Stovetop Science: Exploring the Leidenfrost Effect

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Those of you who frequent the kitchen may be familiar with the experience of dripping water on a hot pan to check its temperature. If you sprinkle water on a mildly heated pan, it will slowly boil and eventually evaporate. But something curious happens if you wait for the pan to really heat up. Water on a hot enough pan will congregate into small droplets, hover above the surface, and skittle around as if on a frictionless plane. Why does this happen?

leidenfrost effect

At a high enough temperature, called the Leidenfrost point (this exact number depends on the water content and nature of the surface), the bottom part of the water droplet that touches the hot pan evaporates faster than the rest of the droplet. This creates a layer of water vapor for the droplet to sit on top of, allowing it ride around the hot surface. Furthermore, steam has poorer thermal conductivity than liquid. This means the vapor layer is a good insulator, and prevents heat from reaching the liquid molecules, allowing the water droplet to live a few seconds longer.

This effect, described by Johann Glottlob Leidenfrost, can also be applied to liquid nitrogen. Liquid nitrogen boils at -196ºC, so any surface at room temperature will be incredibly hot. When you pour out liquid nitrogen on the ground, the droplets skirt around, similar to water on a heated frying pan. This effect also explains why friends at NurdRage were able to do the ALS Ice Bucket Challenge with liquid nitrogen (do NOT try this yourself!).

For more Leidenfrost fun, check out what folks at Bath University have created to make water flow uphill.

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