Onset mechanism for granular axial band formation in rotating tumblers.

The mechanism for band formation of a granular mixture in long rotating tumblers is unresolved 70 years after the phenomenon was first observed. We explore the onset mechanism for axial segregation of a bidisperse mixture of particles of different sizes using the discrete element method. End walls initiate axial band formation via an axial flow due to friction at the end walls. The nonuniform distribution of axial velocity in the flow together with simultaneous radial segregation due to percolation result in small particles being driven further from the end walls, while larger particles accumulate at the end walls. Once this occurs, a cascading effect likely causes other bands to form due to the axial gradient in particle concentrations.