"The gunk in the middle"

Exactly 300 years after Guillaume Amontons produced the classic laws of friction, physicists at The Johns Hopkins University have accounted for the notable endurance of Amontons' equations by identifying the molecular origins of static friction.

It's that stuff in the middle.

"Typically, when someone measures friction, what they will report is the ratio of force to the load, which is the coefficient of friction," said Mark Robbins, professor of physics at Johns Hopkins. "But it's never been understood at the molecular level how we get this linear relationship, or why it should hold for a wide array of materials and geometries. Our work now shows that the gunk in the middle--typically hydrocarbon molecules that are almost always present between two surfaces--provides an explanation."

The findings are reported this week in the June 4 issue of the journal "Science" by Robbins and his colleagues, Gang He and Martin Müser.

Amontons' laws, relied upon extensively and routinely by engineers for three centuries, state that the frictional force needed to slide one body over another is proportional to the load that presses them together and is also independent of the areas of the surfaces. Scientists have argued, however, about the relative importance of surface roughness, chemistry and other factors that might contribute to friction.

Simple theories that leave out the "stuff in the middle" predict that static friction, the force needed to initiate sliding, should vanish between almost any pair of solids. In reality, if that were true, every piece of furniture in a room would slide to one corner and buildings would collapse. The data compiled by Robbins and his colleagues show that hydrocarbon molecules that adsorb on any surface exposed to air resolve this problem. The molecules actually rearrange to lock contacting surfaces together and produ
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Contact: Gary Dorsey
gdd@jhu.edu
410-516-7160
Johns Hopkins University
3-Jun-1999