The sedimentation potential in a dilute suspension

During the sedimentation of charged particles immersed in an ionic solution, a gradient of electric potential forms. The gradient reflects the surface potential of the particles, moderated by the screening effect of the diffuse charge. In the theory developed here the fact that the net current is zero is used to derive an expression for the gradient which is free of ambiguities associated with the shape of the averaging volume, one of the shortcomings of the previous theory. The general expression also differs from those derived by previous investigators who summed the dipole fields for each particle, a procedure which leads to expressions that fail to satisfy the constraint on the current. The effect of the macroscopic field on the sedimentation velocity is found to be significant when the double layer is thick. Changes in the sedimentation coefficient are also reflected in the Brownian diffusivity of suspended particles but here the effect appears less significant.