A study on the mechanism of directional ultrasound-assisted emulsification affecting the oil–surfactant–water interface

The directional impact of ultrasound is increasingly recognized as a critical factor in controlling emulsification efficiency, yet its interfacial mechanism remains poorly understood. This study investigates the effect of ultrasound propagation direction (forward, lateral, and reverse) on surfactant-assisted oil–water emulsification using a combined experimental and molecular dynamics simulation approach. Experiments with four structurally diverse surfactants—cationic cetyltrimethylammonium bromide, zwitterionic lauramide propyl betaine, nonionic alkylphenol ethoxylates, and anionic sodium dodecylbenzene sulfonate. Across all systems, lateral ultrasound consistently outperformed forward and reverse incidence in enhancing emulsification, despite differences in absolute efficiency attributable to surfactant chemistry. Molecular simulations reveal that ultrasound disrupts interfacial stability via intense cavitation and ultrasound-induced molecular motion, thereby promoting oil–water–surfactant mixing while suppressing interface reformation. Notably, lateral ultrasound directly targets the oil–water interface, generating more pronounced and complex molecular displacements compared to forward or reverse impacts. For each individual surfactant, emulsification efficiency is strongly governed by ultrasound propagation direction, with lateral incidence maximizing interfacial energy transfer. These findings reveal that lateral ultrasound maximizes interfacial energy transfer via direct targeting of the oil–water interface, thereby offering a design principle for directional acoustic fields in enhanced oil recovery.