Surface anchoring and temperature variations of the pitch in thin cholesteric layers

The temperature variations of the cholesteric pitch in thin planar layers o f cholesterics and their dependence on the surface anchoring force are inve stigated theoretically. It is shown that the temperature variations of the pitch in a layer are of a universal character. This is manifested in the fa ct that they depend not separately on the parameters of the sample but only on one dimensionless parameter S-d = K-22/dW, where K-22 is the torsional modulus in the Frank elastic energy, W is the height of the surface-anchori ng potential, and d is the thickness of the layer. The investigation is per formed the parameter S-d in a range where the change per unit number of cho lesteric half-turns within the thickness of the layer accompanying a change in the temperature is due to the slipping of the director on the surface o f the layer through the potential barrier for surface anchoring. The critic al values of the parameter S-d (which are most easily attained experimental ly by varying the thickness of the layer), determining the region of applic ability of the approach employed, are presented. The temperature variations of the free energy of the layer and the pitch of the cholesteric helix in the layer as well as the temperature hysteresis in the variations of the pi tch with increasing and decreasing temperature are investigated for the cor responding values of S-d . Numerical calculations of the quantities mention ed above are performed using the Rapini anchoring potential. (C) 2000 MAIK "Nauka/ Interperiodica".