Size segregation in driven granular media*

Granular flows involve grains of different sizes and mass densities, which either remain well-mixed or segregate. We perform discrete element simulations of model granular systems composed of frictionless, bidisperse disks (with diameter D_l and D_s, mass density ρ_l and ρ_s, for large and small disks) under gravity and driven by either simple shear or vibration to understand parameters that control mixing and segregation. We have shown that sheared granular system possess 1) a geometrically-segregated regime, where the system becomes increasingly segregated with decreasing D_s/D_l, 2) a weight-segregated regime, where the system becomes less segregated as D_s/D_l decreases further, and 3) well-mixed states. We identified the boundaries between these three regimes as a function of D_s/D_l and ρ_s/ρ_l. We performed similar studies of vibrated systems to determine if the boundaries between mixed and segregated states depend on the driving method. We show that the large particles rise to the top only if ρ_l<ρ_s, i.e. we only find weight-segregation in vibrated systems. We also show that by driving only large particles in vibrated systems, large particles will rise for intermediate packing fractions (0.7>φ>0.5).