Granular segregation in the double-cone blender: Transitions and mechanisms

We investigate granular segregation in one of the most common industrial devices used in granular processing: the double-cone blender. We report several new and spontaneously occurring segregation patterns, including stripes, bands, and a symmetry-breaking state in which one species vacates half the tumbler. By varying the tumbling speed along with particle size and size ratio, we find that the transitions between segregated patterns are extremely sharp: Changes in fill level or speed of under one percent are sufficient to produce a reproducible qualitative change in the observed pattern. We show that the several distinct segregation patterns observed experimentally can be reproduced from a simplified model in which outward rolling on the convective granular cascade competes against inertial motion of rapidly moving large particles. Finally, we identify a cutoff particle size ratio above which large particles become buried in the cascading flow, and segregation appears to cease. (C) 2001 American Institute of Physics.