Two phase micromixing and analysis using electrohydrodynamic instabilities

Organic–aqueous liquid (phenol) extraction is one of many standard techniques to efficiently purify DNA directly from cells. Effective mixing of the two fluid phases increases the surface area over which biological component partitioning may occur. In this work, two phase mixing has been demonstrated in a three inlet microfluidic device geometry. Mixing between the two phases has been achieved by producing an electrohydrodynamic instability at the liquid–liquid interface between the two phases. The initial instability is modeled by considering the small signal linearized analysis for interfacial stresses from both fluid and electrical stress tensors for both inviscid and viscous models. These models predict the onset of instability and the stability criteria over a range of unstable wavenumbers of the mixing process. These models may be applied to relevant microscale geometries, where the unstable wavenumbers and fastest growth wavenumber are determined. At an applied electric field of ∼8.0×10⁵ V/m an instability is experimentally observed by labeling the organic phase with a fluorescent dye and visualizing interfacial perturbations by microscopy. Increasing the electric field increases the instability growth rate and results in an increase of the level of mixing. These results show an increase in conductive fluid entrainment into the nonconducting fluid core measured as a percentage of area of entrainment into the fluorescently labeled organic phase. The entrainment area is seen to increase from 1.9 to 28.6% as the applied field is increased from 8.0×10⁵ to 9.0×10⁵ V/m. The mixing images are converted into a power spectrum using a fast Hartley transform and the band of unstable wavenumbers of the mixing process are determined. From these results, the theoretical field strengths required to produce these unstable wavenumbers are calculated using the theoretical model, determining the maximum field strength required to excite the largest measured unstable wavenumber. At lower field strengths tested, the theoretically predicted maximum electric field and fastest growth wavenumber compare favorably with the initially applied field and measured fastest growth wavenumber whereas at higher field strengths the theoretical field is much larger than the initially applied field. This is attributed to the larger level of mixing and the ability of the instability to grow beyond the linear range and the field increases as the mixing process occurs due to entrainment of highly conductive fluid decreasing the effective dielectric spacing so that the linearized models underpredict the instability growth rates and interfacial perturbations.