Apr. 1, 1998 The grip of ice on airplane wings, power lines, auto windshields and ships at sea has serious and costly implications for humans. A Dartmouth physicist who has taken a molecular approach to the problem has discovered that applying a small electric voltage across an ice-metal interface can break the bond between ice and metal surfaces.
Thayer School of Engineering Professor Victor Petrenko, who is presenting his research at the meeting of the American Chemical Society in Dallas this week, is already at work adapting his finding to the de-icing of airplane wings. "It may be possible to prevent or significantly reduce icing on the wings of an airplane using a battery no bigger than the one in your car," says Petrenko.
"This is a wonderful example of how basic research on the properties of surfaces can quickly be developed into a useful application," said Jorn Larsen-Basse, National Science Foundation program director. "Surface-to-surface interactions are enormously important in manufacturing, machinery and, in this case, airplane wings or your car's windshield."
Petrenko became interested in the physical properties of ice through his work in Russia on semiconductors, a class of solids named for their capacity to conduct electrical currents. A semiconductor -- as the name implies -- falls halfway between a good metal and an insulator as a conductor. Semiconductors such as silicon and gallium arsenide are the microelectronic basis of a wide range of modern technology.
Several years ago, Petrenko became intrigued at similarities in the physical properties of semiconductors and ice. In work supported by grants from the Army and the National Science Foundation, he set out to understand why molecules of ice stick like burrs to most any surface. "In a few years I started to understand something about the nature of this interaction, and to split it into three parts. A simple electrical interaction was the largest part," he says.
Technically, Petrenko says, ice is a semiconductor -- included in a small class of substances in which protons, rather than electrons, carry an electrical current. This is important because of what happens when molecules of water become molecules with regard to their electrical charge. At the surface, however, molecules tend to line up in the same direction: primarily oriented with their protons facing out, or primarily with their protons facing inward, buried in the ice.
The reason for this is unclear, but the effect is significant: a high density of electrical charges -- either positive or negative, depending on the material -- on an ice surface.
When an electrically-charged surface comes into contact with any other surface, the charged surface induces an opposite charge in the facing surface and, because opposites attract, the two surfaces are drawn together. "This simple attraction accounts for most of ice adhesion," says Petrenko.
Breaking the bond between ice and metal, he reasoned, might be as simple as neutralizing the surface charge with an equal amount of its opposite.
In a walk-in freezer of a room in one of Dartmouth's Ice Research Laboratories, Petrenko tested the theory, using a sheet of ice, a globule of mercury -- which stays liquid until temperatures dip below minus-40 degrees Farenheit -- and a small battery with two wires attached. He touched one wire to the ice, the other to the mercury and the mercury drew itself up and away from the ice: the current had loosened its grip. Petrenko repeated the experiments using steel and other solid metals. In each case, the electricity caused the ice to lose adhesion.
The effect could also be reversed, causing a surface to stick more firmly to the ice. "It is remarkable that by changing the polarity and magnitude of the voltage between ice and metal structures one can increase the ice adhesion and also ice friction," says Petrenko.
And why would anyone want to increase friction on ice: "Think about your auto," he says. "You might find it useful to have more friction in your tires on an icy day." Those tires, too, are on the drawing board.
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