Abstract
The dependence of characteristics of the coupling between nematics and substrates on the planar cell thickness was suggested in the literature. The aim of this study is verification of this hypothesis by exploiting well controlled experimental conditions and stable computational algorithms for finding the characteristics of the nematics-substrate coupling together with the anisotropy of diamagnetic susceptibility and the splay and bend elastic constants via solving inverse problems. Wedge cells (of the wedge angle of the order of few milliradians) were used for studying Fréedericksz' transition of splay-bend type. The phase shift between ordinary and extraordinary rays of light normally incident on cell boundary was used as a physical quantity monitoring the state of the director field inside a cell in selected zones, equivalent to flat-parallel cells of different thicknesses. The optical response was measured as a function of voltage or magnetic field applied to a cell. Experiments were performed with several cells, filled with the 4′-pentyl-4-cyanobiphenyl nematic liquid crystal (5CB), of cover coatings made of different polyimides producing strong anchoring. The magnitudes of the anisotropy of diamagnetic susceptibility, the splay and bend elastic constants and the anchoring angle were found through fitting simulated cell optical transmittance characteristics to the measured one for each equivalent cell separately. The apparent dependence of these magnitudes on cell thickness was observed when the fitting was performed to reach the minimal fit error. This effect is interpreted as misleading and corrected by considering the fitting with the same magnitudes of material constants to all measured transmittance characteristics, with a little greater fit errors. The independence of nematics constants and anchoring angle of cell thickness is finally concluded.
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