Sedimentation and self-diffusion in suspensions of spherical particles

We study hindered settling in monodisperse suspensions of spherical particles in an incompressible viscous fluid. The sedimentation and self-diffusion coefficients are calculated in two-body approximation. We show that in a homogeneous and isotropic suspension in which the change of distribution function is not taken into account a transition to plug flow occurs at a volume fraction of about fifteen percent. At higher volume fractions the collective mobility coefficient vanishes. We discuss the relation to the experimental situation.