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Liquid Crystals
Published in Wen-Jei Yang, Handbook of Flow Visualization, 2018
N. Kasagi, R. J. Moffat, M. Hirata
An organic compound is usually optically nonisotropic in its crystallized, solid phase but optically isotropic in the liquid phase at temperatures above its melting point. A particular group of organic compounds, however, exhibit behavior midway between that of an isotropic liquid and a nonisotropic crystalline solid; these compounds are generally termed liquid crystals or mesophases. The special term thermotropic liquid crystal denotes a compound that displays a nonisotropic liquid character at a temperature between its pure crystalline and isotropic liquid states. On the other hand, a lyotropic liquid crystal is a compound that shows optical anisotropy when dissolved in a particular solvent, e.g., soapy water. A liquid crystal phase can also be created by mixing several compounds [7], such as different pure liquid crystal compounds, liquid crystal and nonliquid crystal compounds, or even different nonliquid crystal compounds.
Self-Assembly and Nanochemistry Techniques for the Fabrication of Metamaterials
Published in Filippo Capolino, Applications of Metamaterials, 2017
Virginie Ponsinet, Ashod Aradian, Philippe Barois, Serge Ravaine
The idea of making nanostructured composites by mixing nanoparticles with ordered self-assembled matrices first originated 40 years ago from the community of thermotropic liquid crystals [127,128]. Theoretically, studies indeed anticipated interesting opportunities to develop liquid crystal composites as novel field-responsive fluids. Thermotropic liquid crystals are composed of anisotropic organic molecules that self-organize in fluid structures called mesophases, exhibiting some intermediate degree of order when the temperature varies between the highly ordered crystalline phase and the disordered liquid phase [129]. Figure 32.17 shows the most classical mesophases obtained from the two main classes of molecular architectures, namely, rods and disks. Thermotropic liquid crystals are widely used in liquid crystal display (LCD) displays for their high birefringence, their ability to be aligned by surfaces, and their strong response to moderate external fields.
Modern Applications and Current Status of Liquid and Crystal Nanomaterials in Environmental Industry
Published in Uma Shanker, Manviri Rani, Liquid and Crystal Nanomaterials for Water Pollutants Remediation, 2022
Rachna, Uma Shanker, Manviri Rani
Soft matter namely liquid crystal has become the interest of the world in this century. Liquid crystals are known as the fourth state of matter. These materials have emerged from the condensed matter physics. In our day-to-day life, there are various examples of liquid crystals, such as milk, lipstick, gluten, cell membranes and mineral slurries (Palffy-Muhoray 2007, Mohanty 2003). These are applied in various things fabrication, like calculators, laptops, smartphones and tablets. Liquid crystals are considered as transitional between an isotropic liquid and crystal solids (Yu 2015). The name liquid crystal was given because it possesses the properties of both solids and liquids. Properties such as electrical and optical anisotropy are similar to the solids, while molecular mobility and fluidity are similar to liquids (Demus et al. 1998). Liquid crystals exhibit some of the arrangement of the particles in one particular direction, while others could be in a random state. This striking feature, made of the intermediate phase, is also known as mesophase (Goodby et al. 2008). Classification of liquid crystals depends upon physic-chemical parameters, such as a transition in the phases (thermotropic and lyotropic) (Hyde 2001). Effect of temperature or pressure results in the formation of thermotropic liquid crystals. Synthesis of materials in the liquid phase has several benefits over other matters (Feng et al. 2015, Gutsch et al. 2004). Synthesis in the liquid phase is common for preparing nanoparticles; synthesis is possible in a fraction of minutes with desired particles size, and methods are generally cost-effective, simple and common reagents can work in the process (Charitidis et al. 2014, Cheng et al. 2014). Besides these functionalization of material is possible through in situ fabrication and could be applied to the desired field of application. It is easy to control the size and shape of the nanoparticles (Lu and Yin 2012). Synthesis of liquid phased nanomaterials could be achieved through two commonly known approaches, i.e. top down and bottom up. Liquid phased nanomaterials could rest in liquid suspension or collected through simply filtering. Interest in liquid phase material fabrication has increased due to their fast response speed and higher thermal stability (Cui and Zhao 2004, Hayasaka et al. 2008). Modification of liquid materials to crystalline could be achieved through incorporation of other inorganic matrixes (Shibaev 2009). Most of the attention has been paid to liquid materials due to their application in display and photonics. Therefore, methods have been developed for the fabrication of nanostructured materials in liquid crystalline phase. Fabrication in nano-range can result into better thermal stability, improved light response and electric/magnetic field reaction. However, formation of liquid and crystalline nanomaterials is a very difficult task since retention of a phase could be difficult. Hence, only a limited set of nanoparticles are appropriate with different synthesis path (Gerasin et al. 2013, Sanchez and Sobolev 2010). Various nanoparticles of liquid crystalline nature have been fabricated till now and various processes are still going on for its formation.
