Water droplet affected by Leidenfrost Effect: Image by Hullernuc
The Leidenfrost Effect is also known as “film boiling.” This one effect makes water dance on a frying pan, flow uphill, and protects a sausage from burning in hot lead!
Public Safety Message for Testing the Leidenfrost Effect
The Leidenfrost Effect deals with steam and heat. These experiments involve hot objects. Carelessness can lead to personal injury from burns or scalding, or even fires.
Do not perform these experiments while impaired by alcohol, drugs or sleep deprivation. Minors must have parental permission and supervision.
If the heat source has an open flame, be sure that your clothes won’t brush past the flames. Long flowing hair, long loose sleeves, or scarves are all hazards to be eliminated before starting these experiments.
Some types of frying pans will scorch or burn if you heat them with nothing in the pan, so use an old skillet that is no longer the household favorite for this experiment. A heavier frying pan will heat or cool more slowly than a lighter one of the same metal.
Simple Leidenfrost Experiments in a Frying Pan
The minimum lab equipment for this home science experiment is:
- A skillet or frying pan
- A stove top or other heat source
- Clean water at about room temperature
Optional equipment includes:
- An eyedropper to apply the water to the skillet
- A stopwatch to time the effects
- Note-taking supplies, such as a pen or pencil and notebook
- A thermocouple to check the temperature of the skillet
Cool Water and Cool Skillet
Leidenfrost droplet: Image by OgreBot
Set up the dry frying pan on a cold stove. Add a drop of water, from a low height of about two centimetres.
Does the water form a drop and remain in a small puddle? Is the puddle moving anywhere? Did any tiny droplets splash away? It is unlikely anything else of great interest will happen.
Turn Up the Heat
Add enough heat that the water starts to boil; watch for a wisp of steam, or for the droplet’s puddle to shrink.
Remove the skillet from the heat for a moment. Add another drop of water from the same height. Does it sit fairly quietly?
If so, this is experiment number 2. Record the temperature of the skillet (if you have a thermocouple). Use the stopwatch to check how long a new drop of water lasts before it evaporates. Log the results.
More Heat for the Skillet
Keep heating the skillet, and add another drop of water. Watch for the first drop that shows bubbles, but does not “dance around” on the frying pan.
Again, remove the skillet from the heat, check the temperature, and time how long a new drop of water takes to fully evaporate if it bubbles as soon as it lands.
Droplet on a Hot Skillet
Resume heating the skillet, checking with an occasional drop of water until one starts “dancing” when it lands. Do you hear any sounds?
Repeat the “remove from heat and time how long a dancing droplet lasts” test.
The Final Heat of the Race
The final condition is to have the frying pan even hotter. The effects on the drop of water should be louder and more intense than before. Again, record the time.
Clean Up and Cool Down
Unless you plan to fry something, the only responsible action is to cool down the frying pan and ensure no-one comes near the apparatus while anything is hot.
Even if you cook, take care with that frying pan, and allow it to cool somewhat before adding anything to the pan. A wet vegetable or piece of meat might jump out of the frying pan and into the fire, or cooking oil might spatter or heat to its smoking or flash points, if added while the pan is too hot.
The Expected Results of the Hot Skillet Experiments
Boiling water droplet: Image by Mononoke
The surprising effect is that there is an ideal temperature range in which the drop of water will skip and dance for the longest time.
When the frying pan is just above the boiling temperature of water, which is 100C at sea level, the drop of water should boil away fairly quickly. Bubbles in the droplet show that any dissolved gas is being released, and steam is rising through the liquid. As the temperature rises, this process moves ever more quickly.
However, as the temperature rises further, the Leidenfrost effect emerges. The bottom “film” of water evaporates so quickly that it insulates the rest of the droplet and lifts it up from the skillet’s surface. This causes the skittering and dancing effect. The steam is momentarily trapped, and its escape provides the sound effects.
The Leidenfrost Point is the temperature at which the droplet lasts longest. At yet higher temperatures, the nearly instantaneous steam carries enough heat to boil the droplet more quickly.
