Thermal renormalization of the anchoring energy of nematic liquid crystals

Nonmonotonic behavior of the nematic tilt angle in a temperature-induced surface transition

We report on a nonmonotonic temperature behavior of the pretilt in cells containing polymers as alignment layers. The latter are very promising since they enable control of the anchoring of the liquid crystal. We show that such a behavior cannot be interpreted by the standard Landau or mean-field theory. We propose a generalization of the mean-field model, including the temperature dependence of the anchoring due to the polymer layer that enables a description of the nonlinear behavior. The agreement between the predictions of the model and the experimental data is good.

Phenomenological model for the optically induced easy direction

We present a phenomenological model to interprete the optically induced easy direction in a nematic cell in the slab approximation. One of the surfaces of the sample is supposed to give strong anchoring, whereas the other, covered with photosensible material, very weak anchoring. We assume that a surface nematic molecule is submitted to a potential connected with its interaction with the surface easy direction, with the surface nematic field, and with the optical induced anisotropy. The case in which the coupling with the nematic order in the bulk is important is considered too. A differential equation for the time evolution of the surface director is proposed, in which the viscous torque is balanced by the torque related to the surface fields. We show that our theoretical predictions are in agreement with experimental data on the optical induced surface orientation. The dependence of the anchoring energy strength on the irradiation time for dye-doped liquid crystals is also investigated.

Azimuthal and zenithal anchoring of nematic liquid crystals

Temperature dependence of azimuthal and zenithal anchoring energy coefficients of the nematic liquid crystal 4-n-pentyl-4(')-cyanobiphenyl on rubbed nylon is measured using dynamic light scattering. The method is based on observations of director fluctuations in a planarly aligned wedge cell, where the anchoring energy coefficients can be obtained without any external torques acting on the liquid crystal during the measurement. We found that both anchoring coefficients decrease steadily on approaching the nematic-isotropic transition. Moreover, in the whole temperature range of the nematic phase, the ratio between the zenithal and the azimuthal anchoring coefficients is almost equal to the ratio between the splay and the twist Frank elastic constants. The same result is obtained also for the nematic phase of 4-n-octyl-4(')-cyanobiphenyl. This indicates that the aligning nylon layer directly affects only the monomolecular layer at the surface whereas the observed anchoring is governed by the elastic properties of the alkyl-cyanobiphenyl.

Temperature-induced surface transition in nematic liquid crystals oriented by evaporated SiOx

Temperature induced surface transitions observed in nematic liquid crystals oriented by obliquely evaporated SiOx are analyzed. It is shown that the experimental data can be interpreted with a theory for the thermal renormalization of the anisotropic part of the surface energy based on the mean field approximation. By assuming that the nematic scalar order parameter is given by the Maier-Saupe theory, the fit is obtained with only one free parameter. The temperature behaviors of the polar and azimuthal angles are in agreement with the proposed model over the whole temperature range.