Two phase micromixing and analysis using electrohydrodynamic instabilities

Abstract

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 $8.0·10 5 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 8.0·10 5 to 9.0·10 5 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.

Numerical Modeling of Microfluidic Two-Phase Electrohydrodynamic Instability

“…They have also been used in microchannels for the generation of droplets in immiscible fluids and have been experimentally observed using a two-fluid interface by Ozen et al [15] as seen in figure 1.1. Another potential application explored by Zahn and Reddy [3] is aimed at utilizing EHD instabilities towards miniaturizing the organic-aqueous extraction technique for DNA purification.…”

“…Briefly speaking, cells are lysed in an aqueous solution (the lysis buffer), and then mixed with an equal volume of organic fluid (phenol, chloroform and isoamyl alcohol, 25:24:1 by volume). Cell components naturally distribute themselves , termed "biological partitioning", into the two immiscible phases: the membrane components and proteins partition to the organic-aqueous Figure 1.2: Schematic of the set-up for EHD instability in a three-layer stratified flow [3]. The electric field is applied perpendicular to the direction of flow.…”

“…This "mixing" step is particularly challenging when implementing the technique in a microfluidic platform. In a previous work by Zahn and Reddy [3], it was demonstrated that EHD instability could be utilized to break a stream into droplets and increase the interfacial area available for partitioning. A schematic of the experimental set-up of Zahn and Reddy [3] is shown in figure 1.2.…”

“…In a previous work by Zahn and Reddy [3], it was demonstrated that EHD instability could be utilized to break a stream into droplets and increase the interfacial area available for partitioning. A schematic of the experimental set-up of Zahn and Reddy [3] is shown in figure 1.2. We can observe a threeinlet geometry in which an organic solution is sandwiched between two layers of aqueous solutions and an electric field is applied perpendicular to the direction of flow.…”

“…Effective dispersion of one fluid phase in the other increases the surface area over which biological component partitioning may occur, and hence enhances DNA extraction efficiency. Electrohydrodynamic (EHD) instability can be harnessed to achieve this goal and has been experimentally demonstrated by Zahn and Reddy (2006). In this work, analysis and simulation are combined to study two-phase EHD instability.…”