Special Issue "Advances in Cholesteric Liquid Crystals"

A special issue of Crystals (ISSN 2073-4352). This special issue belongs to the section "Liquid Crystals".

Deadline for manuscript submissions: closed (30 June 2019).

Special Issue Editor

Guest Editor
Dr. Michel Mitov

Director of Research at CNRS, Centre National de la Recherche Scientifique, France
Centre d’Elaboration de Matériaux et d’Etudes Structurales, CEMES, Toulouse, France
Website | E-Mail
Interests: cholesteric and chiral liquid crystals; polymer–liquid crystal composites; biological liquid crystals; biomimetic replicas; materials design—structure—optical properties

Special Issue Information

Dear Colleagues,

Most of optical properties of liquid crystals (LCs) are due to chiral structures. With their helical structure, cholesteric LCs (CLCs)—also named chiral nematic LCs—figure prominently in LC science. Selectively reflecting the light is their iconic property. A properly oriented layer of CLC is a unique multifunctional material: It is, at the same time, a reflector, a notch filter, a polarizer and an optical rotator.

The research on CLCs often requires making a continuous description of chiral structures and their properties from the nanometer range to the macroscopic scale. It provides natural links between different disciplines in soft matter science and well beyond. It promises a myriad of applications in the areas of sensors based on color changes; tunable bandpass filters; rewritable color recordings; hyperspectral imaging; thermal printable e-paper; polarizer-free reflective displays; lasing; microlenses; smart windows, and so on. Lyotropic CLC formulations may be chosen for beauty cares to screen UV light or to obtain iridescent visual effects. The CLC structure has very far-reaching implications in technology which have not yet shown their full significance.

The CLC structure is a recurring design in animal and plant kingdoms. The functions of biological CLCs include maximisation of packing efficiency, optical information, radiation protection and mechanical stability. The role of the cholesteric structure in living matter is far from being fully defined, and many biomimetic replicas are still waiting for a realization.

This Special Issue of Crystals on “Advances in Cholesteric Liquid Crystals” covers the formulation, properties, experimental investigations, theoretical aspects and applications of thermotropic or lyotropic compounds, and biological materials.

Original research papers and review articles are considered.

It is my pleasure to invite you to submit a manuscript to this Special Issue.

Dr. Michel Mitov
Guest Editor

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All papers will be peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Crystals is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 1400 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • Cholesteric liquid crystals
  • Structures, defects and physical properties
  • Materials design by physical techniques
  • Chemical synthesis
  • Biological and biomimetic cholesteric liquid crystals
  • Optical devices based on cholesteric liquid-crystalline materials

Published Papers (7 papers)

