Interfacial transitions in vibrated immiscible two-phase flow under microgravity

We investigate how vibration-driven interfacial state transitions regulate heat transport in an immiscible two-phase layer under microgravity. Direct numerical simulations are performed for two immiscible fluid layers subject to a vertical temperature difference and horizontal harmonic vibration, while the Weber number and thermal-expansion contrast are varied systematically. As the Weber number increases, the dispersed phase evolves from a stratified-layer state to an interfacial-bulging state and then to a droplet-dispersed state, and the global Nusselt and Reynolds numbers accordingly separate into three regimes. We identify two critical thresholds associated with the onset of interfacial bulging and droplet detachment; both move to lower Weber number as the thermal-expansion contrast increases. The first threshold is set by the pressure balance between thermovibrational pressure difference and capillary restoring stress, and the second threshold is set by the force balance between the thermovibrational force acting on the lifted liquid volume and the capillary force retaining the remaining neck. When rescaled by the corresponding critical thresholds, the onset of the bulging and detachment transitions collapses across different thermal-expansion contrasts, together with the associated changes in the Nusselt and Reynolds numbers. Our results show that interfacial morphology provides the key link between vibration forcing and transport response, and establishes a compact regime-based framework for regulating microgravity two-phase heat transport.