Does rotational melting make molecular crystal surfaces more slippery?

Abstract

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 C₆₀ flake on the surface of the prototype system C₆₀ 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 ∼2 as the flake breaks its perfect angular alignment with the C₆₀ 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 C₆₀-coated tip, where a remarkable factor ∼2 drop has been reported.