One way to levitate droplets is to place them on a surface heated much higher than the droplet’s boiling point. This creates the Leidenfrost effect, where a droplet levitates on a thin layer of its own evaporating vapor. In this study, the situation is quite different.
Although the underlying pool of liquid — here, silicone oil — is heated, its temperature is well below the boiling point of the water droplet. But the droplet still levitates over the pool, thanks to an air layer fed by convection. Aluminum powder in the oil reveals large-scale convection in the pool; note how the oil moves radially toward the droplet. That movement drags the air in contact with the oil with it, which forms the vapor layer keeping the droplet aloft.
One side effect of this convection-driven levitation is that the droplet hovers over the coldest point in the oil. That fact suggests that users can manipulate the droplet’s motion by tuning the underlying heating. (Image and research credit: E. Mogilevskiy)