Rotational dynamics and friction in double-walled carbon nanotubes.

We report a study of the rotational dynamics in double-walled nanotubes using molecular dynamics simulations and a simple analytical model that reproduces the observations very well. We show that the dynamic friction is linear in the angular velocity for a wide range of values. The molecular dynamics simulations show that for large enough systems the relaxation time takes a constant value depending only on the interlayer spacing and temperature. Moreover, the friction force increases linearly with contact area and the relaxation time decreases with the temperature with a power law of exponent -1.53+/-0.04.