Calorimetric study of the nematic to smectic-\textit{A} and smectic-\textit{A} to smectic-\textit{C} phase transitions in liquid-crystal+aerosil dispersions

Abstract

A high-resolution calorimetric study has been carried out on nano-colloidal dispersions of aerosils in the liquid crystal 4-\textit{n}-pentylphenylthiol-4'-\textit{n}-octyloxybenzoate ($\bar{8}$S5) as a function of aerosil concentration and temperature spanning the smectic-\textit{C} to nematic phases. Over this temperature range, this liquid crystal possesses two continuous XY phase transitions: a fluctuation dominated nematic to smectic-\textit{A} transition with $\alpha \approx \alpha_{XY} = -0.013$ and a mean-field smectic-\textit{A} to smectic-\textit{C} transition. The effective critical character of the \textit{N}-Sm\textit{A} transition remains unchanged over the entire range of introduced quenched random disorder while the peak height and enthalpy can be well described by considering a cut-off length scale to the quasi-critical fluctuations. The robust nature of the \textit{N}-Sm\textit{A} transition in this system contrasts with cyanobiphenyl-aerosil systems and may be due to the mesogens being non-polar and having a long nematic range. The character of the Sm\textit{A}-Sm\textit{C} transition changes gradually with increasing disorder but remains mean-field-like. The heat capacity maximum at the Sm\textit{A}-Sm\textit{C} transition scales as $\rho_S^{-0.5}$ with an apparent evolution from tricritical to a simple mean-field step behavior. These results may be generally understood as a stiffening of the liquid crystal (both the nematic elasticity as well as the smectic layer compression modulus $B$) with silica density.