Science of friction is a bit rough

Lab experiments show limitations of classical equations

It’s no wonder earthquakes are so difficult to predict. Even simple laboratory simulations of the friction breakdowns that send tectonic plates lurching into motion are maddeningly difficult to explain.

NOT SO CUT-AND-DRIED Plastic blocks sliding past each other have challenged the traditional view of friction, revealing earthquake-like rupture waves (shown in illustration above) that spread and break the contacts between blocks (red indicates the most contact, blue the least). © Science/AAAS

SLIP SLIDING AWAY Forces pushing downward and sideways (blue arrows) on a Plexiglas block can create uneven stresses (red arrows) along its surface and rupture (yellow circle) the frictional contacts that prevent the block from sliding. P. Huey/Science

By playing with plastic blocks that stick and slip much like rock, physicists are challenging centuries-old ideas about the nature of friction itself. Seemingly unimportant differences at small scales can have big consequences, an Israeli team reports in an upcoming Physical Review Letters.

“If you want to know how hard you have to push a specific object [to overcome friction], and you want to know to high precision, right now we don’t know what the answer is,” says Jay Fineberg, a physicist at the Hebrew University of Jerusalem.

The force needed to start an object sliding traditionally depends on two things: its weight (as well as any other downward forces) and a number called the coefficient of static friction. This number is typically thought to be dictated by the roughness of an object’s surface and the material it’s made of; rubber’s coefficient is larger than Teflon’s, for instance.

Fineberg and his colleagues put friction to the test by squeezing together pairs of Plexiglas blocks with tremendous forces meant to simulate the crush of colliding pieces of the Earth’s crust. One block is pushed sideways until it slips, while gauges alongside the blocks measure the stresses building up where the blocks touch. These stresses, the researchers found, tend to be unevenly distributed.

“Friction is a tricky thing to measure,” says Chris Marone, a geophysicist at Pennsylvania State University in University Park. “These guys are looking in much more detail than people have looked before.”

Different ways of pushing and pressing on the blocks can change both the pattern of stresses and the strength of the shove needed to get things moving. Tilt one block slightly — one or two hundredths of a degree — and the force needed to slide the other block can be larger or smaller than expected. In hundreds of experiments summarized in the new paper, the coefficient of static friction varied by as much as a factor of about two, depending on, for instance, the shape of the blocks or the distribution of the force pressing down.

“The results are a bit contrary to the intuition we have and stimulate us to look for a new theory,” says Stefano Zapperi, a theoretical physicist at Italy’s National Research Council Institute for Energetics and Interphases in Milan.

Such a theory must take into account the microscopic nature of friction — the tiny points of contact where two bumpy surfaces touch. In the new experiments, the physicists studied these surface interactions by shining laser light through the transparent blocks. The lasers revealed that connections between the two objects don’t all give out at the same time when a block starts to slip. Instead, a rupture forms in one spot and spreads outwards.

The speed at which a rupture moves seems to depend on how stresses build up in small areas. But Fineberg and his colleagues still can’t pinpoint the moment when a given pattern of stress will give way to generate one of these moving cracks.

“Can I predict an earthquake in the laboratory? Not yet,” says Fineberg. “We don’t know what the straw that breaks the camel’s back is.”

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