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Beyond Teflon: Etching a Dryer Metal

Thursday, January 29, 2015

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Hydrophobic coatings, the mainstay of materials protection, may have met their match in a new laser technique that reportedly makes metal surfaces repel water permanently.

Anti-corrosion, self-cleaning and anti-icing technologies are based on super-hydrophobic materials, but many of these rely on chemical coatings, explains Chunlei Guo, a professor of optics in the Hajim School of Engineering and Applied Sciences at the University of Rochester.

And while coatings can make a surface repel water, the effect isn't strong and they don't last forever—unlike the permanent bounce that Guo says his technique achieves.

Permanent Properties

Guo and Anatoliy Vorobyev, from the university's Institute of Optics, say they can change the properties of metals by using a powerful and precise laser-patterning technique that creates an intricate pattern of micro- and nano-scale structures.

Scientists at the University of Rochester say they can use lasers to make metal surfaces extremely water repellent, without the need for temporary coatings.

According to Guo, the laser method can create a multifunctional surface that both repels water and is highly absorbent optically.

The big advantage to this process is that "the structures created by our laser on the metals are intrinsically part of the material surface," which means they won't rub or wear off, Guo explains.

Bouncing Off Water & Dust

How well does the technique work?

"The material is so strongly water-repellent, the water actually gets bounced off," Guo says.

"Then it lands on the surface again, gets bounced off again, and then it will just roll off from the surface."

The entire process takes less than a second.

Guo says his materials are more slippery than Teflon. A Teflon-coated surface has to be tilted to nearly 70 degrees before water slides off. Guo's metals, on the other hand, have to be tilted less than five degrees to get the water rolling off.

This technique also makes the surface self-cleaning, the team says. The water collects dust particles as it bounces off the surface.

To test the self-cleaning property, the team dumped dust from a vacuum cleaner onto a treated surface. About half of the dust particles were removed with only three drops of water, Guo says. A dozen drops made the surface spotless.

Challenges Remain

The new process isn't flawless, though; there are still challenges to address before the process can be scaled up.

For starters, Guo says, it takes an hour to pattern a metal sample that is one inch square. The researchers are also looking into ways of applying the technique to non-metal materials.

Then there is a power issue. The femtosecond laser pulse used by Guo and Vorobyev lasts only a quadrillionth of a second, but that short burst reaches a "peak power equivalent to that of the entire power grid of North America."

University of Rochester
University of Rochester / J. Adam Fenster

Guo says the water not only bounces off, but takes dust particles with it, making the surface self-cleaning.

The team is now focusing on increasing the speed of patterning and studying how to expand the technique to other materials, such as semiconductors or dielectrics.

Potential in Developing Countries

Guo says his technique could have potential in developing countries. His research has garnered interest from the Bill and Melinda Gates Foundation, which has funded the work, along with the U.S. Air Force Office of Scientific Research.

"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 says. "A second application could be creating latrines that are cleaner and healthier to use."

Latrines are hard to keep clean in areas with little water, and super-hydrophobic materials could help keep them clean without the need for flushing.

The two scientists previously used a similar technique to turn metal black. Turning metals black can make them very efficient at absorbing light. If made water repellent, too, the metals could make excellent solar absorbers that don't rust or require much cleaning.

The researchers also previously used lasers to make metals attract water. The materials were so hydrophilic that putting them in contact with a drop of water made "water run uphill."

A paper on the new research, "Multifunctional surfaces produced by femtosecond laser pulses," was published in the Journal of Applied Physics.

   

Tagged categories: Colleges and Universities; Metal coatings; Metals; Research; Self-cleaning coatings; Water repellents

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