Contact electrification (CE) is a common physical phenomenon, and its mechanisms for solid–solid and liquid–solid cases have been widely discussed. However, the studies about liquid–liquid CE are hindered by the lack of proper techniques. Here, a contactless method is proposed for quantifying the charges on a liquid droplet based on the combination of electric field and acoustic field. The liquid droplet is suspended in an acoustic field, and an electric field force is created on the droplet to balance the acoustic trap force. The amount of charges on the droplet is thus calculated based on the equilibrium of forces. Further, the liquid–solid and liquid–liquid CE are both studied by using the method, and the latter is focused. The behavior of negatively precharged liquid droplet in the liquid–liquid CE is found to be different from that of the positively precharged one. The results show that the silicone oil droplet prefers to receive negative charges from a negatively charged aqueous droplet rather than positive charges from a positively charged aqueous droplet, which provides a strong evidence about the dominant role played by electron transfer in the liquid–liquid CE.

中文翻译：

---

与液体或固体接触后，对液滴上的接触-带电诱导电荷转移进行量化

接触带电（CE）是一种常见的物理现象，其在固-固和液-固情况下的机制已被广泛讨论。然而，由于缺乏适当的技术，液-液 CE 的研究受到阻碍。在这里，提出了一种基于电场和声场组合的非接触式方法来量化液滴上的电荷。液滴悬浮在声场中，在液滴上产生电场力以平衡声阱力。因此，基于力的平衡计算液滴上的电荷量。此外，液-固和液-液CE均采用该方法进行研究，重点关注后者。发现液-液 CE 中带负电荷的液滴的行为与带正电荷的液滴的行为不同。结果表明，硅油液滴更喜欢从带负电的水滴中接收负电荷，而不是从带正电的水滴中接收正电荷，这为液-液 CE 中电子转移所起的主导作用提供了强有力的证据。