"It is one of the dirty little secrets of physics," writes David Kessler of Bar-Ilan University today in Nature, "that while we physicists can tell you a lot about quarks, quasars and other exotica, there is still no universally accepted explanation of the basic laws of friction." Indeed, one particular conundrum involves the amount of force required to overcome friction and slide a solid object across some surface. As counterintuitive as it may seem, Coulomb's law of friction states that this force varies with the compressive force pushing the object and surface together¿and not with the amount of contact area between the two.
Traditionally, physicists have explained the puzzle as follows: because no two surfaces are in reality flat¿they are rough on the atomic scale¿the amount of contact between them is much smaller than it appears. Increasing the pressure compressing the surfaces together therefore also increases the contact area. But a letter to Nature from Eric Gerde and M. Marder of the University of Texas at Austin now offers an entirely new explanation, on which Kessler provides commentary. The two mathematicians attribute friction's independence from contact area to microscopic self-healing cracks¿or cracks that open up and then reseal themselves.
The simplest way to grasp the idea is to imagine sliding a rug across the floor. Dragging the rug from one end requires a certain amount of effort. The task becomes much easier, though, if you create a bump in the rug¿in essence, a self-healing crack between the rug and the floor¿and push that bump through the rug from one end to the other until the entire rug has advanced. "We show that when [self-healing cracks] are present at the atomic scale, they result in solids that slip in accord with Coulomb's law of friction," Gerde and Marder write. "We expect that this mechanism for friction will be found to operate at many length scales." Further experiments should soon test their ideas.