Impact of Weak Nanoparticle Induced Disorder on Nematic Ordering

Dejvid Črešnar,Ioannis Lelidis,Christina Kyrou,Samo Kralj,Zdravko Kutnjak,Sylwester Rzoska,Aleksandra Drozd-Rzoska,Szymon Starzonek

doi:10.3390/cryst9030171

Abstract

Dilute mixtures of nanoparticles (NPs) and nematic liquid crystals (LCs) are considered. We focus on cases where NPs enforce a relatively weak disorder to the LC host. We use a Lebwohl-Lasher semi-microscopic-type modeling where we assume that NPs effectively act as a spatially-dependent external field on nematic spins. The orientational distribution of locally favoured “easy” orientations is described by a probabilistic distribution function P. By means of a mean field-type approach, we derive a self-consistent equation for the average degree of nematic uniaxial order parameter S as a function of the concentration p of NPs, NP-LC coupling strength and P. Using a simple step-like probability distribution shape, we obtain the S(p) dependence displaying a crossover behaviour between two different regimes which is in line with recent experimental observations. We also discuss a possible origin of commonly observed non-monotonous variations of the nematic-isotropic phase temperature coexistence width on varying p.

Highlights

In recent years, there has been an increasing interest in homogeneous composites [1] consisting of soft materials doped with appropriate nanoparticles (NPs)
We focus on the impact of NP-induced weak disorder on anomalous host structural behaviour, in which we exploit nematic liquid crystals as a demonstrative test bed
We study thermotropic nematic liquid crystals (LCs) doped with NPs of volume concentration p in the diluted regime (i.e., p 1)

Summary

Introduction

There has been an increasing interest in homogeneous composites [1] consisting of soft materials doped with appropriate nanoparticles (NPs). The main goal is to combine constituents possessing complementary properties to obtain composite materials with anomalously enhanced or even new material properties. Diverse mixtures often exhibit complex behaviour, leading to emergent robust, and in some cases even universal behaviour. In this contribution, we focus on the impact of NP-induced weak disorder on anomalous host structural behaviour, in which we exploit nematic liquid crystals as a demonstrative test bed. Liquid crystals (LC) [2] are typical representatives of anisotropic soft materials. These extraordinary materials combine liquid and crystalline ordering, optic transparency and anisotropy, and softness

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Conclusion