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Metals Absorb Light, Repel Water after Laser Conditioning

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A new laser micropatterning technique turns normally shiny metals black and makes them so hydrophobic that water droplets bounce off.

University of Rochester researchers carved microgrooves into sheets of platinum, titanium and brass using an 800-nm laser firing 65-fs pulses. Nanostructures spontaneously form on top of the grooves, giving the metals high optical absorption properties, as well as superhydrophobicity.

A water droplet slides off a piece of nanotextured metal.
A water droplet slides off a piece of nanotextured metal. Courtesy of J. Adam Fenster/University of Rochester.

"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 professor Dr. Chunlei Guo.

Most commercial hydrophobic and high-optical-absorption materials rely on chemical coatings that can degrade and peel off over time, Guo said. But laser-induced nano- and microstructures should not deteriorate, he said.

"Many people think of Teflon as a hydrophobic surface, but if you want to get rid of water from a Teflon surface, you will have to tilt the surface to nearly 70 degrees before the water can slide off," Guo said. "Our surface has a much stronger hydrophobicity and requires only a couple of degrees of tilt for water to slide off."

Guo and colleagues previously used laser surface conditioning to create superhydrophilic silicon, which absorbs water.

Detailed hierarchical structures on platinum.
A femtosecond laser was used to create detailed hierarchical structures on a platinum surface, as shown in this scanning electron microscope image. Courtesy of the Guo Lab/University of Rochester. 

The new batch of superhydrophobic surfaces could enable devices that resist rust and shed ice, dust and other contaminants. The principle could even be applied to make rain collectors and self-cleaning latrines in area where water is scarce.

Meanwhile, the patterned metals’ light-absorbing properties could enhance sensors as much as solar cells.

None of this will be possible, however, until the technology can be scaled up. It currently takes an hour to pattern a 1 × 1-in. metal sample.

Funding came from the Bill & Melinda Gates Foundation and the U.S. Air Force Office of Scientific Research.

The research was published in the Journal of Applied Physics (doi: 10.1063/1.4905616 [open access]).

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Related: Lasers Produce Bio-Inspired Surface Functions

Photonics Spectra
Apr 2015
Research & TechnologyAmericasNew YorkUniversity of RochesterChunlei Guolasersmaterials processingsuperhydrophobicityhydrophobicityTech Pulsefemtosecond lasers

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