Thermodynamic and transport anomalies near isotropic-nematic phase transition

TL;DR

This study uses computer simulations to analyze thermodynamic and transport anomalies near the isotropic-nematic phase transition in liquid crystals, revealing non-monotonic behaviors linked to pseudo-nematic domains.

Contribution

It provides a theoretical explanation for anomalies near the I-N transition by connecting them to pseudo-nematic domains and translation-rotation coupling at the molecular level.

Findings

Non-monotonic variation of specific heat, sound attenuation, and thermal diffusivity near I-N transition

Cusp-like behavior of sound attenuation coefficient

Anomalies attributed to formation and melting of pseudo-nematic domains

Abstract

A theoretical study of the variation of thermodynamic and transport properties of calamitic liquid crystals across the isotropic-nematic phase transition is carried out by calculating the {\it wavenumber (k) and time (t)} dependent intermediate scattering function of the liquid, via computer simulations of model nematogens. The objective is to understand the experimentally observed anomalies and sharp variation in many thermodynamic and transport properties, namely specific heat $C$, sound attenuation coefficient $\Gamma$, thermal diffusivity $D_T$ and sound velocity $c_s$ are as the I-N transition is approached from the isotropic side. The small wavelength limit of the calculated intermediate scattering function $F(k,t)$ is used to obtain the ratio of specific heats $\gamma$ and other properties mentioned above. We find that all of them show non-monotonic variations near the I-N transition, with $\Gamma$ showing a cusp-like behavior. We suggest that the observed anomalous features are a direct consequence of the existence of pseudo-nematic domains in the system near the phase boundary and the melting and formation of such domains give rise to sound attenuation and also to the observed specific heat anomaly. A theoretical description of these anomalies should invoke translation-rotation coupling at molecular level. While the heterogeneous dynamics observed here bear resemblance to that in deeply supercooled liquids near glass transition, the thermodynamic anomalies articulated here are largely absent in supercooled liquids.