Depressurization-regulated droplet boiling behaviors: Low wall-superheat stability

Reduced-pressure spray cooling exhibits strong potential for aerospace thermal management. However, the underlying cooling mechanisms remain insufficiently understood due to complex droplet-wall interactions across wide pressure ranges. Here, we reveal a counterintuitive phenomenon at low pressure and low wall superheat, where droplets remain hydrodynamically and thermodynamically stable due to an elevated nucleation energy barrier. Using a depressurized chamber featuring a nanofilm electrical heater, four boiling regimes are identified based on surface oscillation patterns and grouped into pinning and breakup regions. Increasing ambient pressure induces a constraint-releasing mechanism through two successive hydrodynamic boundaries, corresponding to early-stage nucleation activation and subsequently fully activated bubble dynamics promoted by increased droplet subcooling. Moreover, a new correlation for the maximum spreading coefficient reveals a reversed Jakob number dependence compared to atmospheric-pressure conditions, highlighting the critical role of ambient pressure in regulating spreading behaviors. Thermodynamically, heat transfer evolves consistently with the hydrodynamic response, shifting from mild and sustained cooling at low pressure to jump cooling with higher heat flux at elevated pressure. Collectively, the pinning-to-breakup transition occurs upon crossing a critical heat flux boundary, thermally characterized by a coupled effect of wall superheat and droplet subcooling, and hydrodynamically manifested in constraint-releasing of bubble dynamics. These findings uncover a previously overlooked regime of enhanced droplet stability and further identify heat transfer deterioration at very low ambient pressures, providing guidance for optimizing reduced-pressure spray cooling.