Shear-induced radial segregation in bidisperse suspensions

In this paper, we discuss experimental evidence for radial particle segregation in a parallel-plate geometry. The motion of coloured tracer particles of a size different from the bulk suspension is followed as a function of time, The tracer particles are seen to experience a constant drift velocity independent of their radial position. This is in addition to the random-walk motion arising from their interactions with other particles in the suspension. These observations are found to be consistent with the shear-induced migration model of Leighton & Acrivos (1987 a,b) as well as the tracer diffusivity measurements of Phan & Leighton (1996). In the experiments of Abbott et al. (1991), it was observed that larger particles migrated radially outward to regions of lower shear stress in a wide-gap Couette device. In our experiments large tracers were also observed to migrate radially outward, In this case, however, the radial migration resulted in migration to regions of higher shear stress, contrary to expectations. This apparent discrepancy is explained in terms of a model that incorporates both the stress-induced migration of earlier studies and a curvature-induced migration flux (which in turn is shear-induced) as an opposing effect.