Inadequacy of Two-Drop Studies in Explaining Chaining in Emulsion Systems under AC Electric Fields

Abstract

Efficient dewatering of crude oil emulsions is often hindered by droplet chaining, yet the mechanisms responsible for the strong tendency to chain at low electric capillary numbers (Ca_e ≤ 0.02) remain unclear. Here, we combine high-speed microscopy and numerical simulations to investigate the frequency-dependent dynamics of multi-droplet chains in AC electric fields. At high frequencies (1 kHz), chaining is shown to be a dynamic process in which transient liquid bridges form through coalescence of interstitial droplets with neighboring primary droplets and subsequently break via Rayleigh–Plateau instability. We propose that these bridges act as temporary electrical shorts, locally eliminating the electric field and thereby the stresses in the bridge, and enabling repeated bridge breakup due to Rayleigh–Plateau instability, while preserving chain integrity. Unlike classical two-drop theories, this mechanism does not require large cone angles or active pulling forces. Reducing the frequency to 1 Hz enhances droplet deformation, electrophoretic motion, and electrohydrodynamic (EHD) flows, leading to misalignment, asymmetric coalescence, and chain collapse, consistent with particle-based simulations. Conductive impurities, such as lint fibers, are further shown to stabilize non-coalescence by providing permanent electrical pathways that suppress dipolar attraction and promote electrophoretic repulsion. These findings establish a microhydrodynamic framework for multi-droplet interactions in emulsions and provide insights for improving the performance of industrial electrocoalescers.