Experimental and theoretical analysis of a spontaneous Leidenfrost transitioning phenomenon

This study offers a thorough experimental and theoretical analysis of a unique droplet behavior known as spontaneous Leidenfrost transitioning (SLT). This phenomenon occurs between stable transitional boiling and Leidenfrost rebound. By creating a novel experimental platform that allows for spatial observations of hydrodynamic and thermodynamic behaviors, we uncover significant insights into SLT. Our experimental observations indicate that the occurrence of SLT is independent of the Bond number. However, a higher temperature is necessary to trigger SLT as the Bond number increases. Initially, SLT expands but narrows with rising Weber number, with larger Bond numbers exhibiting earlier narrowing due to intensified thermal-induced instability. Furthermore, enhanced surface smoothness and hydrophilicity are unfavorable for SLT initiation. We identify three distinct phases of SLT: intensive boiling, consecutive levitation, and stable Leidenfrost rebound. By analyzing three hydrodynamic parameters during the second phase, we propose a mechanism describing the evolution of SLT at increasing temperatures. Our investigations into phase transitions reveal that rapid retraction and the formation of a central lift force drive the transition from intensive boiling to consecutive levitation. We also establish a theoretical model to describe the subsequent transition into stable Leidenfrost rebound, which validates the case-sensitive nature of the proposed mechanism while successfully linking it to droplet deformation and heat transfer behavior. These findings provide valuable insights into the underexplored droplet behaviors between two well-known regimes, enhancing the understanding of transitional boiling instability and the transition from stable transitional boiling to Leidenfrost rebound.