
Water Droplets Don’t Just Hover on a Hot Pan – They Roll - dnetesn
https://www.nytimes.com/2018/09/14/science/leidenfrost-effect.html
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gene-h
This is quite surprising! I'm quite surprised at the fact that no one has
thought to put tracers on leidenfrost droplets until now. Really makes you
wonder at what other interesting phenomenon are happening in plain sight we've
yet to notice. More practically it will be interesting to see if this work
generalizes to other multiphase systems. In particular supercavitating
propulsion, where you surround say a torpedo or boat hull in a layer of air so
that it can move much faster in water. The problem of course becomes steering
such things, which right now can be solved by having fins which extend out of
the bubble. The importance of this work shows how you can get interaction
between the fluid and the gas, so that potentially one could steer without
need for fins that go out of the bubble. Although this phenomenon, may not be
relevant over the scales at which supercavitating systems operate.

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cyphar
> Really makes you wonder at what other interesting phenomenon are happening
> in plain sight we've yet to notice.

It happens all the time, and is both exciting and a little worrying (because
it implies that basic effects that we should already understand, we actually
seem to misunderstand quite often).

As an example of this, we didn't have a full explanation for why alkali metals
seem to explode when you put them on water until YouTube when Phil Mason
(thunderf00t) started working on it[1] -- the classic explanation was that it
was because of the hydrogen gas released, but this didn't make sense because
they would also explode under water. There was a Nature article about it[2],
but the gist of it is that once the alkali metal melts and sheds its oxide
surface due to the high heat of the reaction there are solviated electrons in
the water that attract the metal surface and causes spikes to shoot out from
the metal which then cause more spikes and then it all goes bang (because
reactions can only happen on the interface -- having more surface area causes
more of a reaction) once the surface tension breaks.

This also explains why thermite explodes when you put it on ice and similar
effects. And the solution to stop these sorts of explosions (which are
obviously a hazard for people who work with alkali metals) is to add a weak
surfactant (like soap).

[1]:
[https://www.youtube.com/watch?v=LmlAYnFF_s8](https://www.youtube.com/watch?v=LmlAYnFF_s8)
[2]: [https://www.nature.com/news/sodium-s-explosive-secrets-
revea...](https://www.nature.com/news/sodium-s-explosive-secrets-
revealed-1.16771)

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pg_bot
Is this a new discovery? A few years ago I remember seeing water droplets
navigating a maze in a youtube video but I don't know if they went into enough
detail about how the mechanism worked.

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rzzzt
The Leidenfrost Maze:
[https://youtu.be/w0lMJcAfzU4](https://youtu.be/w0lMJcAfzU4)

~~~
v_lisivka
Can it be used for an automatic small-scale chemical reactor? Many reactions
requires heating anyway.

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anfilt
New discovery? I noticed this many years ago when cooking just a tinny bit of
food in the droplet and you see the food roll around inside. Maybe I should
have written a paper, cool pictures though.

I guess maybe it's more surprising to learn what has not been written about.
Makes you wonder what things were noticed by someone but they never said
anything.

