Altering the degree to which a surface repels water can in turn alter how violently water boils, a finding that could help improve the efficiency of heating.
Altering the degree to which a surface repels water can in turn alter how violently water boils, report researchers, a finding that could help improve the efficiency of heating.
Professor Derek Chan of the University of Melbourne and colleagues report their findings today in the journal Nature.
Say you were to dip your finger in water and then (very!) quickly dip it in molten lead. While I don't recommend you try this at home, you won’t get burnt, thanks to an insulating layer of steam that forms around the finger. The chemists have exploited this phenomenon, known as the Leidenfrost effect, to boil water without making bubbles.
Chan and colleagues, including Dr Ivan Vakarelski of the King Abdul University of Science and Technology in Saudi Arabia, have now found that changing the coating on a hot surface can control whether a Leidenfrost vapour layer forms.
The researchers covered a steel ball with Glaco Mirror Coat, a hydrophobic 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 700ºC and dropping it in room-temperature water spurred boiling, but no furious bubbles.
"That's never been seen before," said Chan of this discovery.
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.
"If you are an engineer designing a boiler to heat water, you want to avoid the formation of a vapour layer because that makes heat transfer inefficient," says Chan.
"Therefore you want to use hydrophilic surfaces which transfer heat much better than superhydrophobic surfaces which always form a vapour layer."
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