Figure 1.

(a) Photos of the symmetric (top) and asymmetric (bottom) ratchet microchannels used in the experiment; (b) Zoomed-in views of the symmetric (top) and asymmetric (bottom) ratchet structures with their corresponding dimensions highlighted; (c) Velocity analysis for a particle traveling towards and away from the ratchet throat, respectively, where the background color shows the electric field contour (the darker, the larger magnitude) and the background lines represent the electric field lines (equivalent to the fluid streamlines).