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Leidenfrost Phenomenon
Name: Mike M.
Status: student
Age: 14
Location: N/A
Country: N/A
Date: 5/5/2004
Question:
I was preparing to cook some soup and I put the saucepan
on the stove to let it heat up. It took me a minute to find the can
opener and by the time it was open I decided to test how hot the pan was
before I added anything that might burn; so I dropped a few drops of
water into the pan and they did not evaporate but the droplets kind of
bounced around then conglomerated into a big drop (the stove sort of
leans to the back)... the big drop did not seem to evaporate,
noticeably anyway. When I moved the pan the drop skidded around and
would not stop, like the motion of a well balanced bike wheel when spun
upside down. I was wondering what keeps the drop from evaporating fast
as one would expect from a pan that hot and what makes the water move
effortlessly and with seemingly much less friction than usual.
Replies:
There are 3 ranges of surface-temperature relevant to boiling water off a surface:
A) below 100C, not boiling but evaporating fast. Water wets surface and spreads out
aggressively.
Heat has to make its way through the liquid to the top surface where vapor
can leave.
B) 100C to about 150C, "roiling", water wets surface and immediately gets heated
untill it blows a steam bubble, which rises through the liquid.
Liquid re-wets behind the rising bubble. Repeating. Active, chaotic, vigorous,
even violent.
C) above 150C, "skidding-droplet", heat from hot surface radiates and gas-conducts
to bottom surface of liquid drop, generating enough steam flow and pressure
to hold the drop slightly up off the surface.
Much of the steam being generated is hissing out from underneath the drop!
Because the water never actually touches the pan,
the evaporation rate is actually slower than in (B),
so you get some time to enjoy the frictionless hovercraft act it is doing as it skids around
the pan.
Do not overheat your mom's best pan doing this...
Starting with a cold dry pan, turn on the heat.
Watching out for splattering hot water, occasionally put a small drop of water on the pan
from only 1-2 inches above it.
Turn down the heat about halfway once it starts doing (C)-type "skidding-droplet" behavior.
Then, if your skidding droplets start sticking to a spot and boiling away (B), It is too cool,
turn it up a little.
This is one way experienced lab people use to tell what temperature their hot-plate has
reached.
Many cooks too, I imagine.
Jim Swenson
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Update: June 2012
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