Nematic - isotropic phase transition in turbulent thermal convection

The nematic-isotropic transition of a liquid crystal (LC) at a temperature $T_{NI}$ is an example of a {\em soft} phase transition, where fluid properties, although discontinuous, change only very little and where the latent heat is small. Understanding thermal convection in the presence of such a phase change is relevant to convection in Earth's mantle where discontinuous changes of the crystalline structure occur. We report on turbulent Rayleigh-B\'enard convection of a nematic LC while it undergoes a transition from the nematic to the isotropic phase in a cylindrical convection cell with aspect ratio $\Gamma$ (height over diameter) of 0.50. The difference between the top- and the bottom-plate temperature $\Delta T =T_b-T_t$ was held constant, while the average temperature $T_{m} = (T_b+T_t) / 2$ was varied. There was a significant increase of heat transport when $T_{NI}$ was between $T_b$ and $T_t$. Measurements of the temperatures along the side wall as a function of $T_{m}$ showed several ranges with qualitatively different behavior of quantities such as the time-averaged side-wall temperature, temperature gradient, or temperature fluctuations. We interpret these different ranges in terms of processes in the thermal boundary layers close to the top and bottom plates.