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Science 14 July 2006:
Vol. 313. no. 5784, p. 144
DOI: 10.1126/science.313.5784.144c
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This Week in Science

Figure 1 As mechanical systems shrink in size, friction and wear must be treated differently than in macroscopic machines; there is less material to wear away before a device fails, and liquid lubricants tend to become viscous in confined spaces (see the Perspective by Carpick). Socoliuc et al. (p. 207) present a dynamic approach for reducing friction. They slide the sharp silicon tip of a friction force microscope over the surface of NaCl and KBr salt crystals while mechanical exciting the tip in the direction normal to the surface. When the frequency of oscillation matched a mechanical resonance of the tip in the normal direction (or half that value), the friction was sharply reduced; excitation of lateral resonances had no effect. The normal motion likely allows the tip to find regions of interaction where friction is still finite but stick-slip motion disappears. Park et al. (p. 186) found that charge accumulation or depletion modified the friction force between a silicon surface and a metal-coated probe tip of an atomic force microscope. When the sample was positively biased, the friction force increase for positively doped regions of the sample but stayed the same in negatively doped regions

CREDIT: PARK ET AL.




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