Sedimentation is the process by which small solid particles settle out of a liquid suspension. New calculations show that irregularly shaped particles can display novel behavior during sedimentation that does not occur for more symmetric objects. Such particles can drift laterally so as to smooth out density variations that occur during settling, a result that potentially resolves a long-standing discrepancy between theory and experiment over the magnitude of such variations. The theory may help explain the progress of sedimentation in both natural and industrial contexts.
Sedimentation is important in water treatment and industrial processes that rely on dense suspensions flowing as fluids, and it also influences the paths of rivers and the accumulation of geological deposits. Variations from place to place in the concentration of particles affect the progress of sedimentation. A volume with fewer particles will have greater overall buoyancy than an equal volume containing more particles, so the denser volume will fall faster. For a random distribution, fluctuations in the number of particles grow with the volume, leading to a theoretical prediction 30 years ago that vertical velocity fluctuations in a sediment should depend on the size of the sedimenting system rather than on intrinsic properties of the sediment itself [1]. Most experimental and observational evidence contradicts this prediction.
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