Fig. 1.

Particle trapping in rectangular microchannels. (a) Two lift forces orthogonal to the flow direction act to equilibrate microparticles near wall in rectangular microchannel upstream from the trapping chambers. The shear-induced lift force F_s is directed down the shear gradient and drives particles toward channel walls. The wall-induced lift force F_w directs particles away from the walls and drives particles toward the channel centerline. The balance of these two lift forces causes particles to equilibrate. At the channel expansion, the fluid is accelerated by the drag force and forms laminar vortices (Moffatt eddies). Since the channel wall is no longer in close proximity, the wall-induced lift force F_w is absent, causing the shear-induced lift force to dominate particle migration into the vortices. The vortices entrain these particles and effectively trap them. (b) Fluorescent image illustrating traces of trapped particles.