Research

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14 pages, 4839 KiB

Open AccessArticle

Photoaligned Tunable Liquid Crystal Lenses with Parabolic Phase Profile

by Svitlana P. Bielykh, Liana Lucchetti and Victor Yu. Reshetnyak

Crystals 2023, 13(7), 1104; https://doi.org/10.3390/cryst13071104 - 15 Jul 2023

Cited by 2 | Viewed by 920

We present a theoretical model of a cylindrical tunable liquid crystal lens based on the modulation of anchoring energy. The latter can be easily obtained using photoalignment techniques. The liquid crystal cell we propose exhibits strong anchoring at the top substrate and anchoring [...] Read more.

We present a theoretical model of a cylindrical tunable liquid crystal lens based on the modulation of anchoring energy. The latter can be easily obtained using photoalignment techniques. The liquid crystal cell we propose exhibits strong anchoring at the top substrate and anchoring energy with a parabolic profile at the bottom substrate. The model describes the dependence of the focal length on the applied voltage and presents a theoretical study of the lens aberrations. The results obtained are of general relevance and can be used to optimize the performances of every type of liquid crystal lens with a parabolic profile. Full article

(This article belongs to the Special Issue Feature Articles in Liquid Crystals from Our Editorial Board Members and Their Colleagues, 2022/2023)

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21 pages, 7219 KiB

Open AccessEditor’s ChoiceArticle

TIC Reorientation under Electric and Magnetic Fields in Homeotropic Samples of Cholesteric LC with Negative Dielectric Anisotropy

by Patrick Oswald, Guilhem Poy and Jordi Ignés-Mullol

Crystals 2023, 13(6), 957; https://doi.org/10.3390/cryst13060957 - 15 Jun 2023

Viewed by 670

Abstract

In this paper, we numerically and experimentally show that the director field orientation degeneracy within the Translationally Invariant Configuration (TIC) of a cholesteric liquid crystal under an electric field can be lifted by imposing a magnetic field

\vec{B}

parallel to the electrodes. [...] Read more.

In this paper, we numerically and experimentally show that the director field orientation degeneracy within the Translationally Invariant Configuration (TIC) of a cholesteric liquid crystal under an electric field can be lifted by imposing a magnetic field

\vec{B}

parallel to the electrodes. The configuration can be either parallel or perpendicular to the magnetic field depending on the values of the sample thickness, pitch, and applied voltage, with two equiprobable orientations in each case. The transition between the parallel and perpendicular orientations has hysteresis, suggesting that it is first order. When

\vec{B}

is slightly tilted with respect to the electrode plane, the indeterminacy on the TIC orientation is removed when the TIC is directed along

\vec{B}

. Full article

(This article belongs to the Special Issue Feature Articles in Liquid Crystals from Our Editorial Board Members and Their Colleagues, 2022/2023)

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11 pages, 7321 KiB

Open AccessArticle

Reconfiguration of Nematic Disclinations in Plane-Parallel Confinements

by Saša Harkai, Charles Rosenblatt and Samo Kralj

Crystals 2023, 13(6), 904; https://doi.org/10.3390/cryst13060904 - 01 Jun 2023

Cited by 1 | Viewed by 973

Abstract

We study numerically the reconfiguration process of colliding

|m| = 1 / 2

strength disclinations in an achiral nematic liquid crystal (NLC). A Landau–de Gennes approach in terms of tensor nematic-order parameters is used. Initially, different pairs

\{m_{1}, m_{2}\}

of [...] Read more.

We study numerically the reconfiguration process of colliding

|m| = 1 / 2

strength disclinations in an achiral nematic liquid crystal (NLC). A Landau–de Gennes approach in terms of tensor nematic-order parameters is used. Initially, different pairs

\{m_{1}, m_{2}\}

of parallel wedge disclination lines connecting opposite substrates confining the NLC in a plane-parallel cell of a thickness h are imposed: {1/2,1/2}, {−1/2,−1/2} and {−1/2,1/2}. The collisions are imposed by the relative rotation of the azimuthal angle

θ

of the substrates that strongly pin the defect end points. Pairs {1/2,1/2} and {−1/2,−1/2} “rewire” at the critical angle

θ_{c}^{(1)} = \frac{3 π}{4}

in all cases studied. On the other hand, two qualitatively different scenarios are observed for {−1/2,1/2}. In the thinner film regime

h < h_{c}

, the disclinations rewire at

θ_{c}^{(2)} = \frac{5 π}{4}

. The rewiring process is mediated by an additional chargeless loop nucleated in the middle of the cell. In the regime

h > h_{c}

, the colliding disclinations at

θ_{c}^{(2)}

reconfigure into boojum-like twist disclinations. Full article

(This article belongs to the Special Issue Feature Articles in Liquid Crystals from Our Editorial Board Members and Their Colleagues, 2022/2023)

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17 pages, 3002 KiB

Open AccessEditor’s ChoiceArticle

New Liquid Crystalline Elastomeric Films Containing a Smectic Crosslinker: Chemical and Physical Properties

by Andraz Resetic, Jerneja Milavec, Alexej Bubnov, Damian Pociecha, Vera Hamplova, Ewa Gorecka, Bostjan Zalar and Valentina Domenici

Crystals 2023, 13(1), 96; https://doi.org/10.3390/cryst13010096 - 04 Jan 2023

Cited by 2 | Viewed by 1698

Abstract

Side-chain liquid crystal elastomers (SC-LCEs) have been designed by using a new smectic crosslinker. Two types of monodomain films were prepared based on polysiloxane chains, with a different relative concentration of both crosslinker and mesogenic comonomers. The mesomorphic behavior of the two SC-LCE [...] Read more.

