Characterization of electrothermal microfluidic tweezers (REP) in bio-relevant media

Rapid electrokinetic patterning (REP) has proven to be a powerful microfluidic tweezer that generates spatially and temporally specified microflow via electrothermal vortices. The ultra-small trapping forces on the scale of femtoNewtons exerted by REP, due to its viscous drag of the vortical flow on the trapped particles, attracts many potential bio-manipulation applications. We report, for the first time, use of isotonic sugar-based media to simulate bio-relevant environment for flow manipulation through REP. A DC field was introduced, in addition to the typical AC field, to enhance the vortical flow induced particle trapping performance. In this work, we study the effects of the magnitude and sign of the DC offset on the vortex characteristics. Results show that addition of the DC component in the electric field has a significant impact on the electrothermal micro-vortex and the electric double layer of the electrodes and the trapped particles. We also observe that an abrupt change in the DC offset destabilized the vortex and washed away some particles as the trap equilibrium re-established. However, the trap remained stable when the step change in the DC offset was smaller than ~500 mV.