Fredericksz transition threshold in nematic liquid crystals filled with ferroelectric nano-particles

A key liquid crystalline property for electro-optic applications is the Frederiks threshold electric field. There has been recent experimental interest in liquid crystal-based colloidal suspensions in which the colloidal nanoparticles both possess a permanent electric polarization and provide strong director anchoring on the particle surface. Such suspensions are sometimes known as Filled Liquid Crystals. Our calculations suggest, in qualitative agreement with experiment, that filling the nematic liquid crystal with ferroelectric nanoparticles can significantly decrease the electric Frederiks transition threshold field

Frederiks transition in ferroelectric liquid-crystal nanosuspensions

We construct a theoretical model of the dielectric properties of a ferroelectric LC nanosuspension (FLCNS), using a generalized Maxwell-Garnett picture. The theory supposes that an FLCNS may as a first approximation be considered as a complex homogeneous dielectric ceramic, thus neglecting positional correlations of the colloidal particles. The FLCNS then consists of an anisotropic matrix with a very low concentration (&lt;1% by volume) of impurity particles. The impurity particles possess both shape and dielectric anisotropy, as well as a permanent electric polarization and strong liquid-crystal director anchoring on the particle surface. We show that the effective dielectric properties for capacitance properties and for effective liquid-crystal free energies do not coincide. We calculate the effect of doping a liquid crystal with ferroelectric impurities on the Frederiks transition. The theory takes account of inclusion shape, dielectric susceptibility, and local field effects. We neglect the possibility of dielectric particle chaining, which appears experimentally not to occur in general. Our calculations suggest, in qualitative agreement with experiment, that doping a nematic liquid crystal with ferroelectric particles, even at very low particle concentration, can in some cases significantly decrease the electric Frederiks threshold field.<br/

Theoretical modeling of heterogeneous liquid crystal systems: nano-suspensions and polymer-stabilized LC lens

Heterogeneous liquid crystal (LC) systems are a new class of anisotropic materials which have been intensively studied over the last decade. This article is concerned with two such systems: ferroelectric LC nano-suspensions and LCs with polymer network. The significant implication of ferroelectric LC-colloids is the possibility of a significant reduction in the threshold electric Fredericksz voltage. Here we present a theory of this phenomenon, which agrees qualitatively with experiment. A second example of heterogeneous LC systems is a gradient-polymer-stabilized LC (G-PSLC) structure, which has been proposed for a tunable-focus lens application. We report on theoretical model that describes the dependence of the G-PSLC lens focal length on applied voltage. A previous model is improved by considering light absorption during the polymerization. Again we find qualitative agreement with the experimental data

Modelling the dynamical behaviour of holographic gratings with nematic film-polymer slice sequence structure

In recent years there has been much interest in the physics and possible technological applications of liquid crystal-based tuneable Bragg gratings. Such optical devices can be realised using special composite materials based on a sequence of polymeric sheets alternated by layers containing only liquid crystal, sometimes known as Policryps and Poliphem. Recent experiments have clearly demonstrated a peculiar dependence of relaxation times on actual external field applied (both amplitude and duration) after switching off the operating voltage. We propose a theoretical model of director reorientation in such composites, in which both bulk behaviour in the LC stripes and interfacial polymer-LC interaction is taken into account. Within this model we explain the dynamical behaviour of holographic gratings characterized by a nematic film-polymer slice sequence structure, as well as other polymer-LC phase separated composites