Kinetic Karnival - Episode 5: The Leidenfrost Effect
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THE LEIDENFROST EFFECT [https://youtu.be/6D2cz0K9i8g] 29 minutes · 12 questions
It is important that you watch this video before showing it to students. It includes an unfortunate, insensitive utterance that landed differently in 1980 than it does today. You might choose to mute it; you might choose to have a conversation about it.
When water drops are sprinkled onto metal heated to a temperature between 100 °C and 200 °C. the drops spread over the metal and evaporate in one or two seconds. If the metal is 200 °C or hotter. the drops should disappear even faster. Instead, they last up to 1.5 minutes by means of the Leidenfrost effect. As a drop approaches a very hot surface, the bottom of the drop evaporates to create a thin but protective vapor layer between the surface and the remaining liquid drop. Although the vapor layer is less than a millimeter thick, it still provides both support and protection for the drop. Because a vapor is a relatively poor conductor of heat, the heat conduction from the hot surface to the liquid is substantially decreased. Up to 1.5 minutes are required before sufficient heat is conducted and radiated up through the vapor layer to evaporate the rest of the water.
The Leidenfrost effect is seen but probably unnoticed in many common settings. When a hot iron is tested with a wet finger, the Leidenfrost effect protects the finger from a burn. Often the temperature of a skillet is tested by sprinkling water into it. If the water drops "bead up," then the skillet must be hotter than 200 °C.
The effect also plays a role in several relatively dangerous demonstrations. In the show I dip my finger into molten lead that has a temperature in excess of 620 °F. I first wet my finger because the stunt's success depends on the water. When it is immersed, part of the water is immediately vaporized by the intense heat from the molten lead. That water creates a protective sheath around my finger, at least for the brief plunge.
Similar protection figures into the demonstration where I pour liquid nitrogen into my mouth. The liquid, which is at a temperature that is no greater than –200 °C, rests on my tongue. The tongue is not frozen, in fact does not even feel cool, because part of the liquid vaporizes to form a protective layer of nitrogen vapor between the liquid and my tongue.
The Leidenfrost effect also plays a role in my walk on hot coals. Part of the sweat on my feet helps quench the coals wherever my feet touch them. With a layer of dead or partially cooled ash touching my skin, I endanger my feet less. Part of the sweat also creates a protective vapor layer between the hot coals and my skin. I could continue to walk across hot coals until I depleted the sweat on my feet or began to repeat the regions of contact on my feet.
The stunts in this show are truly dangerous. I have been seriously hurt with them, especially with the walk on hot coals. Please do not attempt them yourself, and please caution your students about the danger.
Jearl Walker, Professor of Physics Cleveland State University
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