Title: Mixing and segregation of vibrated and sheared granular beds

Author (Snapshot): Philip Wang, Yale University

Abstract:

Large-scale granular flows, such as mudslides and avalanches, occur frequently in nature. In addition, granular flows are used for particle processing in the pharmaceutical and consumer products industries. Granular flows typically 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 diameters Dl and Ds for the large and small disks) under gravity and subjected to quasi-static simple shear and vibrations to understand the parameters that control mixing versus segregation. In preliminary studies, we have shown that sheared bidisperse granular systems possess 1) a geometrically-segregated regime for 1>D_s/D_l >0.4, where the system becomes increasingly segregated with decreasing Ds/Dl, 2) a weight-segregated regime for D_s/D_l <0.4, where the system becomes less segregated as Ds/Dl decreases further, and 3) well-mixed states. We have identified the boundaries between these three regimes as a function of the diameter ratio and mass density ratio between the large and small grains. We carried out similar studies of thermally vibrated frictionless, bidisperse disks under the influence of gravity to determine whether the boundaries between the mixed and segregated states depend on the driving mechanism. For the thermally vibrated systems and over a range of temperatures above kinetic arrest, we show that the large particles rise to the top only if the mass density of the large particles is smaller than that for the small particles, i.e. we only find weight segregation. This result is contrary to that obtained for quasi-statically sheared systems, where both weight and geometric-segregation can occur. Further studies of thermally vibrated systems are necessary to determine whether the geometrically-segregated regime can be obtained at low temperatures. We will also carry out additional studies of sheared systems at finite shear rates to determine whether only the weight-segregated regime occurs in rapid flows.