Thermodynamic spectacle illustrates insulating power of steam

By Rachel Ehrenberg

Web edition: September 12, 2012
Print edition: October 6, 2012; Vol.182 #7 (p. 16)

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BUBBLE-FREE BOIL

A superhot steel ball dropped in water generates furious bubbling (right). But a special coating that holds an insulating layer of steam around the ball can keep bubbles at bay (left).

Credit: Ivan Vakarelski

Dip your finger in a bucket of water and then quickly dip it in molten lead — you won’t get burned, thanks to an insulating layer of steam that forms around the finger. Chemists have now exploited this phenomenon, known as the Leidenfrost effect, to boil water without making bubbles.

The researchers covered a steel ball with Glaco Mirror Coat, a water-hating material, along with some other water-repelling chemicals. This turned the sphere’s exterior into a nanoscale mountain range peppered with deep valleys. Heating the sphere to 400º Celsius and dropping it in room-temperature water spurred boiling, but no furious bubbles, the team reports in Sept. 13 Nature. The water near the sphere became vapor that got trapped in the valleys on the sphere’s surface. Eventually this sheet of vapor slipped off and a new one formed.

Treating the surface of another sphere to make it water-loving had the opposite effect, locking the water in the violent bubbling phase. Manipulating this phase-chemistry could lead to tricks for reducing drag on ships or preventing forceful bubbling explosions in labs or kitchens.