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Science 26 August 2005:
Vol. 309. no. 5739, p. 1297
DOI: 10.1126/science.309.5739.1297p
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This Week in Science

One explanation for the variation in frictional forces along different directions on the same surface is that commensurability between the sliding surfaces can lead to intimate interlocking and high friction, whereas incommensurability should lead to lower friction. Park et al. (p. 1354) compared frictional forces on the twofold symmetric surface of an aluminum-nickel-cobalt quasicrystal. This surface is periodic in one direction but is aperiodic in the normal direction. They avoided the effects of adhesive forces by passivating their atomic force microscope tip with alkane thiols so that only hydrocarbon chains interacted with the surface. Friction along the aperiodic direction was eight times lower than in the periodic direction. They argue that dissipation forces, acting either through the formation of electron-hole pairs or though surface vibrations, are much different in the aperiodic direction for this surface.




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