Two Phase Electrohydrodynamic Instability Micromixing

This work characterizes two phase electrohydrodynamic (EHD) instability micromixing of two immiscible organic and aqueous fluid phases. EHD mixing of the two phase microflow is promoted by creating an unstable flow profile by electrically inducing motion of the phase boundary. The aqueous phase is assumed to be infinitely conducting due to dissolved salt ions, while the organic phase is assumed to be non-conducting. As electrodes are biased, charges accumulate at the aqueous/organic interface. At a critical voltage the interface becomes unstable so that the aqueous and organic layers will mix. This is modeled for both inviscid and viscous flows using linear stability analysis considering the interfacial kinematic and stress conditions which predicts the stability criteria with a range of unstable wavenumbers which may be excited The mixing of an unstable “sausaging” and “kink” modes are visualized using epifluorescent microscopy of the dyed organic phase. The characteristic unstable wavenumbers predicted using linear stability theory are determined from the power spectrum of the captured images and compared to the analytical model. Onset of instability is seen at 40 volts RMS at a frequency of 250 kHz. This voltage corresponds to an electric field of Eo = 8 × 10⁵ V/m across the organic phase. The instability becomes progressively more dynamic as the field strength is increased. The system recovers after the field is removed. At low field strengths the theoretical field and fastest growth wavenumbers for mixing compares favorably with the initially applied field 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 while the electric field increases as the mixing process occurs due to entrainment of highly conductive fluid decreasing the effective dielectric spacing.