A Leidenfrost Experiment Only for Responsible Adults
Restart the experiment, but with a few drops of cooking oil in a cold, dry frying pan. Does the oil stay in a small puddle, or spread out?
Slowly heat the skillet. Does the oil begin to spread out? It is very important that there is barely a thin film of oil in the frying pan; this is not deep frying!
When the skillet has entered the Leidenfrost temperature zone, add one drop of water.
The result should be pretty spectacular, with more noise and quicker boiling than before.
Leidenfrost’s Reason to Avoid Mixing Water and Hot Oil
A very safe experiment is to pour a little cold water and a little cold oil into a small, cool, transparent drinking glass or test tube. It should quickly settle into two layers, with the oil on top.
Long ago, this observation led to the phrase “Oil and water do not mix”.
In the hot oil experiment, the water droplet tried to fall through the layer of oil. As it became engulfed by hot oil, a lot of its surface area boiled. The first experiments demonstrated the Leidenfrost effect on only the bottom of the water drop. This time, almost the whole drop’s surface was the film that boiled.
The Safety Lesson: Water Cannot Extinguish a Cooking Oil Fire
As the Mythbuster television program once demonstrated, a deep fryer with hot, flaming cooking oil cannot be extinguished by dumping in water. The Leidenfrost effect turns that water into a steam explosion, sending burning oil in all directions.
Do not try this at home, or camp, or anywhere. Simply refer to this Mythbuster demonstration:
The Positive Cooking Value of the Leidenfrost Effect
The water droplet dance is a traditional way of testing whether the frying pan is at the right temperature. If the water skitters properly, the frying pan is ready to sear the food.
The End of the Lab, but Not of the Leidenfrost Effect
The following effects are not suited to home science experiments.
Impractical Self-Rising Water
The Leidenfrost Effect has been used to ratchet water uphill. The two tricks are to make a pipe with an internal ratchet structure, and then heat it to the Leidenfrost temperature. Pump the water into the low end, and some water should be propelled uphill by steam.
This process might be reasonable where excess heat is being moved anyway. It does not seem likely to replace an Archimedean screw or any other conventional way of moving water uphill.
Sausages Saved by the Leidenfrost Effect
It is possible to protect delicate objects for brief periods of time using the Leidenfrost Effect. The Mythbusters demonstrated this by briefly dipping sausages, and their own fingers, into boiling lead. (Start at about 1:48).
Improving Ship Propulsion
The Leidenfrost Effect might propel a ship through the water by floating it on steam. Readers are encouraged to compare the energy requirements for a given size and weight of such a “Leidenfrost steam ship” to a hovercraft that simply blows air for lift.
Are Other Materials Subject to the Leidenfrost Effect?
Yes, it seems any liquid that can be boiled rapidly can exhibit similar effects. Liquid nitrogen, for example, will skitter and dance when poured onto a room-temperature surface.
Who was Leidenfrost?
Johann Gottlob Leidenfrost: Image provided by Yorg
Johann Gottlob Leidenfrost was born in Germany on Nov. 27, 1715. The son of a minister, Leidenfrost transferred from theological to medical studies during university.
Some years after graduating, in 1743 he became a professor at the University of Duisburg and maintained a private medical practice. He married and became father to seven children.
Leidenfrost died Dec. 2, 1794.
Leidenfrost was not the first to notice “his” effect. In 1732, Hermann Boerhaave reported this effect but Leidenfrost’s 1756 “A Tract About Some Qualities of Common Water” (“De Aquae Communis Nonnullis Qualitatibus Tractatus” in its original Latin) eventually became the definitive cited opus.
References:
“Johann Gottlob Leidenfrost“, Wikipedia Org. (May 13, 2011). Accessed Aug. 15, 2011.
Linke, Heiner and Taormina, Mike et al. “Self-Propelled Droplets“. University of Oregon. Accessed Aug. 15, 2011.
Pease, Roland. “Scientists make water run uphill“. BBC News. (Apr. 30, 2006). Accessed Aug. 15, 2011.
Walker, Jearl. “Boiling and the Leidenfrost Effect“. Cleveland State University. (1910). PDF accessed Aug. 15, 2011.
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