in monodisperse granular materials. Experiments were carried out in a two-dimensional shear cell using different intruder and media diameters, whose quotient defined a size ratio R that ranged from 1.2 to 3.333. When sheared, the intruders segregated and rotated at different rates, which depended on their R values and depth. The vertical intruder trajectories as a function of time were curved due to nonconstant depth-dependent segregation rates. An analysis that considered the lithostatic pressure distribution and a size ratio dependence was done to capture the trajectories and the general segregation rate behavior. As a result of a strain rate analysis, we observed a greater expansion rate around the intruders when R values were larger, which in turn promoted faster segregation. Experiments with large R values showed that intruder rotation was weak and local shear rates were low. In contrast, experiments with R closer to unity resulted in strong intruder rotation, high local shear rates, and contraction below the intruder. Therefore, an intruder with a diameter close to that of the medium was likely to segregate due to a rotation mechanism. We propose that large particle segregation depends on size ratio, local expansion rate, and, to a lesser extent, the local shear rate. Based on our observations we redefine large particle segregation as two well-defined processes dependent on R and the local strain rate.