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Size bidisperse granular materials in a spherical tumbler segregate into twodifferent patterns of three bands with either small particles at the equatorand large particles at the poles or vice versa, depending upon the fill levelin the tumbler. Here we use discrete element method (DEM) simulations withsupporting qualitative experiments to explore the effect of the tumbler wallroughness on the segregation pattern, modeling the tumbler walls as either aclosely packed monolayer of fixed particles resulting in a rough wall, or as ageometrically smooth wall. Even though the tumbler wall is in contact with theflowing layer only at its periphery, the impact of wall roughness is profound.Smooth walls tend toward a small-large-small (SLS) band pattern at thepole-equator-pole at all but the highest fill fractions; rough walls tendtoward a large-small-large (LSL) band pattern at all but the lowest fillfractions. This comes about because smooth walls induce poleward axial drift ofsmall particles and an equator-directed drift for large particles, resulting inan SLS band pattern. On the other hand, rough walls result in both sizes ofparticles moving poleward at the surface of the flow, but due to radialsegregation, small particles percolate lower in the flowing layer where thereis a return drift toward the equator while large particles remain at thesurface near the pole, resulting in an LSL band pattern. The tendency towardeither of the two band patterns depends on the fill level in the tumbler andthe roughness of the tumbler's bounding wall.

Influence of Rough and Smooth Walls on Macroscale Granular Segregation Patterns