Dynamics and rheology of particulate suspensions undergoing steady and unsteady shear flows

Suspension rheology has applications in fields from cutting edge nano-technology to biotechnology as well as existing industrial processes. Suspension systems exhibit interesting behaviors under flows such as shear-thinning and shear-thickening, but the dynamics of many systems are not well-understood. In this work, I introduce two examples of well-defined, even simple, systems where the dynamics and the rheology surprisingly remain mysterious. In addition, the rheology of suspensions of single-walled carbon nanotubes is studied. The experiments are complimented by Stokesian dynamics simulations for the non-colloidal suspensions of spheres in unsteady shear flow. The rheology of rigid rod suspensions in steady shear flow is experimentally investigated.;The work presented in this dissertation provides a significant contribution towards generating a more comprehensive view of rheology of particulate suspensions undergoing steady and unsteady flows. Moreover, the work demonstrates the potential for rheology to be used as a quantitative tool, rather than simply a qualitative one.;First, the dynamics of a suspension of rigid rods in steady shear flow is studied. Theories predict a steady shear viscosity that is independent of shear rate for a non-colloidal suspension of rods. However, unexpected shear-thinning behaviors are observed, although a well-defined suspension system is used. The shear thinning behaviors become stronger with increasing volume fraction and aspect ratio of the rods. Possible mechanisms such as flocculation are explored. Through direct comparison of processed spheres dispersed in an identical suspending liquid as the rod suspension, flocculation mechanisms are ruled out. Based upon a recent study of Park (2009), residual weak Brownian torque is argued to cause net migration toward the center in a torsional flow, resulting in the observed shear thinning behaviors.;Secondly, the stability and rheology of the single walled carbon nanotube (SWNT) suspensions prepared by interfacial trapping method is examined and compared to a conventional method of ultracentrifugation. Since the rheological properties are sensitive to the suspension microstructure, the change of rheological properties, such as viscosity, can be employed as a systematic standard of stability. The steady shear viscosities have been measured and compared as a function of shear rate and aging time of the suspension. Also, the visual states of the suspension have been observed. The rheology of the SWNT suspensions depends on the preparation of surfactant solution. Also, the interfacial trapping method generated similar behaviors to the SWNT suspension prepared by the ultracentrifugation method.;Finally, the dynamics of non-colloidal spheres in oscillatory shear flow is studied by experiment and simulation. Two distinct scales are observed for the development of the rheology in time. At small total strains, a rapid decay of eta" is observed, while eta' and eta* remain constant. However, the evolution of the complex viscosity is observed over large total strains, indicating microstructural changes over long times. Also, this suspension system shows a non-monotonic dependence of viscosity on strain amplitude. Stokesian dynamics simulations are used to correlate the rheology and microstructures.