Free wheeling water droplets plot their own path off a hot plate

Image of a water droplet on a hot plate.

Enlarge (credit: Wikimedia Commons)

Here are the two best things about fluid dynamics: scaling laws and cool videos. A group of researchers has been studying the behavior of water on a hot plate. You are probably all familiar with the effect: you drop some water on a hot plate, the water hisses a bit, and the drop races crazily over the surface. This is called the Leidenfrost effect.

(Its name has nothing to do with frost; rather, the person who first described it was named Johann Gottlob Leidenfrost.)

The drop’s motion was assumed to come predominantly from the shape of the hot plate’s surface. The plate is almost never completely level and typically not very flat, so the drops just follow the local slope. A trace of the drop, presumably, would map out the local contours of the plate. However, this turns out to be wrong. Droplets of water on a hot plate are self propelling and choose their own destination.  

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