# Landau–de Gennes Model for the Isotropic Phase of Nematogens: The Experimental Evidence Challenge

## TL;DR

This paper challenges the Landau–de Gennes model by analyzing experimental evidence of pretransitional effects in liquid crystals, revealing limitations of the model.

## Contribution

The study provides a new model-based explanation for pretransitional effects in nematogens that the traditional Landau–de Gennes model cannot explain.

## Key findings

- Pretransitional effects vary depending on molecular structural features like dipole moment loci and steric hindrance.
- The interplay between observation and pretransition fluctuations time scales is essential for explaining experimental results.
- A new coherent explanation is proposed for phenomena unaccounted for by the canonical Landau–de Gennes model.

## Abstract

The Landau–de Gennes model is one of the most significant fundamental frameworks in The Physics of Liquid Crystals and Soft Matter Physics. It is validated by the universal parameterisation of the Cotton–Mouton effect, the Kerr effect, and light scattering in the isotropic phase of nematogens. However, as early as 1974, de Gennes identified the first two puzzling problems of this model. Over the following decades, this list has expanded. This report presents the first comprehensive analysis of these issues, with the explicit experimental reference. It focuses on the hardly coherently discussed pretransitional changes in the dielectric constant and the extension in a strong electric field, specifically the nonlinear dielectric effect (NDE). Notably, there are uniquely different pretransitional forms of pretransitional effects, depending on molecular structural features such as permanent dipole moment loci or a steric hindrance. It is tested for 5CB, 5*CB, and MBBA: nematogenic liquid crystalline materials that differ in the above features. The obtained specific pretransitional effects and the evidence for the essential importance of the interplay between observation and pretransition fluctuations time scales led to a new coherent, model-based explanation of all the discussed problems, which cannot be explained within the canonical Landau–de Gennes model.