This is no optical illusion – the water droplets you see here are actually rolling uphill. But how?
What you're looking at is a unique demonstration of the ever-fascinating Leidenfrost effect. Named after 18th Century German doctor Johann Gottlob Leidenfrost, the phenomenon occurs when liquid comes into contact with a surface that is much, much hotter than the liquid's boiling point. Under such conditions, an insulating layer of vapor is created between the droplet and the surface. In the case of a water on, say, a piping hot frying pan, the water droplet can actually float on the layer of vapor. This not only causes the droplet to hover, it also protects the water from boiling outright.
Researchers at the University of Bath recently demonstrated that water dropped on a very hot ridged surface would cause the droplet to not only float, but climb along an inclined surface, as though ascending a flight of stairs. What's more, the researchers found they could actually control the ascent of the droplets by making careful adjustments to the features of the surface, as well as its temperature. We first wrote about this discovery back in 2012, but the video above, produced by the folks at Science Friday, features some really beautiful closeup shots of the effect, like this one here, that we thought were worth sharing:
The Science Friday video also includes footage of the "Leidenfrost Maze" that the researchers created from their machined surfaces. Which is just fantastic to watch in action.
Fun fact: the Leidenfrost effect is also responsible for the bizarre behavior of a red-hot ball of nickel dropped in water. The sound you hear toward the end is the result of the insulating vapor disappearing: