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

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 a strong variation with gel density at low temperatures. The temperature dependence of the amplitude of the static fluctuations further suggests that nematic susceptibility becomes suppressed with increasing quenched disorder. The results overall are well described by a mapping of the liquid-crystal-aerosil system onto a three-dimensional XY model in a random field with disorder strength varying linearly with the aerosil density.

Smectic ordering in liquid-crystal–aerosil dispersions. II. Scaling analysis

“…A detailed theoretical study of QRD effects on smectic ordering in liquid crystals [11] concludes that arbitrarily small amounts of QRD destroy even quasi-LRO and hence, no true smectic phase exists since the smectic correlation length remains finite for all strengths of disorder and all temperatures. This theoretical conclusion is in agreement with a recent x-ray study of octylcyanobiphenyl (8CB)+aerosil dispersions [8], the detailed results of which are presented in the companion paper to this work, denoted hereafter as paper I [12]. This x-ray study reveals a finite, though large, smectic correlation length for all temperatures and densities of silica.…”

“…This is surprising since the critical behavior of the 8CB+aerosil samples is changing with ρ S as seen in Fig. 2 and the saturated parallel smectic correlation length ξ LT is much larger than l o for all ρ S [12]. Thus, we are suspicious that this agreement may be accidental.…”

“…In spite of the loss of smectic LRO, Paper I [12] and an earlier calorimetric study of 8CB+aerosils [4] show that smectic thermal fluctuations still play an important role in LC+aerosil systems. Many concepts from pure materials such as finite-size scaling, two-scale universality, and tricritical-to-XY crossover due to variable de Gennes coupling need to be considered in addition to intrinsic new quenched random effects that dominate at temperatures below an effective transition temperature.…”

“…However, smectic thermal fluctuations still exist above a pseudo nematic to smectic transition at T * (close to but below T o N A for pure 8CB). These smectic fluctuations are expected to remain in the XY universality class but also show crossover behavior from Gaussian tricritical (T C) to 3D-XY with increasing strength of disorder [12].…”

Inhomogeneous relaxation dynamics and phase behaviour of a liquid crystal confined in a nanoporous solid

“…Both the collective orientational (isotropic-to-nematic) and the translational (smectic-toliquid or smectic-to-nematic) transitions have turned out to be significantly affected by finite size and interfacial (solid-liquid or liquid-liquid) interactions introduced by confining walls [1][2][3][4][5][6] or the geometrical constraints in nanoporous media [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24].…”