Boil with no toil Changing the degree to which a surface repels water can 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.
One of the factors determining the efficiency of heating water is the contact that the water has with the heating surface.
The problem is the hotter a surface gets, the more likely it is that efficiency will suffer due to a phenomenon known as the Leidenfrost effect.
A common demonstration of this phenomenon is when drops of water skitter across a hot barbeque plate without evaporating.
The drops don't evaporate because the extreme heat causes an insulating layer of vapour to form between the water drops and the hot surface.
The insulation slows heat transfer and that can be a problem in some situations if you want to heat water quickly, for example in a power station.
"If you want efficient heating you do not want to form a Leidenfrost layer," says Chan.
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.
In their experiment they coated 2-centimetre diameter steel balls in four different coatings ranging from very hydrophilic (water-loving) to superhydrophobic (very water-repellent).
They heated the balls to hundreds of degrees Celsius, then plunged them into water at room temperature and filmed what happened as each ball cooled in the water.
Initially, each ball transferred its heat to the water without causing bubbles.
In this situation, the Leidenfrost effect was operating - the ball was extremely hot and, like the barbeque plate, was causing an insulating vapour layer to form between it and the water.
In the case of the three balls that had the least water-repellent coatings, water around them eventually started to bubble violently.
"As the ball cools down, the water breaks through the vapour layer and makes direct contact with the hot surface," says Chan. "When that happens you get an explosion of bubbles."
But the ball with the most hydrophobic surface behaved differently. It had a surface chemically modified to be water-repelling, but also textured so the water around it effectively sat on a tiny 'bed of nails'.
Chan says a Leidenfrost vapour layer was generated around the ball the minute it hit the water and insulated it from the liquid, slowing down the transfer of heat so it cooled completely without any bubbles forming at all.
"That's never been seen before," he says.
If the new findings apply more generally they could be used to create surfaces that help improve the efficiency of heat transfer in engineering applications, such as power stations.
"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."
On the other hand, superhydrophobic surfaces could be used to bring about a slower gentle bubble-free boiling - perhaps creating safer kettles, Chan muses.
Superhydrophobic surfaces could also be used to coat powerlines, he says, to prevent water from cooling and forming frost on them.
And because vapour layers also reduce the drag between different surfaces, superhydrophobic surfaces might also be useful in the development of self-cleaning coatings on buildings and windows, says Chan.
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