We propose a novel continuous-flow microfluidic particle concentrator with a specified focusing-particle number ratio (FR) at different channel outlets using induced-charge electroosmosis (ICEO). The particle-focusing region contains two floating electrodes of asymmetric widths L-2 and L-1 in the gap between a driving electrode pair, all of which are fabricated in parallel in the main channel. Applying an AC voltage over the driving electrodes, an ICEO flow with two vortexes can be induced over each of the two floating electrodes, and the actuation range of the ICEO vortex is proportional to the respective electrode size. We establish a preliminary physical model for the value of FR: at a moderate voltage and frequency range, FR approaches L-2/L-1 due to the scaled ICEO actuation range; by further modifying the voltage or frequency, FR is freely adjustable because of the variation in ICEO velocity. Furthermore, by connecting multiple focusing regions in series, i.e., high FR = (L-2/L-1)(n) can be conveniently generated in an n-stage flow focusing device. Our results provide a promising method for yielding transverse concentration gradients of particles useful in pre-processing before analysis.
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