On axial segregation in a tumbler: an experimental and numerical study

We report experimental and discrete element method (DEM) simulation results on size segregation of granular materials in a rotating drum. We first define the degree of segregation which allows us to quantify the time evolution of the formation of axial segregation bands. We then focus on the interplay between axial and radial segregation. In experiments, surface particle velocities are measured by particle tracking. There is no significant jump in velocity or velocity gradient at the interface between small to large grains in the radial direction. On the other hand, there is a large difference in flow velocity between large and small particles along the axial direction of the drum, and strong gradients at interfaces between large and small particle bands. Measurements of the diffusion of a localized axial band of small grains show the formation of a radial core from an axial band. This process is not subdiffusive (nor superdiffusive) in our simulation. The time evolution of the axial segregation pattern is strongly affected by radial segregation. Coarsening and oscillatory motion of axial bands involve flows of small particles through the radial core, in particular when the band position or width oscillates. Coarsening of the axial segregation pattern is significantly retarded or eliminated in the absence of a radial core.