Determination of azimuthal anchoring energy in grating-aligned twisted nematic liquid-crystal layers

Abstract

In this work sinusoidal diffraction gratings with a range of pitches and amplitudes are used to align nematic liquid-crystal layers in a twisted homogeneous configuration. The grating profiles are accurately characterized using optical surface plasmon polariton spectroscopy, which then allows a calculation of the anchoring energy as predicted by the simple Berreman expression. The experimental Rapini–Papoular anchoring energy is also obtained by a measurement of the director twist away from the alignment direction (easy axis) at room temperature. A linear relationship is found between the two anchoring energies, except when it falls below 4×10−7 J m−2. Noticeably, the correlation between the two theories is not unity, if room-temperature elastic constants are used in the calculation. This apparent inconsistency is explained if the effect of surface memory on the system is considered. Indeed, if elastic constants, corresponding to a higher temperature at which surface memory effects are absent, are used in the Berreman expression, good agreement between the predicted and experimentally measured energies is found.