Sedimentation of charged colloids in the gravitational field : Relaxation toward equilibrium

Abstract
The sedimentation of charged colloids in the gravitational field is studied. This paper deals with the dynamics of the process, i.e. the relaxation toward equilibrium. In the case of salt-free solutions without hard-sphere repulsion, it is found that electrostatic interactions change the apparent mass and the diffusion coefficient of the colloid. The problem can be studied in terms of relaxation times that have been analytically calculated. The result depends on the initial conditions and the spatial localisation. The hard-sphere repulsion, that has been treated at the Carnahan-Starling level, increases the diffusion forces so that the relaxation toward equilibrium is faster. On the opposite, the hydrodynamic interactions between the particle decrease the sedimentation velocity. We have studied the competition between these effects. It is found that the hydrodynamic interactions compensate for the hard sphere repulsion nearly exactly. The results are consistent with experimental studies.