Introductory Chapter: Nematic Liquid Crystals

Abstract

1. Nematic phase The nematic (N) liquid crystalline phase is technologically the most important of the well-known and widely studied mesophases (nematic, smectic, cholesteric, and columnar). Nematics are also the most used, because they illustrate the best dual nature of liquid crystals. Hence, the molecules that form LC mesophases (intermediate states between the crystalline and liquid phase) display a unique combination of properties between long-range order and mobility, the basis of the numerous technical applications [1]. However, this combination constitutes as well an essential requirement for living matter, considering liquid crystals play a significant role in biomolecule's assembling, e.g., smectic phases (in phospholipid bilayer in the cell, protein filament), columnar phases (in DNA), or nematic phases (in chitin, collagen, cellulose, viruses, and silk) [2, 3]. Moreover, the ability of liquid crystal molecules to respond under weak external stimuli (temperature, electric or molecular adsorbates) stimulated the intellectual collaboration between specialists as chemists, physicists, or electrical engineers. As a result of joining of liquid crystal science and other fields, new opportunities for applications have been developed, involving polymers, colloids, or surfactants [4]. Hence, soft responsive materials based on surface-induced ordering transitions using nematic liquid crystal droplets dispersed within a medium or encapsulated into polymeric shells have been a promising perspective for experimental research [5]. The formation of liquid crystalline phase or mesomorphism implies the transition of a pure compound from an ordered crystalline state to a disordered liquid in two events: by a change in temperature and melting, the case of thermotropic liquid crystals, and by adding a suitable solvent to a mesogen and dissolving, the case of lyotropic liquid crystals. Thermotropic liquid crystalline state occurs mostly in compounds with pronounced molecular anisotropy. Lyotropic systems, representing two-or multi-component, occur when dissolved mesogenic amphiphiles self-assemble into ordered micelles. Because of high mobility, nematic phases show a low viscosity, very similar to isotropic liquids, with the difference that the parallelism of the long axes induces the anisotropy of many physical properties. Hence, the nematic liquid crystals are anisotropic in respect to optical properties (double refraction), viscosity, magnetic and electric susceptibility, and electric and thermal conductivities [6]. The nematic phase is formed when the molecules are oriented to a common direction represented by a unit vector, n, or director. Hence, optical parallelism that depends on temperature is expressed quantitatively by order parameters S, which measures the degree of alignment of molecules' symmetry axes: S = 1 _ 2 (3 co s 2 θ − 1) (1)