We report the observation of two identical Leidenfrost droplets rebounding from each other after a head-on collision on a hot surface, resembling billiard ball collision. The observed phenomenon contradicts the established understanding that predicts the coalescence of two colliding Leidenfrost droplets. The collision dynamics of the Leidenfrost droplets is investigated using pressure and energy balance analysis at the macroscale, and a molecular interaction analysis at the nanoscale. The results suggest that droplet properties and the temperature of the hot surface determine the state of the gas film between the colliding droplets. Droplets with lower surface tension form thicker gas films, while lower surface temperatures mitigate the interaction of vapor molecules within the gas film, both facilitating droplet rebound after their collision. The discovered Leidenfrost droplet phenomenon is highly controllable, and has potential for a variety of applications, such as the selective chemical synthesis as explored in this work.

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• Drop & bubble phenomena
• Heat transfer
• Multiphase flows
• Surface & interfacial phenomena