Dual alignment of liquid crystals under non-equilibrium conditions

Abstract

Abstract During the macroscopic reorientation of several nematic liquid crystal systems in a magnetic field, an unusual phenomenon, namely the presence of preferred macroscopic alignment at two different directions, was observed. This phenomenon is similar to the visual observation of pattern formation in liquid crystals under non-equilibrium conditions, except that our study was made at the molecular level. The technique used was C-13 NMR spectroscopy combined with rapidly decelerated sample spinning. The deceleration subjects the liquid crystal to a rapid change in the aligning torque. When the deceleration rate is slow, the alignment of the liquid crystals is predicted by their anisotropy of the magnetic susceptibility (Δχ). When the deceleration rate is sufficiently high, the dual alignment is observed for some of the liquid crystal systems studied. In these systems, part of the liquid crystal molecules remain with the director in the initial alignment, but the rest rapidly realign to reach a surface almost perpendicular to the axis of initial alignment, similar to the ‘opening’ of an umbrella. The ratio of the molecules with the two kinds of preferred alignment is determined by the non-equilibrium dynamics as well as Δχ of the liquid crystal sample. Several liquid crystal systems were investigated: 4-n-pentyl-4′-cyanobiphenyl (5CB; Δχ>0), 4′-cyanophenyl-l-(trans-4-n-pentylcyclohexane (PCH5; Δχ>0 but smaller than 5CB), and l-(trans-4-n-pentylcyclohexyl)-4′-cyanocyclohexane (CCH5; small Δχ<0). The dual alignment is present in 5CB and CCH5, and it is more pronounced in PCH5. We also prepared several mixtures of 5CB and l-(trans-4-n-ethylcyclohexyl)-4′-cyanocyclohexane (CCH2; small Δχ<0), the Δχ of which varies with the composition. The change in alignment became more pronounced as the magnitude of Δχ approached zero. An explanation for these results is suggested.