Gravity-induced flocculation

Abstract

In polydisperse colloidal systems, flocculation can occur as a result of the differential creaming rates between small and large particles. As an improvement on earlier work, we have rigorously modelled this process of gravity-induced flocculation by incorporating gravitational and interparticle (both attractive and repulsive) forces, as well as hydrodynamic interactions in our analysis. From this analysis capture cross-sections and collision frequencies can be precisely defined and computed. In the absence of electrostatic repulsion, the gravity-induced flocculation rate is approximately proportional to g0.80 (where g is the local acceleration of gravity) and not g as previously predicted. When electrostatic repulsion is significant, particles can flocculate into either a primary or secondary minimum (as described by DLVO theory), or remain dispersed. The possibility of two different types of doublets leads to an interesting phenomenon. When Brownian motion can be neglected, dilute spherical sols can be unstable at low and high gravitational forces, but stable against gravity-induced flocculation at intermediate values.