The influence of pressure on segregation and diffusion in a shear flow

The effect of confining pressure on segregation and diffusion of granular material is studied in Discrete Element Method (DEM) simulations of horizontal planar shear flow, where a feedback scheme maintains a constant shear rate for varying pressure. Both the segregation rate and the ultimate degree of segregation in initially mixed size-bidisperse and density-bidisperse beds decrease with increasing pressure. In contrast, the collisional diffusion is pressure independent. Consequently, segregation is reduced relative to diffusive mixing with increasing pressure. To test these findings, we compare DEM results to predictions of a continuum model that includes a pressure dependent segregation velocity and a pressure independent diffusion term. The model accurately predicts the steady-state segregation for both size and density bidisperse mixtures over a wide range of flow conditions. Additional simulations with segregated initial conditions demonstrate that a high enough overburden pressure reduces segregation sufficiently that significant mixing occurs, implying that manipulation of the shear-pressure state in granular flows could be used to drive particle mixtures to either mixed or segregated states as desired.