Structural lubricity and fractures in liquid crystals

Structural superlubricity, one of the most important concepts in modern tribology, has attracted lots of interest in both fundamental research and practical applications. However, despite promising theoretical models, such as the Prandtl-Tomlinson and Frenkel-Kontorova models, and great success in structural lubricant experiments with two-dimensional materials and colloids, these models have not been directly tested. Here, with the cholesteric liquid crystals confined under the Surface Force Balance, we measure the surface torque during rotational friction, and the molecular rotation from the commensurate to incommensurate configuration, at the onset of structural lubricity. Furthermore, by changing the surface potential, the Aubry transition is confirmed. The results agree well with the description by a modified three-dimensional version of the above models and provide molecular evidence for rupture nucleation during static friction. Our study bridges the gap between theories and experiments, and reinforces the connection between friction and fracture.