The two flows converge and enter the 90 curve. In the curve, the induced Dean flow (inset 2), characterized by double-ring vortices in the cross-sectional plane, sweeps the particles from the top and bottom of the channel toward the center plane of the channel. This step is termed as “microfluidic drifting” and effectively focuses the flowing particles in the vertical direction
(inset 3). In the second step (insets 3 to 4), the particles are pushed into a single line by the horizontal sheath flow C and D. Thus, this two-step procedure results in 3D focusing of the particles within a single-planar microfluidic channel.