4 June 2009
Metal can pumps liquid uphill
Trees pull vast amounts of water from their roots up to their leaves hundreds of feet above the ground through capillary action. Along those lines, and taking nature one step better, there is a way a simple slab of metal can lift liquid using the same principle, but at a much faster speed than nature.
The metal 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.
Chunlei Guo uses the femtosecond laser (behind him) to create nanostructures in metal that can move liquid uphill.
“We’re able to change the surface structure of almost any piece of metal so that we can control how liquid responds to it,” said 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 researchers use an ultra-fast burst of laser light to change the surface of a metal, forming nanoscale and microscale pits, globules, and strands across the metal’s surface, said Guo. The laser, called a femtosecond laser, produces pulses lasting only a few quadrillionths of a second—a femtosecond is to a second what a second is to about 32 million years. During its brief burst, Guo’s laser unleashes as much power as the entire electric grid of North America does, all focused onto a spot the size of a needlepoint.
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, Guo said. Likewise, Guo’s nanostructures change the way molecules of a liquid interact with the molecules of the metal, allowing them to attract 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.
“Imagine a huge waterway system shrunk down onto a tiny chip, like the electronic circuit printed on a microprocessor, so we can perform chemical or biological work with a tiny bit of liquid,” Guo said. “Blood could precisely travel along a certain path to a sensor for disease diagnostics. With such a tiny system, a nurse wouldn’t need to draw a whole tube of blood for a test. A scratch on the skin might contain more than enough cells for a micro-analysis.”
Guo’s team has also created metal that reduces the attraction between water molecules and metal molecules, a phenomenon called hydrophobia. Since germs mostly consist of water, it is all but impossible for them to grow on a hydrophobic surface, Guo said.
Currently, to alter an area of metal the size of a quarter takes 30 minutes or more, but Guo and his assistant Anatoliy Vorobyev are working on refining the technique to make it faster. Fortunately, despite the incredible intensity involved, the femtosecond laser can get its power from a simple wall outlet, meaning when the process is refined, implementing it should be relatively simple.
For related information, go to www.isa.org/manufacturing_automation.