Apparatus and methods are disclosed for separating or fractionating particles of differing sedimentation rates in a fluid. For fractionation, the mixture is introduced into a tube and subjected to a cyclic sequence of alternating axial flows, with optional waiting periods of zero flow. The velocities and time periods of the individual flows in the cycle are adjusted so that particles of greater and lesser sedimentation rates tend to be driven in opposite directions. At the end of each cycle there is a step of resuspension of sedimented particles without axial dispersion and independent of axial flows.
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