Ordering of the 7CB liquid crystal induced by nanoscale confinement and boundary lubrication

Abstract

In this study, the thermodynamic properties of nanoscale-thick 4-heptyl-4′-cyanobiphenyl (7CB) liquid crystals (LCs) confined in carbon nanotubes were investigated using differential scanning calorimetric methods. Both the isotropic-nematic phase transition temperature and the enthalpy increase decrease as the nanoscale confining size decreases, indicating the ordering effect of the 7CB molecules under nanoscale confinement. An average-order parameter <S> is calculated to quantitatively identify the ordering effect of LCs by utilising a simple expression of the Landau–de Gennes (LdG) free energy. The average-ordering parameter <S> of the 7CB LC can be 0.7 within a confinement thickness of ∼25 nm and above 0.5 within a film thickness of up to 100 nm. When lubricating the 7CB LC between a steel ball and chromium-coated glass disk at low speeds, a stable boundary layer with a thickness of 10 nm is formed that is independent of the entrainment speed. At higher speeds, the thickness of the 7CB lubrication film ranges from several tens of nanometres to 100 nm, resulting in a stable velocity-independent friction coefficient of approximately 0.03. The ordered microstructure of the 7CB molecules under confinement provides the underlay mechanism of its boundary lubrication property.