Squeezing to Bending Transitions of Interfacial Electrohydrodynamic Instabilities for Digitization and Mixing of Two-Phase Microflows

External field induced interfacial instabilities have shown significant potential in the miniaturization of flow patterns inside the microfluidic devices. Electric field induced instabilities in a trilayer oil-water microflow is explored with the help of analytical models and computational fluid dynamics simulations. Twin oil-water interfaces undergo either in-phase bending or anti-phase squeezing mode of deformation when a direct current (DC) electric field is applied locally. The selection of modes depends on the magnitudes of applied DC field intensity and oil-water interfacial tension. The growth of the squeezing mode leads to a time-periodic dripping of droplets at lower field intensities, whereas, bending mode develops into `whiplash' ejection of miniaturized droplets having octuplet microvortices inside and outside, at higher field intensities. A transition from purely laminar flow is observed during the switch over to bending mode, resembling von K\'{a}rm\'{a}n vortex street formation. Use of alternating current (AC) electric field with variation in frequency and waveform is also found to create on-demand and time-periodic array of flow features following the mode selection.