Reversible axial segregation of rotating granular media

Experimental measurements of axial segregation of binary mixtures of granular media combined in a horizontal cylinder and rotated like a drum mixer are reported. While in the traditional axial segregation effect the mixture of two different sizes of granular media will separate into bands of relatively pure single concentrations along the axis of rotation, in special cases the homogeneous mixed state can be restored simply by decreasing the speed of rotation. Systematic variation of the relative diameters of the components reveal three classes of behavior: no segregation at any rotation speed, a nonreversible axial segregation, and an axial segregation at high speeds that reverses back into the mixed state at low speeds. Measurements of the dynamic angle of repose of the mixed and segregated phases as a function of rotation speed support a model for the axial segregation effect that involves a diffusion equation with an effective axial diffusion coefficient that can be negative under the conditions for which segregation occurs.