Noncontact friction

One can think of contact friction as being a sort of micro-velcro process—atomic “hills” on a surface scrape past atomic “valleys” from another surface. To observe noncontact friction between two surfaces separated by more than 1 nm, Seppe Kuehn and his colleagues at Cornell University use a single-crystal microcantilever that is 0.25 mm long and only a few thousand atoms thick. Brought vertically downward toward a surface and set in motion like a pendulum, the cantilever will slow down because of the friction it feels from the surface beneath it. It turns out, surprisingly, that the noncontact friction force depends on the chemistry of the sample. By studying that chemical dependence in various polymer materials, the Cornell scientists directly detected friction due to the weak electric-field fluctuations from molecular motion in the samples. The work was motivated by recent efforts to achieve single-molecule magnetic resonance imaging, which requires the detection of very small forces and has been hindered by noncontact friction. (S. Kuehn, R. F. Loring, J. A. Marohn, Phys. Rev. Lett. 96 , 156103, 2006 http://dx.doi.org/10.1103/PhysRevLett.96.156103 .)