Temperature pitch variations in planar cholesteric layers: The role of fluctuations and surface anchoring

The influence of thermodynamic fluctuations on temperature pitch variations in planar cholesteric samples with a finite surface anchoring energy is theoretically investigated in the framework of the continuum theory of liquid crystals. It is shown that taking fluctuations into account allows explanation of experimental observations, namely, the absence of a temperature pitch jump hysteresis in sufficiently thick samples and its existence in thin ones. A description of fluctuations, including two phenomenological parameters, is proposed. It allows us to predict temperature points at which the pitch jumps in the sample between two configurations, with the numbers of director half- turns differing by one, as a function of the anchoring energy, Frank elastic modulus, sample thickness, and temperature ( or fluctuation energy). It is shown that performing precise measurements of the pitch versus the temperature in well- controlled samples should allow determination of the phenomenological constants and then prediction the influence of fluctuations on pitch jump parameters in samples of an arbitrary thickness and/ or surface anchoring energy. The corresponding calculations are performed using the Rapini - Papoular anchoring potential. It is shown that the influence of fluctuations on the pitch variation is only negligible in sufficiently thin layers. It is also noted that the results obtained could be useful for investigating pitch jump dynamics in the future. (C) 2003 MAIK " Nauka/ Interperiodica".