Interaction and motion of two neighboring Leidenfrost droplets on oil surface

Evaporation of droplets over a hot oil surface is a natural phenomenon. However, most existing studies focus on a single droplet, and the evaporation of multiple droplets is insufficiently understood. Here, we explore the Leidenfrost evaporation of two identical FC-72 droplets over a hot oil bath. The oil temperature covers 73.6~126.6 °C, and the evaporation of droplets with an initial diameter of 1.5 mm was recorded by an infrared thermographer and a high-speed camera. The shallow oil depth keeps a relatively uniform oil temperature in the slot compared to the deep liquid pool, due to the larger ratio of the surface area for copper-oil contact to the slot volume. We found that neighboring droplets evaporate in three stages: non-coalescence, bouncing and separation. The radius of neighboring Leidenfrost droplets follows the power law R(t)~(1−t/τ)n, where τ is the characteristic droplet lifetime and n is an exponent factor. Moreover, the diffusion-mediated interactions between the neighboring droplets slow down the evaporation process compared to isolated Leidenfrost droplets and lead to an asymmetric temperature field on the droplet surface, which breaking the balance of the forces acting on the droplets. A simple dual-droplet evaporation model is developed which considers four forces acting horizontally on the droplet, including the Marangoni force resulting from the non-uniform droplet temperature, the gravity component, the lubrication-propulsion force, and the viscous drag force. Scale analysis shows that the Marangoni force and gravity component dominate dual-droplet evaporation dynamics. In the non-coalescence stage, the gravity component induces the droplets to attract each other, while the vapor film trapped between droplets avoids their direct touch. When the droplets get smaller, the gravity component is insufficient to overcome the Marangoni force. Hence, the droplets separate in the final evaporation stage. Finally, we identify the competition between Marangoni force and gravitational force as the origin of the bounce evaporation by comparing the theoretical and experimental transition times at distinct stages. This study contributes to explaining the complex Leidenfrost droplet dynamics and evaporation mechanism.