(116c) Local Segregation Forces and Velocities in Granular Flows

Size, density, and shape segregation for flowing bidisperse granular mixtures depends on the species concentration, relative particle properties, and flow conditions. However, the physics of segregation is not well understood at the particle level or the local flow level. We have used 3D Discrete Element Method (DEM) simulations to measure 1) the segregation force on a single large spherical test particle tethered to a “virtual” spring in a shearing bed of small particles; and 2) the local segregation velocity in the macroscale flow of size-bidisperse and density-bidisperse spherical particles as well as for length-bidisperse rod-like particles. At the particle level in the dilute limit, changing the density of the large test particle such that the buoyancy force matches the test particle weight for different size ratios suggests an upward size segregation force that is a quadratic function of the particle size ratio. Based on this, the net vertical force on a single large particle with zero vertical velocity in a sea of shearing small particles is the sum of this upward size ratio dependent force, the upward buoyancy force, and the downward weight of the particle. Focusing on macroscale flows away from the dilute limit, the local segregation velocity for one species is approximately proportional to both the local shear rate and the local concentration of the other particle species for size-bidisperse mm-size spherical particles. The coefficient of proportionality in this relationship, which can be thought of as a segregation length scale, depends logarithmically on the ratio of the particle sizes up to a size ratio of three. Surprisingly, even though the linear dependence of the segregation velocity on the shear rate and concentration derives from a kinetic sieving model for size-bidisperse particles, it also works just as well for density-bidisperse spherical particles and length-bidisperse rod-like particles. In these cases, instead of a logarithmic dependence on the size ratio, the segregation length scale is a logarithmic function of the particle density ratio and rod length ratio, respectively. While these results provide significant insight into the local nature of segregation for particles of various types, the connection between the forces on an individual particle and the local segregation velocity needs further study.