NMR IMAGING OF SHEAR-INDUCED DIFFUSION AND STRUCTURE IN CONCENTRATED SUSPENSIONS UNDERGOING COUETTE-FLOW

This note describes nuclear magnetic resonance (NMR) imaging of two concentrated suspensions undergoing flow between rotating concentric cylinders (wide-gap, annular Couette flow). Suspensions of both monodisperse (50% by volume) and bidisperse (60% by volume) spheres are studied. We find that particles migrate from the higher shear rate regions near the rotating inner cylinder to the lower shear rate regions near the stationary outer wall, establishing large concentration gradients after only a short time. In addition, the large particles in the bimodal suspension form concentric cylindrical sheets, parallel to the axis of the Couette device, which rotate relative to each other. In the Couette devices used in these studies, no significant axial migration of the particles is observed: the dispersion of particles is almost entirely in the radial direction. This particle migration and structure formation is believed responsible for torque reductions and other anomalous behavior witnessed during rheological testing of concentrated suspensions reported by Leighton and Acrivos (1987).