Reverse Buoyancy in Shaken Granular Beds

Large grains are well known to rise to the top of granular beds. In this Letter, we report to the contrary on a shaking regime in which large, heavy grains rise, but equally large, light grains sink in a granular bed. This behavior occurs in deep beds with high amplitude vibrations—precisely the regime encountered during transportation of industrial solids mixtures. We examine a simple one-dimensional model that captures 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 light objects, by contrast, can fluctuate wildly, preventing this infiltration.