Back in the 18th century, a German doctor called Johann Gottlob Leidenfrost described how water behaves when it hits a mass considerably hotter than its boiling point. But first, watch the 3000-frames-per-second video and see if you can realise what’s happening.
As you can see, the water doesn’t boil right away. You may have seen the same effect in your own kitchen. If not, here’s the experiment: Get a dry pan, make it extremely hot, and drop a bit of water on it. If the temperature is high enough, you will see that the water drops will dance around the surface for some time before they disappear. If it’s just below 100C, however, the water will just flatten and evaporate slowly. If it’s above 100C, the water will quickly evaporate on contact. But when it gets to 190C, the droplets will not evaporate for quite a few seconds. In fact, Sir William Fairbairn – a famed Victorian steam boiler designer – found that, while a water droplet evaporated in a hiss at 168C, it survived for an amazing 152 seconds at 202C.
The reason is called the Leidenfrost Effect: When the water touches the pan, a small layer of vapor forms around it. This vapor layer isolates the droplet from the hot surface and protects it from evaporation. [Modernist Cuisine via PopSci]