Interaction and motion of two neighboring Leidenfrost droplets on oil surface

Evaporation of droplets on a hot oil surface is a natural phenomenon. However, most of existing studiesfocus on the evaporation of a single droplet, and the evaporation of multiple droplets is insufficientlyunderstood. Here, we explore the Leidenfrost evaporation of two identical FC-72 droplets on the surface of a hotoil bath. The oil temperature ranges from 73.6 to 126.6 degrees C, and the evaporation of droplets each with an initialdiameter of 1.5 mm is recorded by an infrared thermographer and a high-speed camera. The shallow oil depthkeeps the oil temperature uniform relatively in the slot compared with that in the deep liquid pool due to thelarger ratio of the surface area for copper-oil contact to the slot volume. We find that the neighboring dropletsevaporate in three stages: non-coalescing, bouncing, and separating. The radius of neighboring Leidenfrostdroplets follows the power law R(t)similar to(1-t/T)(n), where t is the characteristic droplet lifetime and n is an exponentfactor. Moreover, the diffusion-mediated interaction between the neighboring droplets slows down theevaporation process compared with the action of isolated Leidenfrost droplet and leads to an asymmetrictemperature field on the droplet surface, thereby breaking the balance of the forces acting on the droplets. Asimple dual-droplet evaporation model is developed which considers four forces acting horizontally on thedroplet, namely, the Marangoni force resulting from the non-uniform droplet temperature, the gravitycomponent, the lubrication-propulsion force, and the viscous drag force. Scale analysis shows that theMarangoni force and gravity component dominate dual-droplet evaporation dynamics. In the non-coalescencestage, the gravity component induces the droplets to attract each other, while the vapor film trapped betweendroplets prevents them from directly contacting. When the droplets turn smaller, the gravity component isinsufficient to overcome the Marangoni force. Hence, the droplets separate in the final evaporation stage.Finally, we conclude that the competition between Marangoni force and gravitational force is the origin of thebounce evaporation by comparing the theoretical and experimental transition times at distinct stages. Thisstudy contributes to explaining the complex Leidenfrost droplet dynamics and evaporation mechanism

Automated summary

This study investigates the Leidenfrost evaporation of two identical FC-72 droplets on a hot oil surface, revealing that neighboring droplets undergo three stages of evaporation: non-coalescing, bouncing, and separating. The asymmetrical temperature field on the droplet surface breaks the balance of forces acting on the droplets. Furthermore, a dual-droplet evaporation model considers four forces and demonstrates that the Marangoni force and gravity component play a dominant role in the dynamics.