First order isotropic - smectic-A transition in liquid crystal-aerosil gels

TL;DR

This study investigates how silica nanoparticle gels affect the short-range order in the isotropic to smectic-A transition of liquid crystals, revealing different effects compared to the nematic to smectic-A transition, with implications for understanding random field influences.

Contribution

It provides the first detailed analysis of the isotropic to smectic-A transition under random fields introduced by aerosil gels, highlighting differences from nematic-smectic transitions.

Findings

Smectic correlation length decreases more slowly with gel density.

Random fields influence the transition differently than in nematic-smectic cases.

Quantitative agreement with random pinning field effects at a first-order transition.

Abstract

The short-range order which remains when the isotropic to smectic-A transition is perturbed by a gel of silica nanoparticles (aerosils) has been studied using high-resolution synchrotron x-ray diffraction. The gels have been created \textit{in situ} in decylcyanobiphenyl (10CB), which has a strongly first-order isotropic to smectic-A transition. The effects are determined by detailed analysis of the temperature and gel density dependence of the smectic structure factor. In previous studies of the continuous nematic to smectic-A transition in a variety of thermotropic liquid crystals the aerosil gel appeared to pin, at random, the phase of the smectic density modulation. For the isotropic to smectic-A transition the same gel perturbation yields different results. The smectic correlation length decreases more slowly with increasing random field variance in good quantitative agreement with the effect of a random pinning field at a transition from a uniform phase directly to a phase with one-dimensional translational order. We thus compare the influence of random fields on a \textit{freezing} transition with and without an intervening orientationally ordered phase.