Particle concentration effects on shear coagulation in the presence of repulsive interparticle forces

Abstract

The shear stability of monodisperse polystyrene latices, both in the presence and absence of surfactants, was observed as a function of particle concentration in the range of particle volume fractions from 4.29 × 10 −5 to 6.86 × 10 −4. Two different latices were tested; one containing ionizable groups chemically attached to the particle surface and one without such groups. All latices were tested in a Couette device capable of generating shear rates as high as 29000 s −1. Both the Smoluchowski theory and the more recent trajectory analysis approach predict that the coagulation rate should increase linearly with the particle volume fraction. This behavior was observed for latex particles which possessed only weak repulsive forces. For latex particles with significant repulsive forces, whether arising from adsorbed surfactants or chemically bound surface groups, the shear coagulation rate was observed to decrease with increasing particle volume fraction. This decrease in rate must be due to a decrease in the collisional efficiency with increasing particle volume fraction. A more definitive explanation of the effect must await future improvements in the theoretical description of the shear coagulation process.