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Sci-Tech

Lasers have turned this metal super hydrophobic

Researchers have used lasers to create an extremely hydrophobic material with potential applications from sanitation to solar panels.

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J. Adam Fenster/University of Rochester

The ability to repel water has so many potential uses. It can help prevent corrosion as well as the growth of organisms; it means objects need to be cleaned less often, since water can't cling long enough to evaporate and leave behind residue; and it can prevent ice from forming.

Current methods usually involve a hydrophobic coating -- a technique that is prone to damage, since the coating is usually ultrathin and needs to be reapplied every few years. A new technique developed by researchers at the University of Rochester, however, can make a surface hydrophobic without the use of coatings.

How? With a precise and powerful laser that etches a pattern of micro- and nanoscale structures onto the surface of metal -- building on earlier work that used laser-patterning to absorb light to render metal black -- creating a material that both absorbs light and repels water.

The material is much more slippery than Teflon, with the added benefit that the structures are part of the material -- which means they won't easily wear away. You also don't have to tip the surface for the water to roll off, like you do with Teflon.

"The material is so strongly water-repellent, the water actually gets bounced off. Then it lands on the surface again, gets bounced off again, and then it will just roll off from the surface," said Chunlei Guo, professor of optics in the University of Rochester Hajim School of Engineering and Applied Sciences.

It's also self-cleaning: As the water bounces on the surface of the material, it will pick up and carry away with it any dust, a theory tested by covering the surface with the dust from a vacuum cleaner bag. Three drops carried away half the dust particles; the surface was completely cleaned with just 12 drops, leaving it both dry and dust-free.

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J. Adam Fenster/University of Rochester

It is this property that has Guo excited about its potential sanitation applications in developing regions, where water can be scarce and therefore precious. It could help clean, for instance, latrines with a minimum of water wastage.

"In these regions, collecting rain water is vital and using super-hydrophobic materials could increase the efficiency without the need to use large funnels with high-pitched angles to prevent water from sticking to the surface," Guo said. "A second application could be creating latrines that are cleaner and healthier to use."

Yet another application could be the development of solar thermal collectors, which absorb solar energy in the form of heat and light, since the surface is highly efficient at absorbing, rather than reflecting, both. This type of solar panel would neither rust nor require much cleaning.

There's still a little more R&D to do though, Guo said. It currently takes an hour to etch a square inch of metal surface using an extremely powerful laser that reaches peak power equivalent to that of North America's entire power grid (luckily each pulse lasts only 1 femtosecond -- 1 quadrillionth of a second). It is obviously not yet an efficient material to manufacture.

The next step is increasing the speed, so that the etching process doesn't take quite so long; as well as developing the technique to etch other materials, such as superconductors and dielectrics for the development of water-repellent electronics.

You can find the team's full paper online in the Journal of Applied Physics.