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January 21, 2015

Water bounces off laser-treated metals

Superhydrophobic properties could lead to applications in solar panels, sanitation, and as rust-free metals

droplets of water bouncing off a metal surface

Rochester scientists have used lasers to transform metals into extremely water-repellent, or superhydrophobic, materials without the need for temporary coatings.

Superhydrophobic materials are desirable for a number of applications, such as rust prevention, anti-icing, or in sanitation uses. However, as Chunlei Guo says, most current hydrophobic materials rely on chemical coatings.

In a paper published in the Journal of Applied Physics, Guo, professor of optics, and his colleague at the Institute of Optics, Anatoliy Goodbye, describe a powerful and precise laser-patterning technique that creates an intricate pattern of micro- and nanoscale structures to give the metals their new properties. The work builds on earlier research by the team in which they used a similar laser-patterning technique that turned metals black. Guo says that by using the technique, they can create multifunctional surfaces that are not only superhydrophobic but also highly absorbent optically.

Guo adds that one of the big advantages of his team’s process is that “the structures created by our laser on the metals are intrinsically part of the material surface.” That means they won’t rub off. And it’s these patterns that make the metals repel water. “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,” says Guo. The whole process takes less than a second.

The materials Guo has created are much more slippery than Teflon—a common hydrophobic material that often coats nonstick frying pans. Unlike Guo’s laser-treated metals, the Teflon kitchen tools are not superhydrophobic. The difference is that to make water roll off a Teflon coated material, you need to tilt the surface to nearly a 70-degree angle before the water begins to slide off. You can make water roll off Guo’s metals by tilting them less than five degrees.

As the water bounces off the superhydrophobic surfaces, it also collects dust particles and takes them along for the ride. To test the self-cleaning property, Guo and his team took ordinary dust from a vacuum cleaner and dumped it onto the treated surface. Roughly half of the dust particles were removed with just three drops of water. It took only a dozen drops to leave the surface spotless. Better yet, it remained completely dry.

Guo says he is excited by potential applications of superhydrophobic materials in developing countries. It is this potential that has piqued the interest of the Bill & Melinda Gates Foundation, which has supported the work.

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

Guo’s team had previously blasted materials with the lasers and turned them hydrophilic, meaning they attract water. In fact, the materials were so hydrophilic that putting them in contact with a drop of water made water run “uphill.”

His team is now planning to focus on increasing the speed of patterning the surfaces and to study how to expand the technique to other materials, such as semiconductors or dielectrics—opening up the possibility of water-repellent electronics.

Funding was provided by the Bill & Melinda Gates Foundation and the United States Air Force Office of Scientific Research.

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