Laser-etched metal so water-repellent, liquid bounces off

Researchers at the University of Rochester announced they’ve discovered a laser-patterning technique that renders any metal surface super hydrophobic (water-repellent). So much so, in fact, the material can no longer get wet. According to Chunlei Guo, professor of optics at the University of Rochester, this imparted property could lead to some game changing, if not comic, results.

“The material is so strongly water-repellent, the water actually gets bounced off,” said Guo, who is lead author of a paper describing the technique, published this week in the Journal of Applied Physics.

As described in the research paper, the process uses a femtosecond laser to etch “an array of parallel microgrooves” measuring 75 μm deep and approximately 100μm apart. The grooves increase the metal surface’s contact angle, which in turn causes water droplets to round and roll off more easily. Water droplets falling onto a treated surface, the paper reports, retain 30 percent of their kinetic energy and rebound repeatedly, like kids on a trampoline.

In addition to water-repellent, the treated metal also becomes self-cleaning, in that the repelled water picks up and removes dust and other contaminates as it rolls off. And, unlike coatings, the treatment doesn’t wear off, since the laser etching transforms the material’s surface itself with a complex, nano-scale pattern.

The technique is similar to a laser etching process developed by researchers at the University of British Columbia in preparation for the 2010 Olympics. By mimicking the nanostructure on the surface of lotus leaves, known for their efficient water-repellent quality, UBC professor Savvas Hatzikiriakos and his team laser-treated stainless steel blades to create super hydrophobic ice skates with a reported 25 percent reduced friction coefficient over conventional skates.

Beyond the hockey rink, super-hydrophobic materials could have multiple applications, Guo said, including rust prevention and anti-icing for airplane wings as well as rain collection and sanitation in water-scarce locations.

“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,” he said. “A second application could be creating latrines that are cleaner and healthier to use.”

In addition to super hydrophobisity, the team’s process also significantly increases a metal’s ability to absorb light and heat, Guo said, opening the door to rust-proof, self-cleaning and highly efficient solar absorbers.

While the process shows potential, it’s also time consuming. Patterning a 1 x 1 inch metal patch, Guo says, currently takes an hour. The next step, the researchers say, will be to increase the speed of the patterning process and study how to apply it to other materials such as semiconductors or dielectrics, to possibly develop water-repellent electronics.
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