Motion of flocs of two or three interacting colloidal particles in a hydrodynamic medium

Abstract

Doublets and triplets of secondary-minimum flocculated, DLVO-type colloidal particles have been simulated by the computational technique of Brownian dynamics, and aggregate lifetimes have been determined as a function of well depth. It is found that a pair of particles may “jump” out of a secondary minimum of depth 10 kT within a matter of seconds. Flocs of DLVO-type particles typically “dissociate” on a timescale shorter than that for floc rotational diffusion, and so a harmonic-dimer potential has been used to model the rotational and translational diffusion of a stable doublet. For particles that can approach closely (within, say, 5% of a particle radius), it is necessary to use exact interparticle hydrodynamic expressions rather than the approximate Oseen form. With triplets there is an increasing tendency towards linear “dissociation” as the well depth increases. The simulated doublets are considerably less stable than the real doublets of polystyrene latex particles which they are meant roughly to represent. Inevitable ambiguities in defining words like “dissociation” and “aggregate” are discussed.