  We report, through experimental observations and computer simulations, that
atomic lattice interlocking can determine whether an object rolls or slides on
a surface. We have quantitatively manipulated carbon nanotubes (CNTs) on a
variety of substrates with an atomic force microscope (AFM) and observe rolling
to occur only on graphite. We measure the forces when the CNT is in-registry
with the graphite lattice, and observe rolling only in this lock-in state.
Atomistic computer simulations identify the energy barriers for sliding and
rolling, elucidate atomic-scale features of slip-roll motion, and explain the
details of the lateral force data in terms of the intrinsic faceting of
multiwall CNTs.
