When polystyrene 'latex' beads are centrifuged to equilibrium in gradients of Nalcoag 1030, Nalcoag 1034A and Ludox 130M, which are commercial formulations of colloidal silica, the beads band at densities that may be markedly lower or higher than the bulk density. Addition to the gradient ofsmall amounts ofcertain polymers restores the banding densities towards the expected value. These findings closely mimic previously observed 'density shifts' of biological particles. A possible model for this anomalous behaviour is discussed.
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