Shear-induced Coagulation Research Articles

The effect of Brownian diffusion on shear-induced coagulation of colloidal dispersions

The effect of a small amount of Brownian diffusion on shear-induced coagulation of spherical particles has been calculated. This has been accomplished by considering the binary collision process between a test sphere and identical spheres interacting with the test sphere through induced-dipole attraction, electrostatic repulsion and hydrodynamically induced forces. The effect of diffusion is found by means of an expansion in inverse Péclet number. Specific calculations were performed for uniaxial extension and for laminar shear flow. It is found that Brownian diffusion, the effect of which is nonlinearly coupled with flow type and strength, can act to increase or decrease the coagulation rate.

Effects of hydrodynamic and colloidal forces on the coagulation of dispersions

Carefully controlled experiments have been conducted to study the effects of hydrodynamic forces on stability of colloids. Results are compared, where possible, to the theory presented earlier, as well as to other experiments. Experiments were performed in a cylindrical Couette device with a dilute suspension of monodisperse (diameter = 500 nm) polystyrene spheres. Both Brownian and shear-induced coagulation experiments were performed. Suspensions with volume fractions of the order of 10 −5 were sheared at shear rates ranging from 10 2 to 2 × 10 3 sec −1. A low-angle laser light scattering technique was used to determine the degree of coagulation for various times of shearing. From combinations of theory and experiment one can estimate the value of the Hamaker constant. For both rapid Brownian and rapid shear-induced coagulation experiments, the Hamaker constant A ≌ 2 × 10 −21 J. When electrostatic repulsion is appreciable, the rate of shear-induced coagulation is greatly decreased. Under these conditions it appears that effects of Brownian and hydrodynamic forces must be considered simultaneously. It was also found that, under conditions of slow coagulation, the stability ratio is dependent on aggregate size, a result predicted by the theory.