Metal disks of different size and density were placed at the bottom of a bed of monodisperse granu- lar material. The system was vibrated sinusoidally in the vertical direction. It was observed that, if the angular ac- celeration of the shaking was slightly greater than that of gravity, the metal disks rose to the top of the bed. This result has been known for over sixty years, but a basic understanding of the mechanism responsible for the rise of the disks is still a subject of debate. Our experiments and theoretical model show that the ascent speed of the disk is proportional to the square root of the disk densi- ty, approximately proportional to the disk size, and is a function of the disk's depth in the bed.
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