State of Leidenfrost droplets: Equilibrium, oscillation and trampolining

The recent discovery of Leidenfrost droplet trampolining deviates from the traditionally accepted steady-state assumption and updates our understanding of Leidenfrost droplet state. However, the conditions of trampolining and its effect on heat transfer have not been fully understood. To address these issues, this study numerically investigates the dynamic behavior and heat transfer characteristics of Leidenfrost droplets under varying liquid viscosity and droplet size. A regime map for Leidenfrost droplet state with respect to Bond number and Ohnesorge is presented. Particularly, an unreported oscillation regime is discovered between equilibrium regime and trampolining regime. The results indicate that low viscosity and moderate droplet size favor the observation of trampolining. Besides, the oscillating droplet is modeled via a mass-spring-damper system in both equilibrium and oscillation regimes, with damping coefficient, spring constant and oscillation period quantitatively correlated with liquid viscosity and droplet size by simple scaling laws. In the trampolining regime, an intriguing phenomenon is observed as the maximum vapor layer thickness demonstrates two local maxima with increasing droplet size. We also quantitatively unravel that reducing liquid viscosity and increasing droplet size can lead to a thicker vapor layer thickness, thus inhibiting the heat transfer to the droplet. © 2024