  Compared to nanoscale friction of translational motion, the mechanisms of
rotational friction have received less attention. Such motion becomes an
important issue for the miniaturization of mechanical machineries which often
involve rotating gears. In this study, molecular dynamics simulations are
performed to explore rotational friction for solid-state gears rotating on top
of different substrates. In each case, viscous damping of the rotational motion
is observed and found to be induced by the pure van-der-Waals interaction
between gear and substrate. The influence of different gear sizes and various
substrate materials is investigated. Furthermore, the rigidities of the gear
and the substrate are found to give rise to different dissipation channels.
Finally, it is shown that the dominant contribution to the dissipation is
related to the excitation of low-frequency surface-phonons in the substrate.
