FIG. 1.

Schematic (a) and device layout (b). Particles flowing in a low aspect-ratio microchannel first migrate to top and bottom walls undergoing a shear-induced lift force F_S (stage 1); once the shear-induced lift force balances with a wall-induced lift force F_W, particles further migrate toward the centers of horizontal walls experiencing a rotation-induced lift force F_Ω (stage 2). Due to the strong size-dependent migration velocity, the larger particles complete the two-stage migration and equilibrate to the two long-face-centered positions much faster than the smaller particles. By setting the channel length to the focusing length of the larger particles, they can be filtered from the smaller particles through the central outlet.