Rheology of noncolloidal suspensions of spheres in oscillatory shear flow and the dynamics of suspensions of rigid fibers

The work presented in this dissertation provides a significant contribution to understanding the rheological behavior of suspensions systems. Specifically, this work provides results from a comprehensive set of experiments on the rheology of suspensions of noncolloidal spheres undergoing oscillatory shear flow. The experiments are complimented by results from the first dynamic simulation of suspensions of noncolloidal spheres in unsteady shear flows. Additionally, results from experiments on the steady shear rheology of model suspensions of rigid ellipsoids are presented.; Experiments on suspensions of spheres undergoing oscillatory shear flow show that the rheology is strongly influenced by the applied strain amplitude. At each amplitude, the steady value of the complex viscosity decreases with total strain for high strain amplitudes and increases for low amplitudes. The transition point at which the qualitative behavior changes occurs at an amplitude-to-gap ratio between 0.1 and 0.5 and is independent of the particle size distribution and suspension system. The steady state value of the complex viscosity is a nonmonotonic function of the applied strain amplitude, with a minimum viscosity observed at an amplitude-to-gap ratio of 1. The experiments suggest that shear-induced migration is of no consequence, and that the observed behavior is instead due to changes in the suspension microstructure. The rheology observed in the experiments is largely confirmed by Stokesian dynamics simulations. Simulations of suspensions of noncolloidal spheres undergoing unsteady simple shear flows are used to evaluate the evolution of the stresses with time along with the corresponding microstructural development. Similar to the experiments, the shear stress drifts with total strain before attaining a steady state that depends upon the applied strain amplitude. The steady state viscosity obtained from the shear stresses exhibits a nonmonotonic dependence on the applied shear rate that agrees qualitatively with the experimental results. An analysis of the suspension microstructure at steady state reveals three distinct microstructures that correlate to the observed rheology. Hydroclusters, ordered layers, and crystalline structures can all be induced by simply altering the applied strain amplitude. Results from both simulations and experiments indicate irreversibility over the range of strain amplitudes studied.; The rheology of semi-dilute suspensions of rigid polystyrene ellipsoids at rotational Peclet numbers greater than 103 was studied for two diRTMerent aspect ratios. The ellipsoid suspensions exhibit shear thinning behavior for both aspect ratios. Although in agreement with previous experiments, rate dependent rheology is not predicted by theories and simulations. Direct comparison of the results with rheological measurements on suspensions of spheres with material properties identical to the ellipsoids suggest that colloidal interactions are improbable sources of the shear thinning behavior. A mechanism for the observed shear thinning behavior is proposed which involves a competition between particle drift due to shear-induced migration, and thermal diffusion. For the largest value of the shear rate, the relative viscosity scales linearly with dimensionless number density regardless of the aspect ratio. At the lowest shear rate, the relative viscosity does not scale linearly with dimensionless number density and a dependence on the ellipsoid aspect ratio is apparent.