REVERSIBLE AXIAL SEGREGATION OF ROTATING GRANULAR MEDIA

Experimental measurements of axial segregation of binary mixtures of g ranular media combined in a horizontal cylinder and rotated like a dru m mixer are reported. While in the traditional axial segregation effec t the mixture of two different sizes of granular media will separate i nto bands of relatively pure single concentrations along the axis of r otation, in special cases the homogeneous mixed state can be restored simply by decreasing the speed of rotation. Systematic variation of th e relative diameters of the components reveal three classes of behavio r: no segregation at any rotation speed, a nonreversible axial segrega tion, and an axial segregation at high speeds that reverses back into the mixed state at low speeds. Measurements of the dynamic angle of re pose of the mixed and segregated phases as a function of rotation spee d support a model for the axial segregation effect that involves a dif fusion equation with an effective axial diffusion coefficient that can be negative under the conditions for which segregation occurs.