Scientists at Bell Labs, the research and development arm of Lucent Technologies have discovered an new way to control tiny liquid droplets by applying electrical charges to specially engineered silicon surfaces that resemble blades of grass. The new technique of manipulating fluids has many potential applications, including thermal cooling of integrated circuits for powerful computers, novel photonic components for optical communications, and small, low-cost "lab-on-a-chip" sensor modules.
The breakthrough that made this possible was a technique that Bell Labs scientists developed for processing silicon surfaces, so that these surfaces resemble a lawn of evenly cut grass, with individual "blades" only nanometers in size.
This new capability to process silicon surfaces to produce "nanograss" lets liquids interact with surfaces in a novel way, thereby providing a way to precisely control their effects. In everyday experience, fluids tend to wet surfaces and stick to them. For example, a raindrop sticks to a car's windshield; when water is spilled, it splatters every which way. The individual blades of the nanograss are so small, however, that liquid droplets sit on top and can be easily manoeuvred.
"Physically, this technique reduces the surface area that the droplet feels, and reduces the interaction between the liquid and the substrate by a factor of a hundred to a thousand," said Tom Krupenkin, the Bell Labs scientist who led the research.
Krupenkin and his team coated the nanograss with a non-stick, water-repellent material, and when the droplets are put on the surface, they can move about without wetting it. By applying a small voltage, however, the team could tailor the behaviour of droplets, making them sink in and wet the surface as directed. The droplets also respond to a change in temperature, allowing for thermal cooling applications.
"Such behavior may be harnessed to cool computer chips," Krupenkin said. "A droplet could be sent to a hot spot on the chip, where it would sink in and absorb the heat, and then go on its way, avoiding the expense and inefficiency of applying a coolant or a heat sink to an entire chip."
Another application for this technique may be in optical networking. For example, moving a droplet of fluid into a nanograss surface can alter the physical properties of the transmitting medium through which light signals are sent, and this may lead to better methods for optical switching. Novel optical components, such as filters, could be created by moving the fluid into and out of nanograss areas, Krupenkin said.
Bell Labs and the New Jersey Nanotech Consortium are also exploring using the technique to create powerful, next-generation reserve micro batteries. Conventional batteries have electrochemical reactions proceeding at some level all the time, even when batteries are not being used. Over time, the batteries degrade. By using the Bell Labs technique to isolate the liquid electrolyte so that electrochemical reactions do not take place until power is actually needed, nanograss-based micro batteries may be ideal for long-term, higher capacity battery applications, especially where bursts of power are needed. Examples would be sensors out in the field that only need a lot of power when they detect something and need to transmit the information as a wireless signal.
Yet another application for the nanograss may be "lab-on-a-chip" devices. "Potentially, one can envision lab-on-the-chip devices that use thousands of different reagents, each deposited in a small spot at the bottom of the nanograss, thus providing novel devices for combinatorial chemistry, genetic analysis, and so on," Krupenkin said. "Some other possible applications where nanograss can be used may be for low-friction torpedoes, self-cleaning windshields, and faster boats where the fluid-repellent properties of the nanograss would be important."