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Author Topic: Why does water behave differently when it hits surfaces of different temperature?  (Read 845 times)

Offline thedoc

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Martin Fennell asked the Naked Scientists:
   Hi, while cleaning a wok, I got it to a very high temperature and spent some time splashing drops of water in to it. I am sure you could get some sort of kitchen science out of it.

Somethings I observed (and that I don't fully understand)

1 - below a critical temperature, balls don't form, you get a sheet of water that evaporates quickly.

2 - above the critical temperature you get the balls of water that do not evaporate for a long time

3 - If there is a burnt bit of food, that the ball hits, the ball goes to a flat sheet, and evaporates quickly

So I am assuming that surface tension is keeping the balls together, and because there is a small area of contact, the heat of the pan does not get into the balls, so they don't evaporate.
If the surface tension is broken, the ball turns to a sheet and so there is a large area in contact with the heat, so it evaporates quickly.

What I don't understand is why balls form when the pan is hot, and not when it is colder. I am guessing that when the water hits the pan, a small part quickly turns to steam, and the balls are supported on that, but below the temperature, not enough steam is generated before the ball collapses.

I'd love to see some high speed or infrared photography to try and see what is really happening.

The scientist in me wants to fully understand what is happening.

The engineer in me is trying to think of an application.

The boy in me just loves the playing with heat and water.

Thanks for a great show

Martin
What do you think?
« Last Edit: 16/03/2016 12:50:01 by _system »


 

Offline chiralSPO

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You're on the right track. Rather than explain what's going on myself, I will direct you to this wikipedia article on the subject:

https://en.wikipedia.org/wiki/Leidenfrost_effect



Liquid nitrogen does the same thing on surfaces that are "room temperature"

https:   //www.youtube.com/watch?v=YZMPyz_fTPA
 

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