Structurally coloured organic–inorganic hybrid silica films with a chiral nematic structure prepared through a self-templating approach
Published in Liquid Crystals, 2021
Weiwei Liu, Haohao Wei, Hongkun Li, Yong Wang, Limin Wu, Baozong Li, Yi Li, Yonggang Yang
Thermotropic liquid crystals have been successfully applied as sensors and for display. Up to now, varieties of liquid crystalline mesophases have been identified, such as blue, nematic, cholesteric, columnar and smectic phases. These mesophases are usually stabilised by the non-covalent interactions. To fix the molecular organisation structures at the mesophases, an in-situ polymerisation approach was developed [10–18]. Up to now, blue [10–12], cholesteric [13–17] and smectic A [18] phases have been stabilised. These materials can be used as reflective films and polarisers. With the developing external templating approach, polybissilsesquioxane films with a chiral nematic structure have been prepared using nanocrystalline cellulose (NCC) as the templates [19]. Herein, a self-templating approach for the preparation of organic–inorganic hybrid silica film with a chiral nematic structure was developed. A liquid crystalline organosilane was synthesised. The molecular organisation structure at the mesophase was fixed by polycondensation at a certain temperature.
Setting things straight in ‘The twist-bend nematic: a case of mistaken identity’
Published in Liquid Crystals, 2020
Ivan Dozov, Geoffrey R. Luckhurst
The nematic phase, in its different variants (e.g. uniaxial nematic, NU, chiral nematic or cholesteric, N*, blue phases, BP I, II and III) is the best studied thermotropic liquid crystal (LC) and has now found significant technological applications. It is well known that the macroscopic structure, symmetry and properties of the LC phase are closely related to the molecular symmetry and chemical structure of the mesogen. For example, highly biaxial mesogenic molecules are expected to show a biaxial nematic phase, which was predicted 50 years ago by Freiser [1] but is still elusive (as a thermotropic nematic). Naturally, highly curved molecules have also attracted attention in the search for exotic LC phases. The 1990’s were marked by the discovery of smectic phases from banana-shaped molecules with spontaneous breaking of the chiral symmetry and striking ferroelectric properties [2–4]. Ten years later, interest also spread to the nematic phases of such curved molecules, in particular to the highly curved mesogenic dimers (here the dimers are curved because the spacer linking the two rod-like groups had an odd number of groups). Indeed, in the early 2000’s, several research groups reported for this kind of compound the existence of a second, lower-temperature nematic phase [5–8]. At that time this phase, whose structure was unknown, was provisionally given the NX-phase as its mnemonic, in the expectation of its future identification and characterisation.
Cholesteric and blue-phase liquid photonic crystals for nonlinear optics and ultrafast laser pulse modulations
Published in Liquid Crystals Reviews, 2018
Liquid crystals are composed of anisotropic organic molecules that self-assemble themselves into a rich variety of ordered phases intermediate between solid crystal and fluid. Their physical and optical properties such as birefringence, dielectric constants, elastic constants, viscosities, absorption spectra, transition temperatures, and dielectric-, thermal- and conductivity-anisotropies are consequences of the constituent organic molecules and the ordered structures [1-4]. Among the so-called thermotropic liquid crystals that exhibit ordering as a function of temperature, three exemplary and widely studied phases are depicted in Figure 3: Nematic, Cholesterics and Smectics. In nematics, the molecules are directionally correlated but disordered in position, and generally aligned along a particular direction represented by the so-called director axis n. Smectics possess positional – as well as directional – ordering, exhibiting layered structures. Cholesteric liquid crystals (CLC) are chiral nematic liquid crystals; they possess similar physical properties as NLC except that the molecules self-assemble in a helical manner around a particular axis. As a result of the spatially periodic variation in the dielectric constant, the dispersion and transmission spectral of CLC exhibit bandgaps and 1-D photonic crystal properties. In this review, we also discuss the so-called cholesteric Blue Phase liquid crystals (BPLC) that exhibit 3-D photonic crystal-like properties [1-5] .