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"Origin of Stability in Sedimentation" Full Paper

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    The problem is to understand the flow dynamics of spheres as they settlein liquids. It has previously been found that the dynamical properties of the particles vary as they settle, often a steady state behavior is not observed by the time all the particles are on the bottom of the cell. To get around this, we employ another way of studying  sedimentation, one in which the particles are constantly settling, but never reaching the bottom. This is done in a Fluidized Bed in which liquid is pumped upwards through the particles at a rate that matches the settling-- in this way the system is 'perpetually settling' and steady state profiles are eventually established. 

    In our experiments the particles are large enough that Brownian motion is negligable, and they settle at such slow rates, V~1 mm/s, that the Reynolds number is <<1 so that the linear equations of creeping flow should apply. As a single particle settles, it sets up a flow field around it which varies as U~1/r, i.e. a power law decay that lacks any characteristic decay length, i.e. it is of infinite range. Consequently in real samples at finite concentrations calculating the flow fields present in the fluid as the particles settle is a complex many-body problem.

Heres an example of how, in a stable Fluidized bed, the particle concentration is not uniform with height.