Scuff across the floor, and you'll excite tiny sound waves that quickly whimper away as heat. Those waves cause most friction, some physicists say, but in certain cases, the resistance of small electric currents--generated when two objects rub against each other--may also slow things down. First evidence for this electrical friction appears in today's Physical Review Letters. Physicists have found that solid nitrogen on a lead surface loses half its normal friction when the lead loses its electrical resistance.
To find an example of electrically generated friction, physicist Jacqueline Krim and two students at Northeastern University in Boston performed "the simplest experiment we could think of," Krim says. They laid a thin layer of solid nitrogen, cooled to 85 kelvins, onto a small quartz crystal coated with lead. Atoms in the crystal vibrate at a particular frequency when zapped with an electric current. If the nitrogen sticks to the lead, then the quartz crystal labors under the weight and vibrates at a lower frequency, so that frequency is a measure of the friction.
When the apparatus was cooled below 7.2 kelvins, the temperature at which lead becomes a superconductor--in other words, when it loses all electrical resistance--any electrical friction should disappear. In Krim's experiment, the frequency of the vibrating crystal abruptly dropped by half, suggesting that half the friction between the lead and nitrogen was due to electrical currents.
The dramatic effect has experts scratching their heads. "I don't know what to make of it," says Uzi Landman, a physicist at the Georgia Institute of Technology in Atlanta. It's weird, he says, that the vigorous vibrations of the quartz crystal don't provide enough energy to completely destroy the superconductivity and keep the friction high. "Whatever is going on, it's very exciting," says Landman, and will generate lots of heated debate.