Side-chain liquid crystal elastomers (SC-LCEs) have been designed by using a new smectic crosslinker. Two types of monodomain films were prepared based on polysiloxane chains, with a different relative concentration of both crosslinker and mesogenic comonomers. The mesomorphic behavior of the two SC-LCE systems was investigated by differential scanning calorimetry and polarized optical microscopy showing a different mesomorphic behavior: in one case, we obtained a nematic SC-LCE film, in the other case, a Smectic A SC-LCE film. In both systems, the mesophases were stable in a wide temperature range. Moreover, the SC-LCE films possess a relatively high orientation at room temperature. The physical-chemical properties, such as the local orientational ordering, structural organization, and dynamics of SC-LCEs’ constituents were studied by means of static and dynamic ²H NMR experiments, small-angle X-ray, and wide-angle X-ray diffractions. The relevant physical properties, such as the thermo-elastic and thermo-mechanic behaviors, are reported and discussed in view of the practical applications. Full article

(This article belongs to the Special Issue Feature Articles in Liquid Crystals from Our Editorial Board Members and Their Colleagues, 2022/2023)

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8 pages, 2143 KiB

Open AccessArticle

Effect of Rod-like Nanoparticles on the Dielectric Susceptibility of Nematic Nano-Composites: A Molecular Theory

by Mikhail A. Osipov, Alexey S. Merekalov and Alexander A. Ezhov

Crystals 2022, 12(12), 1827; https://doi.org/10.3390/cryst12121827 - 15 Dec 2022

Viewed by 1549

Abstract

The effect of rod-like nanoparticles on the high-frequency dielectric susceptibility of the nematic nano-composites has been investigated in the framework of a molecular theory. Analytical expressions for the components of the effective polarizability of a rod-like nanoparticle in the nematic host have been [...] Read more.

The effect of rod-like nanoparticles on the high-frequency dielectric susceptibility of the nematic nano-composites has been investigated in the framework of a molecular theory. Analytical expressions for the components of the effective polarizability of a rod-like nanoparticle in the nematic host have been obtained and used in the calculations of the dielectric susceptibility of the composites as functions of the nanoparticle volume fraction. Numerical calculations of the susceptibility have been undertaken using the nematic liquid crystal 5CB as a host doped with either gold or silver particles for different values of the concentration of nanoparticles. It has been shown that the rod-like nanoparticles have a much stronger effect on the components of the dielectric susceptibility of the nano-composites including, in particular, the one with gold nanoparticles in the vicinity of the plasmon resonance. The main conclusion is that at sufficiently large concentration of nanoparticles, the anisotropy of the dielectric susceptibility of the nano-composites may even change the sign with an increasing concentration which may be important for various applications. Full article

(This article belongs to the Special Issue Feature Articles in Liquid Crystals from Our Editorial Board Members and Their Colleagues, 2022/2023)

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8 pages, 301 KiB

Open AccessEditor’s ChoiceArticle

Effective Chiral Interactions between Nonchiral Rigid Macromolecules in a Chiral Solvent and the Induced Cholesteric Liquid Crystal Phase

by Mikhail A. Osipov and Alexey S. Merekalov

Crystals 2022, 12(9), 1318; https://doi.org/10.3390/cryst12091318 - 18 Sep 2022

Cited by 1 | Viewed by 1569

Abstract

It has been shown that a nonchiral anisotropic macromolecule embedded in a chiral dielectric solvent possesses an effective optical activity proportional to the optical activity of the solvent. As a result, there exists an effective chiral interaction between the macromolecules, which creates a [...] Read more.

It has been shown that a nonchiral anisotropic macromolecule embedded in a chiral dielectric solvent possesses an effective optical activity proportional to the optical activity of the solvent. As a result, there exists an effective chiral interaction between the macromolecules, which creates a torque acting on the primary axes of the two interacting molecules. A general expression for the effective chiral interaction potential has been derived in terms of the effective polarizability and the effective gyration tensor of the macromolecule in the chiral solvent. Explicit expressions for the components of the effective polarizability and the gyration have been obtained using the model of a hard rod filled with anisotropic dielectric and embedded into the isotropic chiral dielectric medium. The theory predicts the formation of the cholesteric helical structure in the nematic polymer liquid crystal phase induced by a chiral solvent. Full article

(This article belongs to the Special Issue Feature Articles in Liquid Crystals from Our Editorial Board Members and Their Colleagues, 2022/2023)

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11 pages, 18128 KiB

Open AccessArticle

Achievement of Unidirectional Aluminum Tin Oxide/UV-Curable Polymer Hybrid Film via UV Nanoimprinting Lithography for Uniform Liquid Crystal Alignment

by Dong-Wook Lee, Dong-Hyun Kim, Jonghoon Won, Jin-Young Oh and Dae-Shik Seo

Crystals 2022, 12(6), 855; https://doi.org/10.3390/cryst12060855 - 17 Jun 2022

Viewed by 1585

Abstract

A uniform unidirectional nanostructure composed of aluminum tin oxide and ultraviolet (UV)-curable polymer is introduced herein. The nanostructure was produced by UV-nanoimprint lithography (UV-NIL), and the fabricated hybrid film was used as a uniform liquid crystal (LC) alignment layer. Atomic force microscopy and [...] Read more.

