Influence of alignment of crystalline confining surfaces on static forces and shear in a liquid crystal, 4′-n-pentyl-4-cyanobiphenyl

Static force-distance relations as well as linear and nonlinear viscoelastic responses to oscillatory shear were studied in 4′-n-pentyl-4-cyanobiphenyl (5CB) confined between two muscovite mica surfaces at 25 °C. The orientation of the crystallographic axes of the mica sheets was varied from close to perfect alignment to a twist angle of θ>80°, and the sliding direction was kept parallel to the γ optical axis of one mica sheet. The layering was unaffected by twist angle when the film thickness exceeded three molecular layers of 5CB in a planar orientation, but for two and three layers the adhesive minima in the oscillatory force-distance curve decreased in magnitude with increasing θ. In the linear viscoelastic response (obtained with shear deformations of <20% of the film thickness), an elastic response dominated at shear frequencies of 0.13-130 Hz and small θ, whereas a more liquidlike response appeared at low frequency for large θ. Apparent discrepancies between the shear responses of alkylcyanobiphenyl films obtained in other investigations were resolved: during shear deformations of large amplitude, either stick-slip or smooth sliding was observed, depending on the number of layers but not on the surface alignment. At the film thickness of two layers, we observed stick-slip above a maximum limiting strain of 0.5 (at the smallest θ), whereas at three layers, the sliding was continuous with shear thinning at strains larger than 0.7. The effective shear moduli and limiting shear stress decreased with increasing film thickness and misalignment. In contrast to the known friction behavior of muscovite mica in the absence of an intervening fluid layer, no local extrema were observed at θ = 30° and 60°, indicating that the shear response resulted from the structure of the film of anisotropic molecules and not directly from the surface crystal lattice.