REVERSE BUOYANCY IN SHAKEN GRANULAR BEDS

Large grains are well known to rise to the top of granular beds. In th is Letter, we report to the contrary on a shaking regime in which larg e, heavy grains rise, but equally large, light grains sink in a granul ar bed. This behavior occurs in deep beds with high amplitude vibratio ns - precisely the regime encountered during transportation of industr ial solids mixtures. We examine a simple one-dimensional model that ca ptures some aspects of this effect. The central observation underlying this model is that inertia causes heavy objects to lift off the bed, permitting smaller particles to infiltrate beneath; the motion of ligh t objects, by contrast, can fluctuate wildly, preventing this infiltra tion. [S0031-9007(98)07684-4].