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.

中文翻译：

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交流电场下流动聚焦微通道中液滴形成的动力学

摘要 电场已被证明是微流体装置中用于精确操纵微滴的有效主动技术。在本文中，我们通过实验和数值研究了交流电场下流动聚焦微通道中油滴中的水的形成。结合体积分数（VOF）方法和泄漏介电模型建立了三维数值模型，揭示了电场作用下液滴的形成机制。由于电场引起的麦克斯韦应力，正弦波形电场引起液体界面处的振荡，刺激分散相的分裂，从而调整液滴尺寸。我们通过根据数值结果评估的毛细管数 CaE 来分析现象。电压和频率的增加都可以提升CaE。随着电压的增加，当CaE > 1时，产生的液滴变小，液滴形成变得不稳定。分散相和连续相之间的压差的主导作用从最初的流体动压转移到后来的电场在电压的演变过程中感应出一个。在相对较高的电频率（f ≥ 5000 Hz，CaE > 1）下，液滴形成状态在恒定的水动力条件下从滴落转变为喷射。数值结果表明，电体力大小的激增倾向于拉伸液体界面处的分散相，从而导致转变。