June 4, 2009 Gravity can make it difficult to move liquid uphill but scientists at the University of Rochester have created a simple slab of metal that lifts liquid using the same wicking process that trees employ to pull vast amounts of water from their roots up to their leaves. The metal could be used to pump microscopic amounts of liquid around a medical diagnostic chip, cool a computer's processor or turn almost any simple metal into an anti-bacterial surface.
By using an ultra-fast burst of laser light, Chunlei Guo, associate professor of optics at the University of Rochester, and his assistant, Anatoliy Vorobyev, were able to form nanoscale and microscale pits, globules, and strands across the metal's surface. These 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 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 found in the metal. This wicking process is similar to the phenomenon that draws spilled milk into a paper towel.
Adding laser-etched channels into the metal further enhances control of the liquid and could be used on electronic circuits or to direct blood 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," says Guo. "A scratch on the skin might contain more than enough cells for a micro-analysis."
Altering an area of metal the size of a quarter takes 30 minutes or more, but Guo and Vorobyev are working on refining the technique to make it faster.
The laser used to alter the surface of the metal is called a femtosecond laser. It 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 the laser unleashes as much power as the entire electric grid of North America, all focused onto a spot the size of a needlepoint, Guo says. Yet, despite the incredible intensity involved, it can still be powered by a regular wall outlet.
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's all but impossible for them to grow on a hydrophobic surface, says Guo.
They have also developed a femtosecond laser processing technique that can create incandescent light bulbs that use half as much energy, yet produce the same amount of light. While in 2006, Guo's team used the femtosecond laser to create metal with nanostructures that reflected almost no light at all, and in 2008 the team was able to tune the creation of nanostructures to reflect certain wavelengths of light - in effect turning almost any metal into almost any color.
The research from Guo's team at the University of Rochester will be detailed in an upcoming issue of Applied Physics Letters.
Darren Quick
