PARTICLE MIGRATION IN A COUETTE APPARATUS - EXPERIMENT AND MODELING

Suspensions comprised of neutrally buoyant spheres in Newtonian fluids undergoing creeping flow in the annular region between two rotating, coaxial cylinders (a wide-gap Couette) display a bulk migration of par ticles towards regions of lower shear rate. A series of experiments ar e performed to characterize this particle migration, including the inf luence of particle size, surface roughness, and volume fraction. Littl e, if any, effect of particle surface roughness is observed. An existi ng continuum diffusive-flux model [Phillips et ni. (1992)] for predict ing particle concentration profiles in monomodal suspensions is evalua ted using the current series of experimental data. This model predicts a dependence of the migration rate on the square of the suspended par ticles' radius, a(2). whereas the present experiments indicate that sy stems with average particle volume fractions of 50% display a rate tha t scales with a(3). Previous use of the diffusive-flux model has assum ed constant values far diffusion coefficients which serve as tuning pa rameters in the phenomenological equation. Here the experimental data are used to investigate variations of the model in which the diffusion coefficients depend upon either the local or global particle volume f raction. For initially uniform suspensions, the coefficients are found to be best modeled as functions of the local particle volume fraction . (C) 1998 The Society of Rheology. [S0148-6055(98)01002-5].