Fig. 1.

Dean flow phenomena in curved microchannels. (A) Idealized Dean flow mediated rotation sequence (i and o denote the inner and outer channel walls). At low κ (Upper), two parallel streams of different species (yellow and black) entering a curved microchannel segment experience minimal perturbation to the laminar flow. At κ ∼ 10 (Lower), the transverse flow generated by the counterrotating Dean vortices in the upper and lower halves of the channel transport the inner (yellow) stream toward the outer wall while the outer (black) stream is pulled inward, causing the positions of each species to be transposed. (B Upper) Schematic of the curved microchannel geometry investigated (100 μm wide; 29 μm tall; 630 μm radius of curvature). The transverse flow field was examined at the entrance to the curved segment and at a location 1.5 mm downstream. Analytically computed velocity and concentration profiles are shown (Left Lower) beside confocal cross-sectional images of the transverse flow in the microchannel (Right Lower) at flow rates ranging from 2.6 < Re < 45.1 (0.7 < κ < 12.1). The boxed area represents conditions under which the transverse flow induces ≈90° rotation in the upper and lower halves of the channel. (C) Top-view images of aqueous streams labeled with blue and yellow dye in a curved microchannel segment (200 μm wide; 29 μm tall; 630 μm radius of curvature). At κ = 1.0 (Upper) the streams flow in parallel along the entire length, whereas at κ = 14.2 (Lower) the blue stream is transported from the inner to the outer wall.