Continuous particle separation with ac electro-osmosis and dielectrophoresis in a microchannel

This paper describes a particle-separation device combining AC electroosmosis and dielectrophoresis under pressure-driven flow. The whole device comprises an initial hydrodynamic-focusing compartment with Y junction and an electrohydrodynamic compartment with interdigitated coplanar ITO electrode arrays. In the electrohydrodynamic compartment, the electrode arrays on the bottom of the microchannel are inclined at a 10 degree angle with regard to the direction of channel. A lateral flow is generated by AC electro-osmosis flow triggered by a low-voltage AC electric field on the surface of the electrode. Superimposed upon the axial pressure-driven flow applied by the external syringe pump, AC electro-osmosis flow induces a depressed vortical flow above the electrodes. We find that when homogeneously suspended micro polystyrene particles with different sizes (0.86 μm and 5 μm) in the KCl solution (0.1 mM) are transported through the vortical flow region, the small particles, 0.86 μm, successfully become trapped in the lateral flow above the electrode arrays under the combination of AC electroosmosis and positive DEP, whereas the large particles, 5 μm, completely pass through the vortices. The effectiveness of this separation is investigated for different axial flow rates and amplitudes of the applied voltage. It is shown that with increasing flow rate, it becomes hard for the small particle to get trapped. The possibility of trapping, however, is enhanced by increasing the amplitude of the applied voltage. In addition, we found that the effectiveness of particle separation is frequency dependent, tending to zero at both low and high frequencies. The peak of the effectiveness happens at a so-called characteristic frequency which depends on the conductivity and geometry of the electrodes. We expect that this electrohydrodynamic method can be used to separate the particles with high effectivity for various applications in microsystems.