  Molecular dynamics simulations of coaxial carbon nanotubes in relative
sliding motion reveal a striking enhancement of friction when phonons whose
group velocity is close to the sliding velocity of the nanotubes are resonantly
excited. The effect is analogous to the dramatic increase in air drag
experienced by aircraft flying close to the speed of sound, but differs in that
it can occur in multiple velocity ranges with varying magnitude, depending on
the atomic level structures of the nanotubes. The phenomenon is a general one
that may occur in other nanoscale mechanical systems.
