Does rotational melting make molecular crystal surfaces more slippery?

The surface of a crystal made of roughly spherical molecules exposes, above its bulk rotational phase transition at T = T-r, a carpet of freely rotating molecules, possibly functioning as " nanobearings" in sliding friction. We explored by extensive molecular dynamics simulations the frictional and adhesion changes experienced by a sliding C-60 flake on the surface of the prototype system C-60 fullerite. At fixed flake orientation both quantities exhibit only a modest frictional drop of order 20% across the transition. However, adhesion and friction drop by a factor of similar to 2 as the flake breaks its perfect angular alignment with the C-60 surface lattice suggesting an entropy-driven aligned-misaligned switch during pull-off at T-r. The results can be of relevance for sliding Kr islands, where very little frictional differences were observed at T-r, but also to the sliding of C-60-coated tip, where a remarkable factor similar to 2 drop has been reported