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Leidenfrost Effect Demonstrations


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Leidenfrost Effect Demonstrations
This is a diagram of the Leidenfrost effect.

In the Leidenfrost effect, a droplet of liquid is separated from a hot surface by a protective layer of vapor.

Vystrix Nexoth, Creative Commons License
There are several ways you can demonstrate the Leidenfrost effect. Here is an explanation of the Leidenfrost effect and instructions for performing science demonstrations with water, liquid nitrogen, and lead.

What Is the Leidenfrost Effect?

The Leidenfrost effect is named for Johann Gottlob Leidenfrost, who described the phenomenon in A Tract About Some Qualities of Common Water in 1796. In the Leidenfrost effect, a liquid in proximity to a surface much hotter than the liquid's boiling point will produce a layer of vapor that insulates the liquid and physically separates it from the surface. Essentially, even though the surface is much hotter than the boiling point of the liquid, it vaporizes more slowly than if the surface was near the boiling point. The vapor between the liquid and the surface prevents the two from coming into direct contact.

The Leidenfrost Point

It's not easy to identify the precise temperature at which the Leidenfrost effect comes into play -- the Leidenfrost point. If you place a drop of liquid onto a surface that is cooler than the liquids boiling point, the drop will flatten out and heat up. At the boiling point, the drop may hiss, but it will sit on the surface and boil into vapor. At some point higher than the boiling point, the edge of the liquid drop instantly vaporizes, cushioning the remainder of the liquid from contact. The temperature depends on many factors, including atmospheric pressure, the volume of the droplet, and the surface properties of the liquid. The Leidenfrost point for water is about twice its boiling point, but that information can't be used to predict the Leidenfrost point for other liquids. If you are performing a demonstration of the Leidenfrost effect, your best bet will be to use a surface that is much hotter than the boiling point of the liquid, so you'll be sure it's hot enough.

There are several ways to demonstrate the Leidenfrost effect. Demonstrations with water, liquid nitrogen, and molten lead are the most common...

Overview of the Leidenfrost Effect
Water Droplets on a Hot Pan
Leidenfrost Effect with Liquid Nitrogen
Immersing Your Hand in Molten Lead

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