If you need to cool a metal surface, one efficient way of doing it is to boil water off it. The change of phase from liquid to vapour requires a large amount of heat, so boiling is a good way of drawing energy out of a material. The effects can easily be experienced by licking a finger and blowing on it; the cooling effects derives from the evaporation of saliva. Cooling is important in many technologies; nuclear reactors must be cooled to remain safe, and the microprocessors in electronic devices produce a great deal of heat in operation that needs to be effectively removed to keep them running. Some laptops, tablets and smartphones even contain water-filled tubes to facilitate cooling.
The hydrophobic surface, right, allows many small vapour bubbles to form, more efficiently removing energy from the hot metal. Image Purdue University image/Taylor Allred
Mechanical engineers at Purdue University in Indiana have found that superhydrophobic surfaces, previously thought to be useless for boiling liquids, in fact have highly favourable properties under the right conditions.
"Superhydrophobic surfaces were previously thought to be bad for boiling because they can't stay wet," said Taylor Allred, a Purdue doctoral student in mechanical engineering and first author of a paper on the research in Physical Review Letters. "You get a blanket of vapour on the surface, and because vapour is a very good insulator of heat, you are insulating the surface instead of cooling it."
"We realised that if we could perform one key step prior to boiling from the superhydrophobic surface, we could get the best of both hydrophobic and hydrophilic surface behaviors all in one," said Prof Suresh Garimella, director of the Purdue's cooling technologies research centre.
Allred first submerged a piece of metal with a surface patterned with a hydrophobic texture in water, then boiled the water without separately heating the metal. This removed the layer of air that is normally trapped in the texture of the hydrophobic surface. It is this trapping of air that gives the surface its properties, so removing the air allows the water to wet the surface.
The result of this is that when the surface is heated to boil water, vapour bubbles are "pinned" into the textured surface as they form, preventing them from spreading out as they expand. Instead, they simply detach from the surface, allowing water to move in behind and keep the surface wet. Another effect is that many small bubbles form on the surface, which improves cooling efficiency as each bubble requires energy to form.
"There are numerous papers on making superhydrophobic surfaces and using them for a variety of applications. With this research, we've opened up a whole new area where they can be deployed," Garimella said.
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