Passive fluidic particle manipulation techniques are useful for particle handling in microfluidic devices. Created using specifically designed channel structures, these systems are advantageous due to their low energy requirements and simple operation. Here we present results on a passive fluidic element termed an asymmetric trap, that is based on obstacle/particle steric interactions. The asymmetric behavior-capturing particles during downward flow with respect to the trap and releasing or passing particles during upward flow-is induced by the asymmetric configuration of the trap elements. The critical particle diameter for the asymmetric behavior in the trap array was theoretically obtained by numerically solving the conservation of flow equations through critical gaps between trap array elements. Based on the physical dimension of the array, five different trap/particle interaction regimes are predicted: symmetric passage (I), asymmetric passage (IIa), symmetric capturing (IIb), asymmetric capturing (III), and channel clogging (IV). The presence of these regimes was experimentally confirmed using micron-sized particles (20.3 mu m and 10.1 mu m) at low Reynolds number (Re < 0.1).
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