The microrheology of colloidal dispersions VII. Orthokinetic doublet formation of spheres

Summary A theory is developed for the rate of doublet formation in dilute dispersions of spheres subjected to a simple shear flow of gradient G when Brownian motion can be neglected but taking account of both hydrodynamic and interparticle interactions. When Coulombic repulsive forces are negligible, the capture frequency is found to be approximately proportional toG^0.82; when they dominate the van der Waals attractive forces, the capture frequency is zero. When both attractive and repulsive forces are important no simple relationship exists, but the capture frequency can be calculated numerically. Experiments are described which are in qualitative agreement with the theory. Hamaker constants for polystyrene in water calculated from the measured doublet formation agree with those calculated by means of the present theory from experimental data of other workers, but are somewhat higher than those obtained from perikinetic coagulation rates.