Determination of nematic liquid crystal material parameters by use of wedge cells

摘要

Three wedge cells of the wedge angle of order of few milliradians were used to measure the phase shift between ordinary and extraordinary ray of transmitted light, for the stationary states induced by magnetic or electric field. A nematic liquid crystal PCB, filling the cell, was of the planar alignment enforced by the previous treatment of the flat boundary plates. Two polyimides, MP9 and PM2, and clean glass were used as aligning substances. A system of interference fringes appeared in each cell placed in normally incident light between analyser and polariser crossed. In the neighbourhood of each fringe a cell was considered as a flat-parallel one and hence it was equivalent to a system of flat cells of different precisely determined thickness. The dependence of the phase shift on applied external fields was measured for twenty-six interference fringes. The nematic liquid crystal material parameters (the splay and bend elastic constants, the anisotropy of diamagnetic susceptibility and the boundary tilt angle as a function of torque) were determined by comparing the calculated characteristics to the measured ones i.e. by solving the inverse problem simultaneously for all measured characteristics as input data. Each of studied cells represented distinct case of anchoring: strong, medium or weak.