The Leidenfrost effect

We have all observed what happens when water drops fall on a hot skillet. Rather than simply boiling off, they skitter around for awhile before disappearing in a puff.

This phenomenon is due to something called the Leidenfrost effect. If you place a drop of water on a surface, it gets flattened and just stays there due to gravity. But when placed on a surface whose temperature is higher than the boiling point of water, a thin layer of water vapor forms almost immediately that partially insulates the drop from the hot surface and also raises it off the surface, making it almost spherical again as well as reducing the frictional forces on it, enabling the drop to move around freely in response to the turbulent air currents that surround it.

Sophie Bushwick looks at recent research (published in the July 19, 2012 issue of Physical Review Letters) on the Leidenfrost effect that explains what is happening in more detail, determined by using high-speed cameras to view in slow-motion what happens to the drops.

Apropos the recent post on fire-walking, some readers brought up the possibility of the Leidenfrost effect as an explanation of why people’s feet might not get burned. The idea is that moisture on the feet might create a vapor insulating layer, reducing the heat transfer to the soles of the feet. While that theory was at one time popular as the dominant protective effect in fire-walking, over time belief in its importance has decreased with respect to the other factors listed in that post.


  1. Scott says

    I had read the reason firewalkers don’t (always) get burned is because the coals that the fire has burned down to, while hot, don’t transfer their heat effectively, so walking across the coals quickly, rather than sauntering lazily across them, is the key to walking and not getting burned.

    Standing on hot coals? That would be a neat trick.

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