Magazine article Science News

Model May Expose How Friction Lets Loose

Magazine article Science News

Model May Expose How Friction Lets Loose

Article excerpt

Friction is at play wherever surfaces meet, and it always begins with atoms. Atomic theories of the phenomenon usually focus on interatomic bonds, shared vibrations, and other surface-to-surface interactions as friction's ultimate source.

Now, two Texas-based physicists have modeled surface slippage--friction's retreat--as bands of atoms in the top surface momentarily leaping up from the underlying surface. Millions of such ripples propagate simultaneously along the interface when, for instance, a book slides on a table, they say.

Scientists have long known that friction on the scale of books and tables obeys simple laws. As French physicists Guillaume Amontons and Charles-Augustin de Coulomb established in the 17th and 18th centuries, the sideways force needed to overcome the friction between surfaces is proportional to the forces, such as weight, pressing the surfaces together. Surprisingly, the size of the friction-defeating force is independent of the area of the surfaces in contact.

For years, physicists have tried to explain this large-scale behavior in terms of atomic-scale events. They've had some success by portraying surfaces as jagged on an atomic scale. That way, very little material actually touches. However, scientists still struggle to explain why protrusions from two surfaces would stick together at all.

There's incentive to find out. A better understanding of friction could improve scientists' grasp of countless phenomena, such as engine performance and tool wear. Moreover, friction is particularly vexing for developers of micromachines (SN: 7/22/00, p. 56).

In the new mathematical model, Eric Gerde and Michael P. Marder, both of the University of Texas at Austin, build upon the physics of how cracks form and propagate through solids. Think of a bump in a rug, says Marder. As people know from everyday experience, pushing such bumps along can move a big rug over a floor. …

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