4 ms·

Water Droplets Don’t Just Hover on a Hot Pan – They Roll

50 points by dnetesn 8y ago

pg_bot 8y ago
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.
- rzzzt 8y ago
  The Leidenfrost Maze: https://youtu.be/w0lMJcAfzU4 https://youtu.be/w0lMJcAfzU4
  - v_lisivka 8y ago
    Can it be used for an automatic small-scale chemical reactor? Many reactions requires heating anyway.
- sandworm101 8y ago
  This seems less about the motion of the drop and more about the internal motion of water within the drop. The maze demo deals with steam, not the liquid water within the drop before it becomes steam.
- gene-h 8y ago
  The maze example required a special shaped substrate to achieve motion, this work achieved droplet motion without a shaped substrate
gene-h 8y ago
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.
- cyphar 8y ago
  > 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-revealed-1.16771 https://www.nature.com/news/sodium-s-explosive-secrets-revea...
anfilt 8y ago
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.

50 points by dnetesn 8y ago

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.

rzzzt 8y ago
The Leidenfrost Maze: https://youtu.be/w0lMJcAfzU4 https://youtu.be/w0lMJcAfzU4
- v_lisivka 8y ago
  Can it be used for an automatic small-scale chemical reactor? Many reactions requires heating anyway.
sandworm101 8y ago
This seems less about the motion of the drop and more about the internal motion of water within the drop. The maze demo deals with steam, not the liquid water within the drop before it becomes steam.
gene-h 8y ago
The maze example required a special shaped substrate to achieve motion, this work achieved droplet motion without a shaped substrate

gene-h 8y ago

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.

cyphar 8y ago
> 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-revealed-1.16771 https://www.nature.com/news/sodium-s-explosive-secrets-revea...

anfilt 8y ago

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.