The goal of this study is to enhance fundamental understanding of the flow of a concentrated bimodal suspension through an abrupt contraction-expansion. Contraction-expansion flows arise in materials processing operations and have the potential to significantly enhance particle size separation or mixing effects occurring in simple geometries such as tube flow. In previous studies of monomodal suspensions in the contraction-expansion geometry, we consistently observe the formation of distinct core-annular structures downstream of the expansion section.

In this study, suspensions of neutrally buoyant, noncolloidal spheres in viscous, Newtonian liquids undergo pressure-driven flow in an abrupt, axisymmetric 1:4 contraction-expansion. Nuclear magnetic resonance imaging (NMRI) is used to measure the steady-state particle concentration distribution and flow field. By adopting varied particle materials (i.e. rigid polymer vs. hydrogel) we can monitor particle size separation through the contrast in the spin-spin (T2) relaxation time among rigid solids, gels and suspending fluids. Image analysis methods for obtaining quantitative concentration profiles of two particle types from NMRI data are under active development. The role of particle and flow properties (e.g. relative particle volume fractions, particle size ratio, and inlet conditions) and a comparison to results from monomodal suspensions will be presented.