Krampf@aol.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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