Electric field mediated squeezing to bending transitions of interfacial instabilities for digitization and mixing of two-phase microflows

Electric field mediated instabilities in a tri-layer oil-water flow inside a microchannel have been explored with the help of the analytical models and computational fluid dynamic simulations. The twin oil-water interfaces undergo either in-phase bending or antiphase squeezing mode of deformation when a direct current (DC) electric field is applied locally inside the channel. The selection of modes largely depends on the magnitudes of the electric field intensity and oil-water interfacial tension. The instability modes grow to form an array of miniaturized oil droplets with a significantly higher surface to volume ratio. While squeezing mode leads to a time-periodic dripping of droplets at relatively lower field intensities, the bending mode develops into a whiplash ejection of miniaturized droplets at higher field intensities. Subsequently, a transition from purely laminar to chaotic flow is observed, resembling the von Karman vortex street from a flow past immersed body, suitable for augmented heat, mass, and momentum transport inside a microfluidic channel. Under these conditions, the simulations also reveal the formation of multiple microvortices inside and outside the droplets, which helps in increase in the local Reynolds number for a better mixing efficiency in such microflows. Use of alternating current electric field instead of DC is also found to create on-demand flow features in a time-periodic manner following the mode selection. The amplitude, frequency, and waveform of such electric field is found to generate miniaturized oil-droplets along with the formation of an array of flow features, namely, thread, slugs, plugs, among others. Published under license by AIP Publishing.