Small load against black ice
The frosty layer of ice on aircraft wings, power cables, windshields of cars and ships at sea causes serious problems and high costs. A physicist has discovered that a small electrical potential at the interface between ice and metal can weaken the bond between the two materials. Victor Petrenko of the Thayer School of Engineering previously presented his findings at the American Chemical Society meeting in March 1998 and is now busy applying them to an aircraft de-icing system. "It may be possible to prevent or at least reduce wing icing with a battery no larger than the one in your car," he believes.
Petrenko became interested in the physical properties of ice while working with semiconductors in Russia. As the name suggests, semiconductors behave half like a real metal and half like an insulator. They form the basis of modern technology. A few years ago, Petrenko became fascinated by the similarities in the properties of semiconductors and ice. Technically, he explains, ice is a semiconductor in which protons, rather than electrons, carry the charge. This difference is important because in water or ice, the molecules align themselves with their partially charged ends. On the surface, however, they tend to all orient themselves in the same direction. The protons either point primarily outwards or inwards. The reason for this behavior is still unknown. However, the result is a high density of electrical surface charge, which can be positive or negative.
When an electrically charged surface comes into contact with another surface, it induces an opposite charge in the other surface. And since opposites attract, the two fabrics cling together. "This simple attraction is largely responsible for ice binding," says Petrenko.
He imagined that the attraction between metal and ice could be broken simply by neutralizing the surface charge. He tested his theory experimentally in a cooling chamber: he placed a drop of mercury, which remains liquid even at temperatures just below the freezing point of water, on a sheet of ice. When he connected a small battery with one terminal to the ice and the other terminal to the mercury, the metal on the ice contracted to minimize the contact area. Also in experiments with other metals, the adhesive force decreased when the voltage was applied.
The effect could also be reversed when polarity was changed. But why would anyone want more grip on the ice? "Just think about your car," Petrenko replies."You might find it quite useful on a frosty day if your tires have more friction." And of course he already has this project in the works.
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