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Liquids: background theory
Published in Michael de Podesta, Understanding the Properties of Matter, 2020
One special type of variation of n has important applications in the use of liquid crystals in digital displays, and is known as a cholesteric liquid crystal. The term cholesteric derives from two words: chole which is the Greek prefix for the bile or gall duct from which these chemicals were originally derived; and stereo which in this context refers to the three-dimensional waxy nature of the substance. Within any layer of the structure, the substance looks like a nematic liquid crystal structure, but its key feature is that the director rotates from one region of the crystal to another. As illustrated in Figure 8.13, the rotation of the director is perpendicular to the plane containing the nematic director. Molecules such as those illustrated in Figure 8.7 would be unlikely to form a cholesteric phase liquid crystal.
Visual Displays
Published in Julie A. Jacko, The Human–Computer Interaction Handbook, 2012
Christopher M. Schlick, Carsten Winkelholz, Martina Ziefle, Alexander Mertens
Cholesteric LCDs: A cholesteric liquid crystal is a type of liquid crystal with a helical structure. Cholesteric liquid crystals are also known as chiral nematic liquid crystals. They appear in layers with no positional ordering within the layers, but they do have a director axis that varies with each layer. The variation of the director axis tends to be periodic in nature. The period of this variation (the distance over which a full rotation of 360° is completed) is known as the pitch. The pitch varies with temperature and can also be affected by the boundary conditions when the chiral nematic liquid crystal is sandwiched between two substrate planes (Yeh 2009).
Liquid Crystals
Published in Wen-Jei Yang, Handbook of Flow Visualization, 2018
N. Kasagi, R. J. Moffat, M. Hirata
Since a cholesteric liquid crystal phase reflects a certain component of circularly polarized light and allows the other components to pass through it, the background beneath the liquid crystals must be completely black; otherwise, the color change will not be visible. Any black paint can be used, as long as the paint and its solvent do not damage liquid crystal capsules chemically. Water-based paints are less apt to interfere with liquid crystals than are hydrocarbon-based paints.
Broadband reflection cholesteric liquid crystal film fabricated by near-infrared photothermal response technology
Published in Liquid Crystals, 2021
Limin Zhao, Weiting Shi, Ping Gan, Xuetao Zhang, Hui Cao, Huihui Wang, Dong Wang, Zhou Yang, Wanli He
Cholesteric liquid crystal (CLC) has aunique periodic helical structure. It is classified into left- and right-handed CLCs. Circularly polarised light of aspecific wavelength with the same rotation direction as CLC will be reflected. This property of CLC is called selective reflection [1]. At normal incidence, the selective reflection centre wavelength of CLC (λ) is given by λ = nP, where Pand ndenote the pitch and average refractive index (RI) of the CLC, respectively. The average RI, n = (no+ne)/2, is the mean of the ordinary RI (no) and extraordinary RI (ne) of the CLC. The reflection bandwidth of CLC is given by Δλ = ΔnP, where Δnis the birefringence of the CLC, which is determined by Δn = ne − no [2]. Owing to selective reflection, CLCs are widely applied in reflective displays, energy-saving doors and windows, laser protection apparatus and other applications [3–5].
Effect of chiral monomer containing D(+)-camphoric acid on the optical properties and phase behaviours of side-chain cholesteric liquid crystal polymers
Published in Liquid Crystals, 2021
Ya-Ru Ma, Xue-Song Zhang, Xuan Xie, Yue-Jiao Huang, Xiao-Zhi He, Yue-Hua Cong, Bao-Yan Zhang, Ying-Gang Jia
Generally, a cholesteric liquid crystal can be obtained by doping a chiral molecule into a nematic liquid crystal. The long axis of cholesteric liquid-crystal molecules can revolve around the helix axis to form a helix structure. Because its refractive index changes periodically, it can selectively reflect the circularly polarised incident light with the same helicity as the helix axis [69]. At normal incidence, the average reflection wavelength λ obeyed the Bragg condition that can be governed by the following Formula 2, in which p called as cholesteric pitch that corresponding to the distance required for liquid-crystal molecules to rotate 360°, θ is the incident angle of the beam, and n can be expressed by Formula 3 as the average of ordinary (no) and unusual (ne) refractive indices of cholesteric liquid crystal.
Broadband reflection induced by chiral dopant-loaded mesoporous polystyrene-divinylbenzene microspheres via diffusion in polymer stabilised cholesteric liquid crystals
Published in Liquid Crystals, 2023
Yuzhen Zhao, Tingting Lang, Yaqin Chu, Chaonian Li, Yang Zhao, Huimin Zhang, Zhun Guo, Zongcheng Miao, Dong Wang
The cholesteric liquid crystal (N*-LC), which can be formed by doping a nematic liquid crystal (N-LC) with chiral molecules, has attracted considerable attentions due to its unique reflection characteristics [1–4]. The refractive index of N*-LC is regularly modulated along the helical axis because of the particular arrangement of the molecules [5–7]. In recent years, the broadband reflective liquid crystal films based on the N*-LC have been applied in many fields, such as brightness enhancement films for LC displays [8], coloured reflective displays [9], switchable smart windows and other optical elements [10,11].