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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 toG0.82; 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.

An experimental technique has been developed to study the deposition of colloidal particles under well controlled hydrodynamic conditions. The deposition process is observed under a microscope and recorded on video tape for further analysis. Fluid flow conditions in the experimental set-up were determined by numerical solution of the Navier-Stokes equations. Mass transfer equations were solved numerically (taking into account hydrodynamic, gravitational, electric double layer, and dispersion forces) for the stagnation point region. Also, some analytical solutions are presented. Deposition has been studied of 0.5µm polystyrene latex particles on cover glass slides used as collectors. From an analysis of the shape of the coating density vs. time curves and independently from the distribution of the particles on collector surfaces, it was found that one particle is able to block an area of about 20 to 30 times its geometrical cross-section. The initial flux of particles to the collector for a given salt concentration was found to depend strongly on the method of cleaning the collector surface. In general the flux and the escape of particles to and from the collector surface are sensitive to the interaction energy at small separations. The direct method of observing particle deposition and detachment could lead to important insights into the nature of particle-wall interactions at near contact.