A uniform unidirectional nanostructure composed of aluminum tin oxide and ultraviolet (UV)-curable polymer is introduced herein. The nanostructure was produced by UV-nanoimprint lithography (UV-NIL), and the fabricated hybrid film was used as a uniform liquid crystal (LC) alignment layer. Atomic force microscopy and line profile analysis were performed to confirm a well-ordered nanostructure with 760 nm periodicity and 30 nm height. X-ray photoelectron spectroscopy analysis was also conducted to examine the chemical modifications to the hybrid film surface during UV exposure. Optical transmittance investigation of the nanopatterned hybrid film revealed its compatibility for LC device application. Stable, uniform, and homogeneous LC alignment on the hybrid film was confirmed by polarized optical microscopy observance and analysis of LC pretilt angle. The unidirectional structure on the film surface enabled uniform LC orientation along with surface anisotropy property. Hence, we expect that the proposed UV-NIL process can be applied to fabricate high-resolution unidirectional nanostructures with various inorganic/organic hybrid materials and that these nanostructures have high potential for next-generation LC systems. Full article

(This article belongs to the Special Issue Feature Articles in Liquid Crystals from Our Editorial Board Members and Their Colleagues, 2022/2023)

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9 pages, 1675 KiB

Open AccessEditor’s ChoiceArticle

Influence of Surface Relief on Orientation of Nematic Liquid Crystals: Polyimide Doped with WS₂ Nanotubes

by Natalia Kamanina, Andrey Toikka, Yaroslav Barnash, Alla Zak and Reshef Tenne

Crystals 2022, 12(3), 391; https://doi.org/10.3390/cryst12030391 - 14 Mar 2022

Cited by 8 | Viewed by 2201

Abstract

Among the different methods for orienting liquid crystal (LC) molecules, adding nanoparticles into the matrix of the substrate material towards modifying its surface, is actively pursued. In this context, the influence of the nanoparticle content on the texture of the surface of polymer [...] Read more.

Among the different methods for orienting liquid crystal (LC) molecules, adding nanoparticles into the matrix of the substrate material towards modifying its surface, is actively pursued. In this context, the influence of the nanoparticle content on the texture of the surface of polymer film used as the substrate for the LC orientation is of particular interest. Thus, in the current paper, WS₂ nanotubes were used to dope the polyimide (PI) substrate-film in order to modify and control its surface morphology/roughness and properties. The modified organic surface structure is applied in order to achieve a new means for controlling the orientation of the LC molecules. This tool adds to the classical methods for controlling the orientation of the LC molecules, such as the display technique. Full article

(This article belongs to the Special Issue Feature Articles in Liquid Crystals from Our Editorial Board Members and Their Colleagues, 2022/2023)

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Review

Jump to: Research

13 pages, 2507 KiB

Open AccessReview

Photosensitive Alignment: Advanced Electronic Paper-Based Devices

by Vladimir Chigrinov, Aleksey Kudreyko and Jiatong Sun

Crystals 2022, 12(3), 364; https://doi.org/10.3390/cryst12030364 - 09 Mar 2022

Cited by 5 | Viewed by 1939

Abstract

In this review we describe the reversible photoalignment effect imposed on the director in nematic liquid crystals that provides an approach for fabrication of advanced optically addressed devices. Several new concepts have been developed to render photosensitive materials during the past decade. Functional [...] Read more.

In this review we describe the reversible photoalignment effect imposed on the director in nematic liquid crystals that provides an approach for fabrication of advanced optically addressed devices. Several new concepts have been developed to render photosensitive materials during the past decade. Functional soft azo dye compounds exhibiting distinct functionalities in response to polarized light are highly desirable for fabrication of optically rewritable electronic paper. An optically rewritable element base using simple and inexpensive materials can potentially enable the development of novel environmentally friendly, paper-like gadgets with improved functionality over regular electronic paper. We argue that an optically rewritable technique is relevant for some applications, where conventional paper might be irrelevant. In particular, we have tested and discussed several techniques of color and 3D image formation. This strategy for fabrication of novel devices offers versatile methods for visualization. We also show that the intensity modulation of the irradiation light has a potential to generate improved grayscale visualization. This principle is based on the statistical distribution control of photosensitive azo dye molecules, driven by the incident polarized light. Additionally, we discuss the functional characteristics of the developed prototypes. Full article

(This article belongs to the Special Issue Feature Articles in Liquid Crystals from Our Editorial Board Members and Their Colleagues, 2022/2023)

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