The microrheology of colloidal dispersions: XII. Trajectories of orthokinetic pair-collisions of latex spheres in a simple electrolyte

Abstract

Suspensions of 2.6- and 4.0-μm-diameter polystyrene latex spheres in aqueous glycerol containing KCl were subjected to Poiseuille flow in 200-μm-diameter tubes and the trajectories of particles undergoing two-body collisions recorded on cine film. The results were analyzed using theoretical equations describing the translational and rotational motions of interacting charged spheres in a shear flow. From the asymmetry of the paths of approach and recession of the spheres forming a transient doublet, it was possible to detect net attractive or repulsive forces of the order of 10 −13 N acting for 0.1 to 0.35 sec while the surfaces approached closer than 100 nm. The interaction forces were interpreted using the DLVO theory of colloid stability. Thus, in 50% aqueous glycerol, at 1 m M KCl, net repulsive forces due to double layer interaction acted during the collision, while at 10 m M KCl, the existence of a net attractive force provided evidence that the spheres passed through the secondary potential energy minimum. Here, trajectory analysis yielded a value of the Hamaker constant of 0.003 aJ and the retardation parameter of the van der Waals force of 200 nm. In 93% glycerol, however, double layer interaction did not appear to be involved in the pronounced repulsion observed during some collisions. It may be that this effect was due to surface roughness or the presence of hydration forces physically preventing approach of the two surfaces closer than 10 nm.