Electrohydrodynamics of a concentric compound drop in an AC electric field

The dynamics of a compound drop suspended in another immiscible fluid in the presence of an AC electric field is investigated experimentally and using analytical theory. A closed-form analytical expression for the mean deformation and amplitude of deformation at cyclical steady state is derived in the small deformation limit. Experiments were performed with 0.1M NaCl/castor oil compound drops suspended in highly viscous silicone oil. In this case, both the core and the shell deform into prolate spheroids. The effect of two independent variables was investigated, namely, the ratio of the core radius to the shell radius and the frequency (omega) of the applied AC field. In the limit of omega -> 0, the present analytical model reduces to the DC electric field model for the compound drop. It was observed that the size of the core significantly affects the dynamics of the compound drop. The mean and the amplitude of deformation of the shell increase considerably with an increase in the radius ratio. Since the present model is valid for a small deviation from a spherical shape, an excellent quantitative agreement is found between analytical and experimental results at low deformation, whereas, at large deformation, the match is only qualitative. It was also observed that the relative phase difference between the core and the shell decreases with an increase in the radius ratio and frequency of the applied electric field. Published by AIP Publishing.