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Research

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Open AccessArticle
Liquid Crystal Based Head-Up Display with Electrically Controlled Contrast Ratio
Crystals 2019, 9(6), 311; https://doi.org/10.3390/cryst9060311
Received: 9 May 2019 / Revised: 7 June 2019 / Accepted: 14 June 2019 / Published: 18 June 2019
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Abstract
With the growing demand for driving safety and convenience, Head-Up Displays (HUDs) have gained more and more interest in recent years. In this paper, we propose a HUD system with the ability to adjust the relative brightness of ambient light and virtual information [...] Read more.
With the growing demand for driving safety and convenience, Head-Up Displays (HUDs) have gained more and more interest in recent years. In this paper, we propose a HUD system with the ability to adjust the relative brightness of ambient light and virtual information light. The key components of the system include a cholesteric liquid crystal (CLC) film, a geometric phase (GP) liquid crystal lens, and a circular polarizer. By controlling the voltage applied to the GP lens, the contrast ratio of the virtual information light to ambient light could be continuously tuned, so that good visibility could always be obtained under different driving conditions. Full article
(This article belongs to the Special Issue Advances in Cholesteric Liquid Crystals)
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Open AccessCommunication
Electro-Optical Properties of a Polymer Dispersed and Stabilized Cholesteric Liquid Crystals System Constructed by a Stepwise UV-Initiated Radical/Cationic Polymerization
Crystals 2019, 9(6), 282; https://doi.org/10.3390/cryst9060282
Received: 20 March 2019 / Revised: 8 May 2019 / Accepted: 22 May 2019 / Published: 29 May 2019
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Abstract
Polymer-dispersed liquid crystal (PDLC) and polymer-stabilized liquid crystal (PSLC) are two typical liquid crystal (LC)/polymer composites. PDLCs are usually prepared by dispersing LC droplets in a polymer matrix, while PSLC is a system in which the alignment of LC molecules is stabilized by [...] Read more.
Polymer-dispersed liquid crystal (PDLC) and polymer-stabilized liquid crystal (PSLC) are two typical liquid crystal (LC)/polymer composites. PDLCs are usually prepared by dispersing LC droplets in a polymer matrix, while PSLC is a system in which the alignment of LC molecules is stabilized by interactions between the polymer network and the LC molecules. In this study, a new material system is promoted to construct a coexistence system of PDLC and PSLC, namely PD&SChLC. In this new material system, a liquid-crystalline vinyl-ether monomer (LVM) was introduced to a mixture containing cholesteric liquid crystal (ChLC) and isotropic acrylate monomer (IAM). Based on the different reaction rates between LVM and IAM, the PD&SChLC architecture was built using a stepwise UV-initiated polymerization. During the preparation of the PDS&ChLC films, first, the mixture was irradiated with UV light for a short period of time to induce the free radical polymerization of IAMs, forming a phase-separated microstructure, PDLC. Subsequently, an electric filed was applied to the sample for long enough to induce the cationic polymerization of LVMs, forming the homeotropically-aligned polymer fibers within the ChLC domains, which are similar to those in a PSLC. Based on this stepwise UV-initiated radical/cationic polymerization, a PD&SChLC film with the advantages of a relatively low driving voltage, a fast response time, and a large-area processability is successful prepared. The film can be widely used in flexible displays, smart windows, and other optical devices. Full article
(This article belongs to the Special Issue Advances in Cholesteric Liquid Crystals)
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Open AccessArticle
Nematic and Cholesteric Liquid Crystal Structures in Cells with Tangential-Conical Boundary Conditions
Crystals 2019, 9(5), 249; https://doi.org/10.3390/cryst9050249
Received: 27 March 2019 / Revised: 7 May 2019 / Accepted: 9 May 2019 / Published: 14 May 2019
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Abstract
Orientational structures formed in nematic and cholesteric layers with tangential-conical boundary conditions have been investigated. LC cells with one substrate specifying the conical surface anchoring and another substrate specifying the tangential one have been considered. The director configurations and topological defects have been [...] Read more.
Orientational structures formed in nematic and cholesteric layers with tangential-conical boundary conditions have been investigated. LC cells with one substrate specifying the conical surface anchoring and another substrate specifying the tangential one have been considered. The director configurations and topological defects have been identified analyzing the texture patterns obtained by polarizing microscope in comparison with the structures and optical textures calculated by free energy minimization procedure of director field and finite-difference time-domain method, respectively. The domains, periodic structures and two-dimensional defects proper to the LC cells with tangential-conical anchoring have been studied depending on the layer thickness and cholesteric pitch. Full article
(This article belongs to the Special Issue Advances in Cholesteric Liquid Crystals)
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Open AccessArticle
Surface Anchoring Effects on the Formation of Two-Wavelength Surface Patterns in Chiral Liquid Crystals
Crystals 2019, 9(4), 190; https://doi.org/10.3390/cryst9040190
Received: 26 February 2019 / Revised: 25 March 2019 / Accepted: 26 March 2019 / Published: 2 April 2019
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Abstract
We present a theoretical analysis and linear scaling of two-wavelength surface nanostructures formed at the free surface of cholesteric liquid crystals (CLC). An anchoring model based on the capillary shape equation with the high order interaction of anisotropic interfacial tension is derived to [...] Read more.
We present a theoretical analysis and linear scaling of two-wavelength surface nanostructures formed at the free surface of cholesteric liquid crystals (CLC). An anchoring model based on the capillary shape equation with the high order interaction of anisotropic interfacial tension is derived to elucidate the formation of the surface wrinkling. We showed that the main pattern-formation mechanism is originated due to the interaction between lower and higher order anchoring modes. A general phase diagram of the surface morphologies is presented in a parametric space of anchoring coefficients, and a set of anchoring modes and critical lines are defined to categorize the different types of surface patterns. To analyze the origin of surface reliefs, the correlation between surface energy and surface nano-wrinkles is investigated, and the symmetry and similarity between the energy and surface profile are identified. It is found that the surface wrinkling is driven by the director pressure and is annihilated by two induced capillary pressures. Linear approximation for the cases with sufficient small values of anchoring coefficients is used to realize the intrinsic properties and relations between the surface curvature and the capillary pressures. The contributions of capillary pressures on surface nano-wrinkling and the relations between the capillary vectors are also systematically investigated. These new findings establish a new approach for characterizing two-length scale surface wrinkling in CLCs, and can inspire the design of novel functional surface structures with the potential optical, friction, and thermal applications. Full article
(This article belongs to the Special Issue Advances in Cholesteric Liquid Crystals)
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Open AccessArticle
Electro-Thermal Formation of Uniform Lying Helix Alignment in a Cholesteric Liquid Crystal Cell
Crystals 2019, 9(4), 183; https://doi.org/10.3390/cryst9040183
Received: 1 March 2019 / Revised: 21 March 2019 / Accepted: 26 March 2019 / Published: 1 April 2019
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Abstract
We demonstrated previously that the temperature of a sandwich-type liquid crystal cell with unignorable electrode resistivity could be electrically increased as a result of dielectric heating. In this study, we take advantage of such an electro-thermal effect and report on a unique electric-field [...] Read more.
We demonstrated previously that the temperature of a sandwich-type liquid crystal cell with unignorable electrode resistivity could be electrically increased as a result of dielectric heating. In this study, we take advantage of such an electro-thermal effect and report on a unique electric-field approach to the formation of uniform lying helix (ULH) texture in a cholesteric liquid crystal (CLC) cell. The technique entails a hybrid voltage pulse at frequencies f1 and, subsequently, f2, which are higher and lower than the onset frequency for the induction of dielectric heating, respectively. When the cell is electrically sustained in the isotropic phase by the voltage pulse of V = 35 Vrms at f1 = 55 kHz or in the homeotropic state with the enhanced ionic effect at V = 30 Vrms and f1 = 55 kHz, our results indicate that switching of the voltage frequency from f1 to f2 enables the succeeding formation of well-aligned ULH during either the isotropic-to-CLC phase transition at f2 = 1 kHz or by the electrohydrodynamic effect at f2 = 30 Hz. For practical use, the aligning technique proposed for the first time in this study is more applicable than existing alternatives in that the obtained ULH is adoptable to CLCs with positive dielectric anisotropy in a simple cell geometry where complicated surface pretreatment is not required. Moreover, it is electrically switchable to other CLC textures such as Grandjean planar and focal conic states without the need of a temperature controller for the phase transition, the use of ion-rich LC materials, or mechanical shearing for textural transition. Full article
(This article belongs to the Special Issue Advances in Cholesteric Liquid Crystals)
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Open AccessArticle
Effect of Size Polydispersity on the Pitch of Nanorod Cholesterics
Crystals 2019, 9(3), 143; https://doi.org/10.3390/cryst9030143
Received: 13 February 2019 / Revised: 4 March 2019 / Accepted: 4 March 2019 / Published: 10 March 2019
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Abstract
Many nanoparticle-based chiral liquid crystals are composed of polydisperse rod-shaped particles with considerable spread in size or shape, affecting the mesoscale chiral properties in, as yet, unknown ways. Using an algebraic interpretation of Onsager-Straley theory for twisted nematics, we investigate the role of [...] Read more.
Many nanoparticle-based chiral liquid crystals are composed of polydisperse rod-shaped particles with considerable spread in size or shape, affecting the mesoscale chiral properties in, as yet, unknown ways. Using an algebraic interpretation of Onsager-Straley theory for twisted nematics, we investigate the role of length polydispersity on the pitch of nanorod-based cholesterics with a continuous length polydispersity, and find that polydispersity enhances the twist elastic modulus, K 2 , of the cholesteric material without affecting the effective helical amplitude, K t . In addition, for the infinitely large average aspect ratios considered here, the dependence of the pitch on the overall rod concentration is completely unaffected by polydispersity. For a given concentration, the increase in twist elastic modulus (and reduction of the helical twist) may be up to 50% for strong size polydispersity, irrespective of the shape of the unimodal length distribution. We also demonstrate that the twist reduction is reinforced in bimodal distributions, obtained by doping a polydisperse cholesteric with very long rods. Finally, we identify a subtle, non-monotonic change of the pitch across the isotropic-cholesteric biphasic region. Full article
(This article belongs to the Special Issue Advances in Cholesteric Liquid Crystals)
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Review

