We give an overview of some of the experiments currently underway to study the coupling of the microstructure and rheology of concentrated suspensions. Nuclear magnetic resonance imaging, real-time x-ray radiography, and refractive index matching allow the viewing of particles in concentrated suspensions. Both shear flow experiments and falling ball rheometry are reviewed. In the slow flow of these suspensions of large, hard, particles in a viscous Newtonian fluid, colloidal forces are negligible and hydrodynamic forces dominate.
Large local concentration changes are shown to occur rapidly in suspensions of uniform spheres subjected to flow between concentric rotating cylinders. Suspensions of spheres with a bimodal size distribution not only show similar phenomena, but also exhibit particle separation according to size. In addition, the large particles in the bimodal suspension migrate into ordered, concentric, cylindrical sheets, parallel to the axis of the cylinders. These sheets of particles rotate relative to each other. The particle migration and structure formation induced by this inhomogeneous shear flow is believed to be responsible for torque reductions and other anomalous behavior witnessed during the rheological testing of concentrated suspensions reported in the literature. Thus, suspensions may not always be characterized by a viscosity that is a scalar material property.
Suspensions of fibers also show markedly different rheological properties when the particles are aligned by flow. Falling ball rheometry is shown to be an effective tool to determine the bulk viscosity of a suspension while only slightly influencing the microstructure. This is illustrated by showing that falling ball rheometry can isolate the effect of orientation on the viscosity of a suspension of fibers.