(526e) Instabilities in Particle Dispersions

Shear flow of colloidal suspensions can vary enormously; from low viscosity fluids to highly elastic pastes. Their flow properties are controlled to a large extent by the size and concentration of the particles and by the interactions among them. There are two limiting cases of the fluid to solid transition which are well understood: The first is for very low volume fraction and very strong attractive interaction; the particles undergo diffusion limited cluster aggregation (DLCA), forming fractal clusters which ultimately form a continuous, but tenuous network that spans the whole system, thereby becoming a solid that can bear stress. Because the density of a fractal cluster decreases as it grows in size, DLCA will result in gelation at any volume fraction, however low. The second limiting case is for high volume fractions and no attractive interactions; the colloidal particles behave as hard spheres, and crowding can lead to a colloidal glass at volume fraction of 0.58, where the particles become caged and can no longer diffuse freely. The suspension again behaves as an elastic solid, where the elasticity is due to the resistance to distortion of the average particle configurations, and is entropic in nature. Kinetic theories have long been used to investigate suspensions of long-chain and rodlike molecules. Instability in particle dispersions involving aggregation kinetics and jamming at different volume fractions and Peclet numbers are computed and compared to experiments PMMA dispersions.