Understanding the dynamics of segregation bands of simulated granular material in a rotating drum

– Axial segregation of a binary mixture of grains in a rotating drum is studied using Molecular Dynamics (MD) simulations. A force scheme leading to a constant restitution coefficient is used and shows that axial segregation is possible between two species of grains differing by size only. Oscillatory motion of bands is invistigated and the influence of the frictional properties elucidated. The mechanism of bands merging is explained using direct imaging of individual grains. Introduction. – Among the many puzzling phenomena exhibited by granular media, axial segregation [1] a.k.a. banding is one of the least understood. Due to the fundamental interest as well as the numerous industrial applications [2], a great deal of both experimental [3–6] and theoretical [7, 8] work has been devoted to the topic, but full understanding is still lacking. Molecular Dynamics simulation provides new insights in the understanding of the phenomenon since it allows one to vary all parameters and measure any physical property. The only large scale numerical study of axial segregation [9] reports remarkable results but interactions between grains are derived from the Lennard-Jones potential, which is not well-suited for granular material. Our simulation uses a modified spring-dashpot force scheme leading to a restitution coefficient independent of the species of grains colliding. Here we show that axial segregation is possible between two species of grains differing by size only. We observe that a difference in the frictional properties of the two species of grain is not necessary to the onset of banding but does triggers oscillatory instabilites. Finally, the mechanism of bands merging is elucidated using direct imaging of individual grains. Simulation Methods. – This letter presents results based on the Molecular Dynamics method (MD), a.k.a. Discrete Elements Method (DEM). This method deals with soft (but stiff) frictional spheres colliding to one another. The drum is partially filled with a mixture of small and large beads (with respective radius RS = 2.4 mm and RL = 2RS). The total volumes of small and large grains are equal. The number of grains varies from 5.10 to 5.10. The length of the drum, l, varies from 200RS (≃ 50 cm) to 1000RS (≃ 250 cm) and its radius