Dynamics of droplet in flow-focusing microchannel under AC electric fields

Abstract

Electric field has been proven to be an effective active technique in microfluidic devices for precise manipulating of the microdroplet. In this article, we investigate the Water in Oil droplet formation in a flow-focusing microchannel under AC electric field experimentally and numerically. A three-dimensional numerical model is built combining the Volume of Fraction (VOF) method and the leaky dielectric model, which reveals the droplet formation mechanism under the effects of the electric field. Due to the Maxwell stress induced by the electric field, the sine waveform electric field induces the oscillation at the liquid interfaces, which stimulates the breakup of the disperse phase and thus tunes the droplet size. We analyse the phenomena by the electric capillary number CaE evaluated according to the numerical results. The increase of the electric voltage and the frequency both are able to lift CaE. With the increase of the electric voltage, the droplet generated becomes smaller and the droplet formation turns unstable when CaE > 1. The dominating effect of the pressure difference between the disperse phase and the continuous phase shifts from the initially hydrodynamic pressure to the latterly electric field induced one during the evolution of the electric voltage. With a relatively high electric frequency (f ≥ 5000 Hz, CaE > 1), the droplet formation regime transits from dripping to jetting under the constant hydrodynamic conditions. The numerical results show that the surge of the magnitude of the electric body force tends to stretch the disperse phase at liquid interfaces which leads to the transition. This study explored the dynamic mechanism of the droplet formation under AC electric field with different voltages and frequencies which contributes to the in-deep understanding of the coupling effect between the hydrodynamic pressure and the AC electric field induced Maxwell stress and hence, might lead to better control strategy on the promising technology.