dripping water on a hot pot of more than 200℃, you can see that the water droplets are lifted by rapidly formed water vapor and move around on the hot pot. This is the Leighton Frost phenomenon.

this phenomenon itself has been known for many years, but the details have been discovered until now. What we are going to enjoy today is a special Leighton Frost phenomenon, in which water droplets about 1mm in diameter roll spontaneously on a hot plane, like a small liquid wheel.

the corresponding research papers on motion graphs were recently published in the journal Nature Physics. The researchers found that large and small droplets move very differently, also lifted by steam on a hot surface. Droplets with diameters of 2mm move more slowly and generally move in the same direction, due to external factors (such as slightly uneven planes). Small droplets with a diameter of 1mm are very different. They move faster and roll straight in a random direction.

(trajectories of small droplets and large droplets)

the difference in the mode of movement is related to the convection within the droplets. The researchers added tiny particles that can show the direction of convection (you can see them in the motion picture), thus showing the difference of convection between large and small droplets: the larger droplets are flatter and contain convection units in different directions, and the effects of their flow are more likely to counteract each other, leaving the droplets in place. When the diameter is small to a certain extent, the droplet becomes nearly spherical, leaving only one convection unit inside. This causes the droplets to roll in the direction of convection.

(comparison of convective structures between large droplets and small droplets. (the picture is from the original paper)

however, the principle of droplet rolling is not quite the same as that of a wheel. After all, a small droplet is held up by an air cushion and does not move forward through friction with the surface like a wheel. In fact, the small droplet tilts the bottom surface in convection, and the steam pushes the tilted bottom surface to produce a horizontal force, which drives the droplet to roll.

(schematic diagram of the tilt of the droplet. (the picture is from the original paper)

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from the observation, it seems that the rolling of small droplets can be controlled by temperature. If this process can be made manageable, it may also have some potential applications.

finally, there is a summary video:

original paper: the https://www.nature.com/articles/s41567-018-0275-9

video was taken before the paper was published and contributed to the American Physics Society's fluid Gallery (Gallery of Fluid Motion) in 2017