Smectic ordering in liquid crystal - aerosil dispersions I. X-ray scattering

TL;DR

This study uses x-ray scattering to analyze how aerosil gel dispersions affect smectic ordering in liquid crystal 8CB, revealing that disorder suppresses the nematic-smectic transition and introduces static fluctuations modeled by a 3D XY system with random fields.

Contribution

It provides a detailed characterization of smectic correlations in liquid crystal-aerosil dispersions and models the effects of disorder using a 3D XY framework with random fields.

Findings

Disorder destroys the nematic-smectic-A transition.

Correlations remain short-range across all conditions.

Static fluctuations dominate at low temperatures, consistent with a random field model.

Abstract

Comprehensive x-ray scattering studies have characterized the smectic ordering of octylcyanobiphenyl (8CB) confined in the hydrogen-bonded silica gels formed by aerosil dispersions. For all densities of aerosil and all measurement temperatures, the correlations remain short range, demonstrating that the disorder imposed by the gels destroys the nematic (N) to smectic-A (SmA) transition. The smectic correlation function contains two distinct contributions. The first has a form identical to that describing the critical thermal fluctuations in pure 8CB near the N-SmA transition, and this term displays a temperature dependence at high temperatures similar to that of the pure liquid crystal. The second term, which is negligible at high temperatures but dominates at low temperatures, has a shape given by the thermal term squared and describes the static fluctuations due to random fields induced by confinement in the gel. The correlation lengths appearing in the thermal and disorder terms are the same and show strong variation with gel density at low temperatures. The temperature dependence of the amplitude of the static fluctuations further suggests that nematic susceptibility become suppressed with increasing quenched disorder. The results overall are well described by a mapping of the liquid crystal-aerosil system into a three dimensional XY model in a random field with disorder strength varying linearly with the aerosil density.