Droplet behaviors of W/O emulsions under the combination of electric field and medium coalescence

The efficient separation of water-in-oil emulsions plays an important role in many industrial fields. However, the demulsification performance using a single method is limited. In recent years, a novel deep purification method combining direct current electric field and medium coalescence has been proposed. To better understand the mechanism of this process, this work investigated the behavior of water droplets at different conditions. The results show that all injected water droplets initially migrate toward the charging side, while the opposite movement of some droplets is due to collision with the others or breakage from the original ones. The droplets migrated back and forth between electrodes through the polarize-discharge process. However, owing to the heterogeneous electric field near the charging side, some large-size droplets rebounded at a certain distance away from the charging electrode, leading to a counterflow of W/O emulsion during electrostatic coalescence. Moreover, the higher voltage leads to the faster coalescence of dispersed droplets on the medium surface, and the wetted medium can redistribute the electric field. With the collision and the deposition of water droplets, different kinds of water areas were formed and kept a dynamic equilibrium on the medium surface. The subsequent water droplets deposited on the medium surface and then flowing away along these areas. The results proposed in this paper present an in-depth understanding of the mechanism of the electrocoalescence, the medium coalescence, and the combination of them, which provides useful insights for the design of demulsification device and the separation process of W/O emulsion.

油包水型乳状液（water-in-oil emulsions）的高效分离在诸多工业领域中具有重要意义。然而，单一破乳（demulsification）手段的性能存在局限。近年来，一种结合直流电场（direct current electric field）与介质聚结（medium coalescence）的新型深度净化方法被提出。为更清晰地阐明该过程的作用机制，本研究探究了不同工况下水滴的运动行为。结果表明，所有注入的水滴最初均向充电电极侧迁移，部分水滴出现反向运动则系其与其他水滴发生碰撞或自身发生破碎所致。水滴通过极化放电（polarize-discharge）过程在电极之间往复运动。然而，由于充电电极侧存在非均匀电场，部分大尺寸水滴会在距充电电极一定距离处发生反弹，进而导致静电聚结过程中油包水型乳状液出现逆流。此外，电压越高，分散相水滴在介质表面的聚结速率越快，经浸润的介质可对电场进行重新分布。随着水滴的碰撞与沉积，介质表面会形成多种形态的水域并维持动态平衡，后续水滴将沉积于介质表面，并沿这些水域流动脱离。本文研究结果深化了对电聚结（electrocoalescence）、介质聚结以及二者协同作用机制的理解，可为破乳装置的设计与油包水型乳状液分离工艺提供有益参考。