Interfacial instability and failure mechanisms of ferrofluid seals in liquid environments: A ternary phase-field ferrohydrodynamic study

Ferrofluid sealing in liquid environments involves strongly coupled magnetic, interfacial, and hydrodynamic effects that are difficult to resolve using conventional two-phase models. In this study, a ternary phase-field ferrohydrodynamic framework is developed to simulate the interfacial evolution and failure of an air-ferrofluid-water sealing system. The framework couples magnetic-field-induced stresses, ternary phase-field dynamics, interfacial tension, wetting, shaft rotation, and pressure-driven flow within a unified computational model. Its numerical reliability is systematically examined through mesh independence, time step sensitivity, and phase-field interface-thickness sensitivity. The model is further validated against three benchmark problems involving magnetic deformation of a ferrofluid droplet, a three-phase liquid lens, and the critical sealing pressure of a conventional ferrofluid gas seal, showing good agreement with analytical, experimental, and published numerical results. The validated framework is then applied to investigate steady-state interface morphology, leakage-channel formation, and critical sealing pressure in liquid environments. The results demonstrate that liquid-environment sealing exhibits a distinct failure pathway from gas sealing, characterized by progressive interface deformation, liquid penetration, necking, and continuous leakage-channel formation. Increasing magnetic field intensity enhances sealing stability, whereas shaft rotation promotes interface instability. A dimensionless failure regime map based on the magnetic Bond number, Weber number, and critical pressure Bond number is established to distinguish inertia-dominated, transitional, and magnetically stabilized regimes. The proposed framework provides a computational basis for predicting and optimizing multiphase ferrofluid sealing systems.