Phys. Rev. Lett. 101, 125505 (2008)

Friction is a familiar force in everyday life, but its nanometre-scale details are obscure. This is because the fundamental mechanisms are subtle and sensitive to contamination, say André Schirmeisen of the University of Münster, Germany, and his colleagues.

They pushed islands of the element antimony across a graphite surface using the tip of an atomic-force microscope. Some of the particles encountered frictional resistance proportional to their area of contact with the surface; others slid almost friction-free.

The latter state, called superlubricity, has been argued to arise from a mismatch between the atomic-scale corrugations of two surfaces, which, in theory, should be the norm for solids. Schirmeisen and his team conclude that lubricity is undermined by impurities stuck at the interface.