A spontaneous Leidenfrost transitioning phenomenon

The boiling behavior of impacting droplets plays a critical role in spray cooling, directly governing the overall cooling efficiency. Among the various boiling regimes, transitional boiling is particularly significant, as it marks the onset of droplet instability. However, the dynamic interplay between transitional boiling and Leidenfrost rebound remains largely underexplored. In this Letter, we report a universal spontaneous Leidenfrost transitioning (SLT) phenomenon that reveals the coupled evolution of bubble-vapor dynamics, extending the current understanding. Using a custom-designed experimental setup featuring a transparent nano-film heater, we observe that droplets in the SLT regime initially experience vigorous contact boiling following the emergence of a distinctive fingering-crown structure. This stage is followed by repeated contact-levitation cycles, ultimately concluding in Leidenfrost rebound. To explain the formation of the fingering-crown structure, we propose a theoretical model in which a spatial vapor pressure gradient (Δpv) beneath the droplet, which is induced by a hyperbolic vertical vapor velocity distribution, acts as the key mechanism. This model is validated experimentally through combined hydrodynamic (ridge height and dynamic droplet radii) and thermodynamic (heat transfer evolution) analysis. Specifically, our results reveal a characteristic rise-fall pattern between the maximum Δpv and the initial surface temperature, spanning from nucleate boiling to stable Leidenfrost rebound. This trend shows a strong consistency with the predictions of the proposed theoretical model.