Particle segregation and dynamics in confined flows.

Nonlinearity in finite-Reynolds-number flow results in particle migration transverse to fluid streamlines, producing the well-known "tubular pinch effect" in cylindrical pipes. Here we investigate these nonlinear effects in highly confined systems where the particle size approaches the channel dimensions. Experimental and numerical results reveal distinctive dynamics, including complex scaling of lift forces with channel and particle geometry. The unique behavior described in this Letter has broad implications for confined particulate flows.