We experimentally study how the cholesteric pitch $P$ depends on the equilibrium pitch $P_0$ in planar liquid crystal (LC) cells with both strong and semistrong anchoring conditions. The cholesteric phase was induced by dissolution in the nematic LC of the right-handed chiral dopant 7-dehydrocholesterol (7-DHC, provitamin ${\text{D}}_3$) which transforms to left-handed tachysterol under the action of uv irradiation at the wavelength of 254 nm. By using the model of photoreaction kinetics we obtain the dependencies of isomer concentrations and, therefore, of the equilibrium pitch on the uv irradiation dose. The cholesteric pitch was measured as a function of irradiation time using the polarimetry method. In this method, the pitch is estimated from the experimental data on the irradiation time dependence of the ellipticity of light transmitted through the LC cells. It is found that the resulting dependence of the twist parameter $2D/P$ ($D$ is the cell thickness) on the free twisting number parameter $2D/P_0$ shows jumplike behavior and agrees well with the known theoretical results for the anchoring potential of Rapini-Papoular form.

• Received 16 October 2013

DOI:https://doi.org/10.1103/PhysRevE.89.012503