Subtle order in settling suspensions

A dilute suspension of uniform, non-Brownian spheres settles slowly in a viscous solvent. The initially well-mixed system showing Poisson or random occupancy statistics evolves to a system having reduced number fluctuations, but otherwise appearing random. The reduced number fluctuations are consistent with recent measurements of velocity fluctuations in settling suspensions. These experimental results test the assumptions leading to the theoretical predictions by Calflisch and Luke that the velocity fluctuations increase without limit with increasing sample dimension. The theoretical prediction assumes Poisson occupation statistics contrary to our observations.