Non-contact friction at miscroscopic scale

Friction is a daily phenomena and everybody has been used to it and learned how to use it by instinct. And the macroscopic law is usually stunningly simple and universal and relative motion is a necessary condition. On the other hand, how it comes about microscopically has been one among the mootest topics that arouse curiousities. Here is a review on a recent work in this field:

To unravel this tangle of vdW, electrostatic and phononic friction, Kisiel et al. incorporated a second experimental approach that aims to distinguish between phononic and electronic dissipation. In both vdW and electrostatic friction, dissipation ultimately takes place through forced motion of charge in a resistive medium. To separate this effect from phononic dissipation, one can vary the electrical resistivity of one of the two mating materials. This can be done most elegantly in a temperature-dependent experiment in which resistivity is switched on and off by going through a superconductivity transition. An experiment of this type was performed in 1998 by Dayo and colleagues⁵, who used a quartz microbalance to observe increased slip times of nitrogen adhered to a metal surface. Their work triggered significant debate, which continues to this day, because the observed behaviour did not show the predicted temperature dependence around the critical temperature.

In their experiments, Kisiel et al. used a nanoscale cantilevered tip vibrating in close proximity to the surface of a conductor. An atomic force microscope allowed both precise positioning and friction measurements. By positioning a silicon tip close to a niobium surface at temperatures above niobium's superconductivity transition temperature, T_c, they measured friction coefficients down to ~10⁻¹² kg s⁻¹. Moreover, they collected evidence of an electromagnetic origin of friction by verifying the dependence on the distance and applied voltage as predicted by Volokitin and Persson². Going through T_c to lower temperatures, the friction dropped to one third of the initial value. Below T_c, they argue, electronic dissipation is excluded and phononic interaction should govern the friction. Again, the dependence on the distance and applied voltage fitted the prediction, and even the temperature dependence they found is somewhat more gradual than that in the experiments of Dayo et al. [http://www.nature.com/nmat/journal/v10/n2/full/nmat2947.html]