EXPERIMENTAL-OBSERVATIONS OF PARTICLE MIGRATION IN CONCENTRATED SUSPENSIONS - COUETTE-FLOW

Nuclear magnetic resonance (NMR) imaging was used to observe the evolution of radial concentration and velocity profiles of initially well-mixed concentrated suspensions of spheres in viscous Newtonian liquids undergoing flow between rotating concentric cylinders (wide-gap, annular Couette flow). In Couette flow, particles migrate from the high shear-rate region near the inner rotating cylinder to the low shear-rate region at the outer wall. The particle concentration near the outer wall approaches maximum packing for randomly distributed spheres at steady state, and velocity profiles reveal that the suspension is almost stagnant in these regions. For unimodal suspensions of spheres, the shear-induced migration of large particles results in concentric two-dimensional, circular sheets of particles arranged in hexgonal close-packed arrangements extending inward from the outer wall. This paper examines the functional dependence of particle migration in concentrated suspensions undergoing shear flow in a wide-gap Couette. The primary experimental parameters were strain, shear rate, viscosity of the suspending liquid, particle diameter, and degree of polydispersivity of the particle phase. The particle migration is irreversible, even under creeping flow conditions, and is a function of total strain. Particle migration rate increases with the mean particle diameter raised to 2.7 +/- 0.3 for sieved samples and 2.5 +/- 0.3 for unsieved samples. The particle migration does not depend on the strain rate nor on the suspending liquid viscosity. The migration rate was found to depend only weakly on the polydispersivity of the particulate phase.