| Colloidal suspensions compose a major segment of the chemical industry, encompassing paints, pastes, coatings, lubricants, and composite materials to name a few. Economic and environmental pressures demand ever-higher solids loading, solvent specificity, and greater thermal and mechanical stability. However, there are often difficulties associated with processing these materials. For example, concentrated suspensions under a large shear field can exhibit shear thickening, in which the viscosity dramatically increases as a function of shear rate. This is accompanied by an altered microstructure, possibly causing irreversible flocculation or damage to processing equipment.; Recognizing that flow properties are dictated by colloidal-level forces, constitutive equations relating the applied deformation to the resultant stresses have been developed which are based on the nonequilibrium configuration of particles in the suspension. Optical measurements, such as dichroism and birefringence, provide a sensitive measure of both microstructure and elastic stress in many materials. Using Rayleigh-Gans light scattering theory, a new expression for dichroism was derived. This expression, when compared to the micromechanical equations for shear stress, resulted in a stress-optical relation analogous to the stress-optical law found in many polymer solutions. The stress-optical relation equates measurement of the refractive index tensor to the stress tensor for concentrated suspensions of spherical particles.; The goals of this work were: (1) to test statistical mechanical theories which predict the nonequilibrium microstructure through optical and rheological measurements on model hard-sphere and charged-rod suspensions, and (2) to determine the relative contribution of Brownian versus hydrodynamic forces to the stress tensor using the newly-developed stress-optical relation. The results demonstrated that shear thinning in colloidal suspensions is governed by changes in the thermodynamic (Brownian) contributions to the shear stress. The hydrodynamic contribution remains constant, experimentally verifying the theories and simulations for the first time. The converse was found to be true for shear thickening, which is caused by the formation of particle "clusters" generating large lubrication stresses. With an understanding of the microstructural origins of rheological behavior, colloidal forces can be tailored to impart specific behavior. |
