Effect of surfactants on the motion of magnetic microparticles across liquid–liquid interface

This study employed high-speed camera technology to observe the kinetic behaviour of micrometre-sized Fe₃O₄ particles traversing the oil–water interface under the influence of a non-uniform magnetic field. Experiments reveal that Span80 surfactant concentration alters interfacial properties (interfacial tension, viscosity, and oil–water–solid contact angle). As Span80 concentration increases, the Ohnesorge number (Oh) rises while the magnetic Bond number (Bo_m) decreases. Particles exhibit five distinct motion modes at the interface: encapsulation and detachment, interfacial jet formation, cluster ejection, suspension ejection, and steady-state stretching. At low concentrations (＜ 0.42 wt.%), the Marangoni effect predominates, enveloping particles within the oil phase to form magnetic emulsions; near the critical micelle concentration (≈ 0.42 wt.%), particle aggregation intensifies, enabling them to overcome the interfacial energy barrier and traverse the interface; at high concentrations (＞ 0.63 wt.%), liquid column necking is suppressed, with only deformation occurring. This study systematically reveals the mechanism by which surfactant concentration influences particle distribution and interfacial mechanical response through regulation of the Oh and Bo_m, providing theoretical foundations for the application of magnetic particles in multiphase flow and interfacial manipulation.