The slow sedimentation of suspensions of solid particles in a fluid results in complex phenomena that are poorly understood. For a low volume fraction (φ) of particles, long-range hydrodynamic interactions result in surprising spatial correlations1 in the velocity fluctuations; these are reminiscent of turbulence, even though the Reynolds number is very low2-4. At higher values of φ, the behaviour of sedimentation remains unclear; the upward backflow of fluid becomes increasingly important, while collisions and crowding further complicate inter-particle interactions5-8. Concepts from equilibrium statistical mechanics could in principle be used to describe the fluctuations and thereby provide a unified picture of sedimentation, but one essential ingredient - an effective temperature that provides a mechanism for thermalization - is missing. Here we show that the gravitational energy of fluctuations in particle number can act as an effective temperature. Moreover, we demonstrate that the high-φ behaviour is in fact identical to that at low φ, provided that the suspension viscosity and sedimentation velocity are scaled appropriately, and that the effects of particle packing are included.
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