Deposition kinetics of colloidal particles at an interface: Interplay of diffusion and gravity

Abstract

In this paper, we analyze the kinetics of irreversible adsorption of hard spheres from a suspension at rest onto a plane under the influence of diffusion and gravity. We have obtained analytical solutions valid in the low coverage limit of the adsorption kinetics. In order to investigate the adsorption kinetics up to higher coverages, we have also performed nonsequential Brownian dynamics computer simulations. It is shown that the widely employed dimensionless radius R* (or, equivalently, the gravitational Péclet number Pe) cannot alone characterize the relative effect of diffusion and sedimentation in adsorption kinetics. The description of the adsorption process requires the introduction of an additional, independent dimensionless number, Gad, which is a combination of the Péclet number and the bulk volume fraction. The adsorption kinetics is dominated by diffusion for Gad≪1 and by sedimentation for Gad≫1, irrespective of the value of R*. In the case of R*>1 and Gad≫1 the observed kinetics is qualitatively similar to the predictions of the ballistic deposition model, although significant deviations are observed. When Gad≫1, it is also shown that blocking effects due to the interaction with previously adsorbed particles are proportional to the volume fraction so that they can be unobservable until the adsorbing surface is nearly saturated.