Does rotational melting make molecular crystal surfaces more slippery?

Area of Science:

• Materials Science
• Tribology
• Condensed Matter Physics

Background:

• C60 fullerite surfaces exhibit a phase transition with freely rotating molecules above T = Tr.
• These rotating molecules may act as nanobearings, influencing sliding friction.
• Understanding friction and adhesion changes across this transition is crucial for nanoscale applications.

Purpose of the Study:

• To investigate the frictional and adhesion changes of a sliding C60 flake on C60 fullerite.
• To explore the role of molecular orientation and the phase transition on tribological properties.
• To determine the impact of aligned vs. misaligned flake orientations on friction and adhesion.

Main Methods:

• Extensive molecular dynamics simulations were employed.
• The study focused on a sliding C60 flake interacting with a C60 fullerite surface.
• Frictional and adhesion forces were analyzed across the bulk rotational phase transition temperature (Tr).

Main Results:

• A modest 20% drop in friction was observed at a fixed flake orientation across the transition.
• Adhesion and friction decreased by a factor of approximately 2 when the flake's angular alignment with the surface lattice was broken.
• An entropy-driven switch between aligned and misaligned states during pull-off at Tr was suggested.

Conclusions:

• The study reveals significant changes in friction and adhesion of C60 fullerite surfaces at the rotational phase transition.
• Molecular misalignment, driven by entropy, plays a key role in reducing friction and adhesion.
• Findings are relevant for C60-based nanobearings, sliding tips, and understanding friction in similar systems like Kr islands.