Curvilinear self-propulsion of Leidenfrost droplets on an immiscible liquid layer with Bénard–Marangoni cells

The motion of Leidenfrost droplet on a hot liquid surface poses attractive features in self-propelling drops, nanofabrication, and frictionless transport. Nonetheless, the Marangoni instability inside the heated liquid pool was ignored and its influence on the behavior of Leidenfrost droplets remains an open question. Here, we report a remarkable curvilinear self-propulsion of Leidenfrost droplets suspended on an immiscible liquid pool where the Marangoni instability arises. In a thin liquid pool, the motion of Leidenfrost droplets is not random, but rather definitely directs to the center of Bénard–Marangoni cells, across it and traveling toward surrounding cells. With the increase in pool thickness, the propulsion speed of droplet decreases gradually and eventually gets totally trapped among Rayleigh–Bénard–Marangoni cells. Combined with numerical simulations, we show that the curvilinear propulsion of Leidenfrost droplets is induced by the Marangoni effect of liquid pool, which consists of the pool surface fluctuation and the sustaining evaporative cooling effect of droplet on pool surface. Our findings provide deeper physical insights into the dynamics of a Leidenfrost droplet over a heated liquid pool and provide a potential approach of fluid transport in the Leidenfrost state by regulating the surface morphology of liquid pool.