Measurements of zenithal anchoring energy of nematics at the planar interface in hybrid cells

An automated technique for the measurements of the electric field controlled phase retardation of nematic cells has been developed based on a new concept of "virtual devices" (software PhysLab). The latter include a generator of a sine voltage with continuously growing amplitude, a lock-in amplifier and a recorder. A set of equations describing a torque balance for the nematic director was solved numerically for a semiinfinite liquid crystal with allowance made for the finite dielectric anisotropy and different elastic moduli. Two even terms (of 2nd and 4th orders) have been hold in the expansion of the anchoring energy over surface angle. In experiment, an ellipsometry technique has been used for determination of the optical retardation. Experiments were carried out on hybrid nematic cells of various thickness with different treatment of the planar orienting interface providing different anchoring strength: strong (rubbed polyimide) and rather weak (thin SiO layers evaporated at 75 degrees angle).