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The classical theory
Published in Epifanio G. Virga, Variational Theories for Liquid Crystals, 2018
Condition (3.11) applies to both nematics and cholesterics: it derives from a change of frame, irrespective of the material that occupies ℬ. There are, however, transformations of ℬ under which nematics and cholesterics behave differently: they reflect different material symmetries. What makes the difference between nematic and cholesteric liquid crystals on a microscopic scale is the different enantiomorphism of their molecules. While nematic molecules, which resemble rods, remain alike after a reflection, cholesteric molecules, which resemble helical springs, suffer a change in chirality under a reflection: right-handed helices are changed into left-handed helices, and vice versa. It is the common opinion that such a microscopic feature has its counterpart also on a macroscopic scale. When ℬ suffers a reflection, the energy density of nematics must not be affected, whereas that of cholesterics might be.
Liquid crystal displays
Published in John P. Dakin, Robert G. W. Brown, Handbook of Optoelectronics, 2017
Selective reflection of colored light from cholesteric liquid crystals can be used for electronic displays too. Moreover, the cholesteric electro-optic characteristic can be arranged to be bistable, allowing unlimited multiplexibilty using low-cost passive matrix addressing. Among the first optical switching modes to be studied at RCA was the bistable cholesteric [135], the switching mechanism for which was elucidated during the 1970s [136,137]. However, the success of the bistable cholesteric is largely due to the team at Kent State University headed by Doane [138] and the many innovations made by the engineers at the spin-out company Kent Displays Inc. (KDI) [139,140].
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).
Branched schiff base liquid crystalline oligomers: synthesis and properties
Published in Liquid Crystals, 2022
Jiang-Tao Sun, Zi-Yun Zhang, Chun-Yang Li, Ya-Ping Liu, Qing-Qing He, Yi-Min Wang, Ying-Gang Jia, Mei Tian, Dan-Shu Yao
Chiral liquid crystal is characterised in that the liquid crystal molecular structure contains an asymmetric chiral core and the molecule itself does not have mirror symmetry [34]. This kind of liquid crystal has the dual characteristics of optical activity and liquid crystal. Chiral liquid crystal molecules form helical structures [35–38]. So they have many optical properties that ordinary liquid crystals do not have, such as optical rotation, circular dichroism, selective reflection of polarised light, etc [39–45]. Their liquid crystal phase are generally chiral nematic texture[46,47]. The flexible spacer, the structural type of the mesogenic unit and the chiral group all influenced the formation of the cholesteric phase[48]. In addition, the molecular structure also had a certain influence on the properties of liquid crystals [1,3,49]. Recently, Ailincai D et al. [50] prepared Polymer dispersed liquid crystal (PDLC) composites by the encapsulation of cholesteryl acetate (L-ChAc) in polyvinyl alcohol boric acid (PVAB) by microemulsion technique. The results indicated the new polymer dispersed liquid crystal systems as promising materials for biosensors building. In addition, cholesteric liquid crystals had important application in smart windows and thermal printable e-paper, electro-optic displays, lasing, microlenses, and so on[51,52].
Self-diffusion method for broadband reflection in polymer-stabilized cholesteric liquid crystal films
Published in Liquid Crystals, 2022
Xuetao Zhang, Weiting Shi, Rui Han, Hui Li, Hui Cao, Yinjie Chen, Zhou Yang, Dong Wang, Wanli He
Cholesteric liquid crystals are known as chiral nematic liquid crystals and can be formed by adding chiral compounds to the nematic liquid crystals. Due to its unique helical structure, it can selectively reflect incident light of different wavelengths. When circularly polarised light is incident vertically on the N*-LCs, circularly polarised light with the same direction of rotation as that of the N*-LCs is reflected and circularly polarised light with the opposite direction of rotation as that of the N*-LCs is transmitted in a certain wavelength range. The reflection wavelength of the N*-LCs is following the Bragg reflection law. Where λ = n × P, Δλ = Δn×P, where λ represents the reflective wavelength, n represents the refractive index, P represents the CLC pitch, Δλ represents the reflective bandwidth, and Δn represents birefringence [9–11]. Since the value of Δn for colourless organic materials is less than 0.3, the bandwidth of a single-pitch Ch-LC region is less than 100 nm [12]. As the reflective N*-LCs films were not like the polymer-dispersed LC films at the expense of optical transmittance [13–15], the broadband reflective films, especially infrared-reflective films, were very useful in the application of passive solar energy technology in architecture design for the infrared regions of the solar accounting for about 43% of the solar energy [16–21].
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.