Asymmetric temporal variation of oscillating AC electroosmosis with a steady pressure-driven flow

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.

Download Full-text

Characterization of AC Electrokinetic Forces Using Pressure-Driven Flow in a Microchannel

Volume 13: Nano-Manufacturing Technology; and Micro and Nano Systems, Parts A and B ◽

10.1115/imece2008-68984 ◽

2008 ◽

Author(s):

Jorge J. Capurro ◽

Hongseok Moses Noh

Keyword(s):

Glass Plate ◽

Force Balance ◽

Ac Electroosmosis ◽

Ac Electrokinetics ◽

Quantitative Understanding ◽

Versatile Technique ◽

Electrothermal Effects ◽

Pressure Driven Flow ◽

Pressure Driven

Ac electrokinetics is a versatile technique for particle and fluid manipulation in microfluidic environment. However, analyzing and predicting particle motions due to the ac electrokinetic effects is a difficult task because it requires the quantitative understanding of multiple phenomena such as dielectrophoresis (DEP), ac electroosmosis (ACEO), and electrothermal effects (ETE). In this paper we present a force balance approach to analyze ac electrokinetic effects, particularly ACEO. Pressure-driven flows were used to quantify the ACEO and DEP forces acting on a particle. Polystyrene microbeads suspended in KCl solution were introduced in polydimethylsiloxane (PDMS) microchannels attached to a glass plate with gold microelectrodes. The microbeads were initially collected and aligned along the center of the electrodes at 1 kHz and 1 Vp-p, and then a well-controlled pressure-driven flow was introduced resulting in the translation of the particles. Particles moved to a new location where a new force balance is reached. This particle translation on the surface of the electrode was carefully monitored as varying the applied flow rate. The net force due to ac electrokinetic effects at different locations over the electrode was calculated using the experimental data and the force balance relationship.

Download Full-text