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In 1756, a German scientist named Johann Gottlob Leidenfrost reported his observation of an unusual phenomenon. Normally, water splashed onto a very hot pan sizzles and evaporates very quickly. But if the pan's temperature is well above water's boiling point, "gleaming drops resembling quicksilver" will form and will skitter across the surface. It's known as the "Leidenfrost effect" in his honor.
In the ensuing 250 years, physicists came up with a viable explanation for why this occurs. If the surface is at least 400 degrees Fahrenheit (well above the boiling point of water), cushions of water vapor, or steam, form underneath them, keeping them levitated. The Leidenfrost effect also works with other liquids, including oils and alcohol, but the temperature at which it manifests will be different. In a 2009 Mythbusters episode, for instance, the hosts demonstrated how someone could wet their hand and dip it ever so briefly into molten lead without injury, thanks to this effect.
But nobody had been able to identify the source of the accompanying cracking sound Leidenfrost reported. Now, an international team of scientists has filled in that last remaining gap in our knowledge with a recent paper by Mathai et al. in Science Advances.
The answer: it depends on the size of the droplet.
(Score: 3, Interesting) by AthanasiusKircher on Tuesday May 14 2019, @03:01AM
So, if I read this correctly, the main thing they claim to have discovered is the source of the noise -- which apparently comes when drops that are too big or have too many contaminants to cover completely touch the surface and then apparently "explode."
Well, okay. I mean, I guess it's good that someone measured this, but where the heck else would the noise come from? Aliens? Tiny gnomes groaning at having to operate the hovercrafts that cause the drops to swirl around the pan?
No, I think anyone who has ever dropped anything in hot pan and heard a loud crackle probably assumed it came from vast expansion of gas (or "explosion," if you need the drama of TFA) when it made contact with the pan. The fact that it's larger droplets or those with too many particulate contaminants to hover well that hit the pan and crackle... Well, that too seems like common sense.
Again, good for them for measuring this stuff. But TFA acting like this noise is some centuries-old mystery... Nah, I'm not buying that. Anyone who has ever been acccidentally splattered with hot oil after a crackling noise from a pan probably understands what would make the noise here.