Laser processing creates 'super-hydrophobic' metals

26 Jan 2015

Scientists at University of Rochester say properties promise better solar panels, sanitation and rust-free metals.

Scientists at the University of Rochester, NY, USA, have used lasers to transform metals into extremely water repellent (“super-hydrophobic”) materials without the need for temporary coatings. Super-hydrophobic materials are desirable applications such as rust prevention, anti-icing, or even in sanitation uses. However, as Rochester’s Chunlei Guo explains, most current hydrophobic materials rely on chemical coatings.

In a paper published in the Journal of Applied Physics, Guo and his colleague at the University’s Institute of Optics, Anatoliy Vorobyev, describe a laser-processing technique that creates an intricate pattern of micro- and nanoscale structures, giving the remarkable properties.

Permanently hydrophobic

Guo states that this technique can create multifunctional surfaces that are not only super-hydrophobic but also highly-absorbent optically. "One of the big advantages of this process is that the structures we create are intrinsically part of the metal's surface - so they won’t rub off. It is these patterns that make the metals repel water.

“The material is so strongly water-repellent, the water actually bounces off. Then it lands on the surface again, and bounces off again, then it will just roll off the surface,” said Guo, professor of optics in the University’s Hajim School of Engineering and Applied Sciences. "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 super-hydrophobic. The difference is that to make water roll off a Teflon-coated material, it needs to be tilted at nearly 70 degrees before the water begins to slide, whereas water slides of Guo’s metals at a tilt angle of less than five degrees.

As the water bounces off the super-hydrophobic surfaces, it also collects dust particles and takes them along for the ride. To test this 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 remains completely dry.

Funding from Bill Gates

Guo is excited by potential applications of super-hydrophobic materials in developing countries. It is this potential that has piqued the interest of the Bill and Melinda Gates Foundation, which has supported the work.

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

Latrines are a challenge to keep clean in places with little water. By incorporating super-hydrophobic materials, a latrine could remain clean without the need for water flushing.

But challenges still remain to be addressed before these applications can become a reality, Guo states. It currently takes an hour to pattern a 1 inch by 1 inch metal sample, and scaling up this process would be necessary before it can be deployed in developing countries. The researchers are also looking into ways of applying the technique to other, non-metal materials.

How they did it

In the study, the Journal of Applied Physics reports, the research team used an amplified Ti:sapphire laser system that generates 65fs pulses with a central wavelength of 800 nm and at a maximum pulse repetition rate of 1 kHz. The laser beam is focused onto the sample surface by a lens onto a sample mounted on a translation stage.

”The study samples are platinum, titanium, and brass. Each sample is textured with an array of parallel microgrooves covered by extensive nanostructures. The platinum sample is processed at laser fluence of 9.8 J/cm². The titanium sample is processed at laser fluence of 7.6 J/cm².

”Brass is processed at laser fluence of 3.9 J/cm². The orientation of microgrooves is controlled by the scan direction. A scanning electron microscope and a 3D laser-scanning microscope examine the surface structures. Superhydrophobic properties are studied by measuring both water contact angle and the surface tilt angle for water sliding.

”The self-cleaning properties are studied with real-life dust particles collected from a vacuum cleaner. For cleaning, the researchers use rolling and falling water drops. The rolling drops with nearly zero kinetic energy are produced by pipetting water drops near the sample surface, while the falling drops are produced by pipetting drops at a height of 3–8 cm above the sample surface."

Guo is keen to stress that this same technique can give rise to multifunctional metals. Metals are naturally excellent reflectors of light. That’s why they appear to have a shiny luster. Turning them black can therefore make them very efficient at absorbing light. The combination of light-absorbing properties with making metals water repellent could lead to more efficient solar absorbers – which would neither rust nor need much cleaning.

Guo and Vorobyev's work is described in the following University of Rochester video:

About the Author

Matthew Peach is a contributing editor to optics.org.