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Open AccessReview
Liquid-Crystalline Dispersions of Double-Stranded DNA
Crystals 2019, 9(3), 162; https://doi.org/10.3390/cryst9030162
Received: 13 February 2019 / Revised: 12 March 2019 / Accepted: 17 March 2019 / Published: 20 March 2019
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Abstract
In this review, we compare the circular dichroism (CD) spectra of liquid-crystalline dispersion (LCD) particles formed in PEG-containing aqueous-salt solutions with the purpose of determining the packing of ds DNA molecules in these particles. Depending on the osmotic pressure of the solution, the [...] Read more.
In this review, we compare the circular dichroism (CD) spectra of liquid-crystalline dispersion (LCD) particles formed in PEG-containing aqueous-salt solutions with the purpose of determining the packing of ds DNA molecules in these particles. Depending on the osmotic pressure of the solution, the phase exclusion of ds DNA molecules at room temperature results in the formation of LCD particles with the cholesteric or the hexagonal packing of molecules. The heating of dispersion particles with the hexagonal packing of the ds DNA molecules results in a new phase transition, accompanied by an appearance of a new optically active phase of ds DNA molecules. Our results are rationalized by way of a concept of orientationally ordered “quasinematic” layers formed by ds DNA molecules, with a parallel alignment in the hexagonal structure. These layers can adopt a twisted configuration with a temperature increase; and as a result of this process, a new, helicoidal structure of dispersion particle is formed (termed as the “re-entrant” cholesteric phase). To prove the cholesteric pattern of ds DNA molecules in this phase, the “liquid-like” state of the dispersion particles was transformed into its “rigid” counterpart. Full article
(This article belongs to the Special Issue Advances in Cholesteric Liquid Crystals)
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