Abstract

When the particles in a suspension are denser than the suspending solvent, their motions are controlled by sedimentation as well as the customary diffusion due to stochastic thermal fluctuations. As the particle concentration is increased, many-body hydrodynamic interactions strongly reduce the settling and diffusion[1,2]. Specifically, in a uniform concentrated suspension, the motion of the particles induces a compensating back flow of solvent which increases the average drag (i.e., reduces the diffusion and settling rates). However, the drag on any given particle is determined by the instataneous configuration of its neighbors. Therefore, the particles settle with a distribution of velocities, both vertically and horizontally, which on long time scales causes the particle to drift randomly. Recent computer simulations[3,4] and direct observations at low volume fraction in quiescent sedimentation experiments[5] show that the particles settle on the order of 100 interparticle distances before the microstructure rearranges sufficiently to decorrelate the particle velocities. Consequently, even in the absence of thermal fluctuations, on long time scales the particle motions become stochastic with an effective “diffusion” coefficient which is much larger than that due to Brownian motion.

© 1992 Optical Society of America