Scientists at the University of Rochester have created a simple slab of metal that lifts liquid the same way trees pull vast amounts of water from their roots to their leaves hundreds of feet above the ground. Although the principle is the same, the speed at which the metal performs this feat would make nature envious. The scientists use an ultra-fast burst of light from a femtosecond laser to change the surface of the metal so that it attracts, repels, and guides liquids.
The metal, revealed in an upcoming issue of Applied Physics Letters, may prove invaluable in pumping microscopic amounts of liquid around a medical diagnostic chip, cooling a computer's processor, or turning almost any simple metal into an anti-bacterial surface.
"We're able to change the surface structure of almost any piece of metal so that we can control how liquid responds to it," says Chunlei Guo, associate professor of optics at the University of Rochester. "We can even control the direction in which the liquid flows, or whether liquid flows at all."
The wicking process, which on Guo's metal moves at a quick one centimeter per second speed against gravity, is very similar to the phenomenon that pulls spilled milk into a paper towel or creates "tears of wine" in a wineglass—molecular attractions and evaporation combine to move a liquid against gravity, says Guo. Likewise, Guo's nanostructures change the way molecules of a liquid interact with the molecules of the metal, allowing them to become more or less attracted to each other, depending on Guo's settings. At a certain size, the metal nanostructures adhere more readily to the liquid's molecules than the liquid's molecules adhere to each other, causing the liquid to quickly spread out across the metal. Combined with the effects of evaporation as the liquid spreads, this molecular interaction creates the fast wicking effect in Guo's metals.
Adding laser-etched channels into the metal further enhances Guo's control of the liquid.
Guo is also announcing this month in Physical Review Letters a femtosecond laser processing technique that can create incandescent light bulbs that use half as much energy, yet produce the same amount of light.
This research funded by the U.S. Air Force Office of Scientific Research and the National Science Foundation.