Sorting out segregation mechanisms at the interface between densely creeping and energetic granular flows
Seminar Room 1, Newton Institute
Densely flowing granular mixtures segregate die to difference in the size, material density, and other particle properties. In some situations the segregation is simple: smaller particles tend to sink compared to their larger, equal-density counterparts; denser particles tend to sink compared with their equal-sized lighter counterparts. These are often attributed to kinetic sieving and buoyancy, respectively. However, in some situations the segregation is more complex: particles poured into a pile may segregate into stratified layers, and in drums some mixtures will segregate into radial stripes and axial bands. It is difficult to determine the dominant segregation mechanism(s) in each case. In most experimental segregation studies of dense flowing granular mixtures, velocity gradients, volume fraction gradients and gravity simultaneously act on the granular mixture.To isolate the segregation mechanisms, we study segregation in densely sheared granular mixtures experimentally and computationally in two systems: a rotating drum and in a split-bottom cell. In a free surface flow that occurs in the rotating drum, we find the relative importance of each mechanism depends on the position in the flow, the importance likely governed by the local volume fraction. On the other hand, when shear is perpendicular to gravity as in a split-bottom cell, we find gravity-driven segregation by size is significantly reduced; partially where the volume fraction gradient is eliminated. In this regime, segregation mechanisms associated with a variable shear rate dominate. We discuss these results and their relative importamce for segregation in different regimes of relatively dense granular mixtures.