If there exists a density difference between particles and liquid, then gravitational driving forces will set the particles in motion. Complicated fluid-mediated hydrodynamic interactions between the particles lead to convoluted trajectories and give rise to collective phenomena as for example anisotropic self diffusion of particles. The particles slowly settle to the bottom of the container with an average speed that decreases as the volume fraction of particles in the container increases. We have chosen here to follow the convention in most of the literature on sedimentation. We display a typical situation during the batch settling process in Fig. 4 .
Gravity is taken to act in -y direction.
We investigate for an almost monodisperse system. Reynolds number is 100 times larger, i. e. . We have performed simulations and determined the settling velocity as a function of the solid fraction of the particle suspension. Without these lubrication forces, we choose the value of in Eq. The interaction of particle and liquid phase results only from the pointlike frictional drag between the two.