  The irreversible behavior of a highly confined non-Brownian suspension of
spherical particles at low Reynolds number in a Newtonian fluid is studied
experimentally and numerically. In experiment, the suspension is confined in a
thin rectangular channel that prevents complete particle overlap in the narrow
dimension and subjected to an oscillatory pressure-driven flow. In the small
cross-sectional dimension particles rapidly separate to the walls, whereas in
the large dimension features reminiscent of shear-induced migration in bulk
suspensions are recovered. Furthermore, as a consequence of the channel
geometry and the development and application of a single-camera particle
tracking method, three-dimensional particle trajectories are obtained that
allow us to directly associate relative particle proximity with the observed
migration. Companion simulations of a steadily flowing suspension highly
confined between parallel plates are conducted using the Force Coupling Method
which also show rapid migration to the walls as well as other salient features
observed in the experiment. While we consider relatively low volume fractions
compared to most prior work in the area, we nevertheless observe significant
and rapid migration which we attribute to the high degree of confinement.
