Microfluidic mixing by dc and ac nonlinear electrokinetic vortex flows

A novel micromixing strategy is proposed for microfluidic applications. High-intensity nonlinear electroosmotic microvortices, with angular speeds in excess of 1 cm/s, are generated around a small (similar to1 mm) conductive ion-exchange granule when moderate dc and ac electric fields (30-125V/cm) are applied across a miniature chamber smaller than 10 muL. Coupling between these microvortices and the fast electrophoretic motion (similar to1 cm/s) of the granule in low frequency (between 0.3 and 1.0 Hz) ac fields and a slower backflow vortex (velocity similar to1 mm/s) in dc fields produces an intense chaotic micromixing action. Two segregated dye samples, normally requiring nearly 0.5 h to mix by diffusion, are observed to mix within 30 s in the mixing chamber. The effective diffusivity scales as E-2 and is measured to be 2 orders of magnitude higher than molecular diffusivity at reasonable field strengths and optimal frequencies. The main vortices are generated by nonlinear versions of the Smoluchowski slip velocity on the granule surface that result from a nonuniform counterion flux into the granule and the corresponding nonuniform polarization. Although these nonlinear electrokinetic vortices cease after 30 min as the granule saturates with the counterions, this mixing chamber should prove useful for mixing aqueous/electrolyte samples in disposable microchips and in batch microreactors. Mixing by ac field is preferred because of the solute can be better confined within the mixing chamber and contamination by electrode reaction is reduced.