Axial and lateral particle ordering in finite Reynolds number channel flows

In pressure-driven channel flows at finite Reynolds number, suspended particles are known to undergo cross-streamline migration to a specific position off the centerline. We investigate the combined effects of channel geometry and particle concentration on the migration and ordering of particles in the channels with rectangular cross-sections using both experiments and numerical simulations. First we show that under confinement a single particle always migrates to the center of the longer dimension of the cross section for a range of cross-sectional aspect ratios. In case of a dilute suspension, the particles exhibit lateral and axial ordering owing to the hydrodynamic interactions of neighboring particles. Upon increasing the particle loading, a stepwise transition from one to two to many trains of particles in the lateral direction is observed. For a given channel cross-section, we present a criterion for this transition based on the number of particles per unit length.