LEIDENFROST EFFECT

The Leidenfrost effect:

It is also known as film boiling, which occurs when a liquid comes into contact with a solid that is at a temperature well above the liquid’s boiling point. Upon contact, a layer of vapor forms between the liquid-solid interface, creating a barrier between the two. There are many examples of this phenomenon, ranging from something you may have seen in your kitchen (water “dancing” around in a pan) to things you shouldn’t try at home (dipping your hand into a pot of molten lead).

The process of film boiling is attributed to the doctor and theologian Johann Gottlob Leidenfrost 

who first described the effect in a manuscript titled A Tract About Some Qualities of Common 

Water. 

The most common occurrence of the Leidenfrost effect can be found in the kitchen when a droplet of water is placed on a hot frying pan. If the pan is heated to the Leidenfrost point, about 200°C, the water evaporates so quickly that a thin layer of vapor forms between the pan and the water droplet, protecting the water from the burning hot pan. The vapor also causes the water droplet to “dance” around in the pan as the vapor propels it in different directions. The droplet will of course evaporate over a certain amount of time, but it will take significantly longer than if the pan were heated to a temperature above boiling, but below the Leidenfrost point.

 

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 a vapor.

At some point higher than the boiling point(When exceeds the Leidenfrost Point), the edge of the liquid drop instantly vaporizes, cushioning the remainder of the liquid from contact. The resulting gas suspends the rest of the water droplet just above it, preventing any further contact between liquid water & hot plate. As steam has poorer thermal conductivity, further heat transfer between the pan & droplet is slowed down dramatically. This also results in the drop being able to skid around the pan on the layer of gas just under it.

                                  

The temperature depends on many factors, including atmospheric pressure, the volume of the droplet, and the surface properties of the liquid.

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.

Leidenfrost Effect Demonstration - Water on a Hot pan:

                                     

The simplest way to demonstrate the Leidenfrost effect is to sprinkle droplets of water on a hot pan or burner. In this instance, the Leidenfrost effect has a practical application. You can use it to check whether or not a pan is hot enough to be used for cooking without risking your recipe on a too-cool pan!