Effect of Aerosil Gels on de Gennes Nematic−Smectic Coupling

Abstract

There is a well-known de Gennes coupling between the scalar nematic order parameter S and the smectic density-wave amplitude /Psi/, which causes the nematic-smectic-A phase transition to cross over from second order to first order via a tricritical point. This sort of crossover is observed for numerous liquid crystals (LCs) with small to moderate nematic ranges. Recent studies of LC+aerosil systems, where the aerosil particles form a soft thixotropic gel, have shown that such aerosil gels can greatly reduce or completely suppress this coupling. The underlying LC critical exponents once de Gennes coupling is absent are discussed and compared with theory.

Calorimetric study of the nematic to smectic-Aphase transition in octylcyanobiphenyl-hexane binary mixtures

“…The N-SmA phase transition is a non-trivial member of the 3D-XY universality class due to the anisotropy of its critical fluctuations parallel and perpendicular to the director [18,25,26]. The N-SmA critical behavior is strongly effected by the coupling between the smectic order parameter ψ( r) = ψ 0 exp(i q 0 .…”

In Memory of Pierre-Gilles de Gennes

“…The paper by Barry et al reports on a direct observation of the finite-scale penetration of twist at the edge of a single isolated smectic A layer of virus particles. de Gennes also showed how smectic and nematic order are coupled; however, this coupling can sometimes be eliminated by the incorporation of gels, as shown in the work of Garland and Iannacchione . Consequences of nematic ordering are readily detected in magnetic resonance line shapes and can be exploited to obtain precise phase diagrams of lyotropic liquid crystals, as shown by Smith and Freed .…”