Friction at a Distance

Friction at a distance, the friction between close objects that aren't in contact, is poorly understood. Seppe Kuehn and his colleagues at Cornell have set out to change this.

First, what does contact mean? Kuehn (607-254-4685, sk288@cornell.edu) suggests that when two objects are less than about 1 nanometer apart they are said to be in contact. One can think of contact friction as being a sort of micro-velcro process -- atomic "hills" in one surface scrape past atomic "valleys" from the other surface. To observe non-contact friction, the friction between two surfaces separated by more than 1 nanometer, the Cornell researchers use a tiny single-crystal microcantilever less than a millimeter long and only a few thousands of atoms thick.

Brought vertically downwards toward a surface, and set in motion, the cantilever will slow down in proportion to the friction it feels from the surface beneath. Surprisingly, the friction force between the cantilever and sample depends on the chemistry of the sample.

By studying this dependence of non-contact friction on the chemistry of the sample the Cornell scientists have made the first direct, mechanical detection of non-contact friction arising from the weak electric fields caused by motions of molecules in the samples. The samples included various polymer materials.

This work is motivated by recent efforts towards single-molecule MRI which require the detection of very small forces -- efforts that have been hindered by non-contact friction.

Kuehn, Loring, and Marohn, Physical Review Letters, 21 April 2006
Contact Seppe Kuehn, Cornell University, 607-254-4685, sk288@cornell.edu