A team of physicists at the University of Rochester has used lasers to make metal surfaces extremely water repellent, or super-hydrophobic.
A water droplet bouncing off a laser-treated platinum surface. Image credit: J. Adam Fenster / University of Rochester.
Super-hydrophobic materials are desirable for a number of applications such as rust prevention, anti-icing, or even in sanitation uses. However, most current hydrophobic materials rely on chemical coatings.
Prof Chunlei Guo and Dr Anatoliy Vorobyev of the University of Rochester’s Institute of Optics have developed a new laser-patterning technique that creates an intricate pattern of micro- and nanoscale structures to give the metals water repellent properties.
“One of the big advantages of the 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 is these patterns that make the metals repel water,” said Prof Guo, who is a co-author of the paper published in the Journal of Applied Physics.
“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.”
The materials the scientists have created are much more slippery than Teflon (polytetrafluoroethylene).
Unlike the team’s laser-treated metals, the Teflon kitchen tools are not super-hydrophobic. The difference is that to make water to 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 the team’s metals by tilting them less than 5 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, the scientists 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.
“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 used in practice.”
The scientists are also looking into ways of applying the technique to other, non-metal materials.
They use extremely powerful, but ultra-short, laser pulses to change the surface of the metals. A femtosecond laser pulse lasts on the order of a quadrillionth of a second but reaches a peak power equivalent to that of the entire power grid of North America during its short burst.
The researchers are 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 – solar absorbers that don’t rust and do not need much cleaning.
The 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.
Prof Guo and Dr Vorobyev are now planning on focusing on increasing the speed of patterning the surfaces with the laser, as well as studying how to expand this technique to other materials such as semiconductors or dielectrics, opening up the possibility of water repellent electronics.
_____
A.Y. Vorobyev & Chunlei Guo. 2015. Multifunctional surfaces produced by femtosecond laser pulses. J. Appl. Phys. 117, 033103; doi: 10.1063/1.4905616
