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Many recipes and procedures call for bringing water to a roiling boil—from making the perfect cup of tea to generating electric power. But the bubbles that denote the rapid transformation of water from a liquid to a vapor, otherwise known as steam, actually slow the process. The normal, microscopic imperfections—holes, gaps and voids—on the surfaces of everything from industrial boilers to pots and pans create pockets where air is trapped and liquid water can become steam. But the process in each void ends after a steam bubble develops and travels to the surface, because water subsequently fills the gap where it formed.
But researchers report in the engineering journal Small that water in pots coated with tiny copper rods—just 450 nanometers tall and 40 to 50 nanometers around (one nanometer is 40 millionths of an inch)—may speed the process by creating more air pockets and, so, more bubbles.
Mechanical engineer Nikhil Koratkar of the Rensselaer Polytechnic Institute in Troy, N.Y., and his colleagues discovered this by coating the bottom of a copper pot with a layer of copper nanorods. By depositing these rods at an oblique angle, the scientists created an uneven film with various gaps in it. These nanoscale imperfections triggered faster-forming, more furious bubbles because they provided more air-trapping pockets where liquid water could be transformed into a gas.
"The density of the bubbles you create was 30 times more when we had these rods," Koratkar says. (Translation: the coating produced 30 times more bubbles than an ordinary pot.)
The more bubbles, the more efficiently and quickly the water boils. Koratkar says the discovery paves the way for development of pots and pans in which water would heat up in a jiffy. "Depositing the nanorods across a five- by five-inch [12.7- by 12.7-centimeter] vessel is something we can do right now," he says. "If we can provide these features to the base of a vessel that you use at home, then the potential for saving energy is enormous."
He believes that cooling computer chips might be the first place to utilize the new technology, however, given that such chips already use copper interconnects and often overheat as they continue to become ever tinier.
"The techniques we use to deposit nanorods are compatible with what you use to make chips," Koratkar notes. "The smaller the application, the more likely it is for this to be cost-effective," noting that coating the entire base of an industrial boiler in nanorods using current technology would be too costly to make it worth the high price tag.
Another snag: before copper nanorod pots become common in kitchens, researchers must first make sure they are safe. After all, the reason the effect diminishes with each repeated boil could be because the nanorods are actually detaching from the bottom of the pot, leaching into the contents as they are exposed to the turbulent boiling water. "If these rods start coming off the surface," Koratkar says, "then we have to think of a way of attaching them more strongly," such as coatings that bind the nanorods together. "If you're using pans to cook food and you have copper nanorods that are coming off the surface, that's a major problem."
