Krampf
Experiment of the Week
- #137
Floating Water Drops
http://www.krampf.com
This week's experiment should be familiar to any of you that have ever
cooked pancakes. As my mother taught me, and as you will find in most
cookbooks, in order to tell if the skillet is hot enough, you dip your
fingers into some water and then shake a few drops onto the skillet. If
the drops just sit there or if they hit the skillet and boil, then it
is not hot enough. As the temperature of the skillet increases, you
reach a point where the drop of water seems to bounce and glide around
the skillet. Then you know that the skillet is hot enough for pancakes.
This is called the Leidenfrost Effect, and that is what we want to
observe now.
This experiment uses the stove, so be sure that you have permission and
that you are very careful. Remember that safety always comes first.
Start with a clean, dry skillet. Place it on the burner of your stove
and heat it. Every 15 seconds or so, test the skillet with a few drops
of water, until you reach the point where the drops seem to float
across the surface. This occurs when the temperature of the skillet
reaches 320 F.
After you have observed a few drops, you can turn off the heat, unless
you want to make some pancakes or griddlecakes. Why does the drop
behave that way? The first drops that you let fall on the skillet just
sat there. The metal was not very hot and so the drop just sat there
until it got hot enough to boil. As the metal got hotter and hotter,
the drops boiled more and more quickly. Finally, we reached the point
where the water vaporizes before it actually hits the skillet. This
produces water vapor, which acts as a cushion between the drop and the
skillet. Enough heat is transferred from the skillet to the drop to
cause it to continue to produce enough water vapor to keep the drop
floating.
The Leidenfrost Effect is also seen when working with liquid nitrogen.
When nitrogen, the main gas in the air around you, is cooled to 320
degrees below zero, Fahrenheit, it becomes a liquid. When this liquid
is spilled on a table top, it behaves in the same way as water on a
very hot skillet. Over the past few weeks, I have been working with
liquid nitrogen, developing some demonstrations for a new show, which
is what gave me the idea for this week's experiment. I think it is
wonderful how observations of cryogenic liquids tie into what I learned
about cooking pancakes as a child.
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