Photoacoustic Simultaneous Studies of Thermal Conductivity, Diffusivity and Heat Capacity Over the Smectic A-Nematic Phase Transition in Alkylcyanobiphenyls with Varying Nematic Range

Abstract

In thermotropic liquid crystals (L.C.) the thermal parameters may depend strongly on temperature in the vicinity of their phase transitions. An accurate study of their critical behavior thus requires a technique which introduces small temperature perturbations in the investigated sample. In the photoacoustic (PA) technique in the gas microphone configuration [1] an adequate signal to noise ratio may be achieved while introducing temperature oscillations in the sample of only a few mK. Also, the d.c. temperature gradients within the probed depth introduced by the a.c. laser source was also negligible. Finally the technique allows the simultaneous determination of the specific heat c, thermal conductivity k and diffusivity D. In this work the smectic A-nematic (A-N) transition in liquid crystal samples with different nematic temperature range was studied. The second order A-N transition in liquid crystals was placed in the same universality class as a superfluid, namely the three dimensional XY like transition [2]. Experimentally [3], observations carried out in alkylcyanobiphenyl compounds and mixtures having different nematic temperature range showed that the critical exponent α of the specific heat ranged between the tricritical value, 0.5 and −0.03, a value close to the XY model one (−0.007). This was interpreted as due to a crossover between tricritical and SY like critical behavior as the nematic range increased. Even in compounds whose value approached the one predicted by the XY model, however, the critical exponents γ for the susceptibility and v for the correlation length remained inconsistent with the XY model predictions [4].