Crystals

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3 pages, 152 KiB

Open AccessEditorial

Early Career Stars of the Decade

by Charles Rosenblatt and Helen F. Gleeson

Crystals 2021, 11(1), 52; https://doi.org/10.3390/cryst11010052 - 11 Jan 2021

Viewed by 1715

Abstract

The field of liquid crystals is constantly reinventing itself, evolving from measurements of elastic constants and devices such as the simple nematic twist cell in the 1970s to topological defects, nanoparticle inclusions, and smart sensors today [...] Full article

(This article belongs to the Special Issue Early Career Stars of the Decade)

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33 pages, 99480 KiB

Open AccessArticle

Time Dependent Lyotropic Chromonic Textures in Microfluidic Confinements

by Anshul Sharma, Irvine Lian Hao Ong and Anupam Sengupta

Crystals 2021, 11(1), 35; https://doi.org/10.3390/cryst11010035 - 30 Dec 2020

Cited by 14 | Viewed by 3924

Abstract

Nematic and columnar phases of lyotropic chromonic liquid crystals (LCLCs) have been long studied for their fundamental and applied prospects in material science and medical diagnostics. LCLC phases represent different self-assembled states of disc-shaped molecules, held together by noncovalent interactions that lead to [...] Read more.

Nematic and columnar phases of lyotropic chromonic liquid crystals (LCLCs) have been long studied for their fundamental and applied prospects in material science and medical diagnostics. LCLC phases represent different self-assembled states of disc-shaped molecules, held together by noncovalent interactions that lead to highly sensitive concentration and temperature dependent properties. Yet, microscale insights into confined LCLCs, specifically in the context of confinement geometry and surface properties, are lacking. Here, we report the emergence of time dependent textures in static disodium cromoglycate (DSCG) solutions, confined in PDMS-based microfluidic devices. We use a combination of soft lithography, surface characterization, and polarized optical imaging to generate and analyze the confinement-induced LCLC textures and demonstrate that over time, herringbone and spherulite textures emerge due to spontaneous nematic (N) to columnar M-phase transition, propagating from the LCLC-PDMS interface into the LCLC bulk. By varying the confinement geometry, anchoring conditions, and the initial DSCG concentration, we can systematically tune the temporal dynamics of the N- to M-phase transition and textural behavior of the confined LCLC. Overall, the time taken to change from nematic to the characteristic M-phase textures decreased as the confinement aspect ratio (width/depth) increased. For a given aspect ratio, the transition to the M-phase was generally faster in degenerate planar confinements, relative to the transition in homeotropic confinements. Since the static molecular states register the initial conditions for LC flows, the time dependent textures reported here suggest that the surface and confinement effects—even under static conditions—could be central in understanding the flow behavior of LCLCs and the associated transport properties of this versatile material. Full article

(This article belongs to the Special Issue Early Career Stars of the Decade)

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12 pages, 2351 KiB

Open AccessArticle

Degradation-Induced Actuation in Oxidation-Responsive Liquid Crystal Elastomers

by Mahjabeen Javed, Seelay Tasmim, Mustafa K. Abdelrahman, Cedric P. Ambulo and Taylor H. Ware

Crystals 2020, 10(5), 420; https://doi.org/10.3390/cryst10050420 - 25 May 2020

Cited by 10 | Viewed by 5172

Abstract

Stimuli-responsive materials that exhibit a mechanical response to specific biological conditions are of considerable interest for responsive, implantable medical devices. Herein, we report the synthesis, processing and characterization of oxidation-responsive liquid crystal elastomers that demonstrate programmable shape changes in response to reactive oxygen [...] Read more.

Stimuli-responsive materials that exhibit a mechanical response to specific biological conditions are of considerable interest for responsive, implantable medical devices. Herein, we report the synthesis, processing and characterization of oxidation-responsive liquid crystal elastomers that demonstrate programmable shape changes in response to reactive oxygen species. Direct ink writing (DIW) is used to fabricate Liquid Crystal Elastomers (LCEs) with programmed molecular orientation and anisotropic mechanical properties. LCE structures were immersed in different media (oxidative, basic and saline) at body temperature to measure in vitro degradation. Oxidation-sensitive hydrophobic thioether linkages transition to hydrophilic sulfoxide and sulfone groups. The introduction of these polar moieties brings about anisotropic swelling of the polymer network in an aqueous environment, inducing complex shape changes. 3D-printed uniaxial strips exhibit 8% contraction along the nematic director and 16% orthogonal expansion in oxidative media, while printed LCEs azimuthally deform into cones 19 times their original thickness. Ultimately, these LCEs degrade completely. In contrast, LCEs subjected to basic and saline solutions showed no apparent response. These oxidation-responsive LCEs with programmable shape changes may enable a wide range of applications in target specific drug delivery systems and other diagnostic and therapeutic tools. Full article

(This article belongs to the Special Issue Early Career Stars of the Decade)

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10 pages, 5394 KiB

Open AccessArticle

Interactions on the Interface between Two Liquid Crystal Materials

by Rok Geršak and Simon Čopar

Crystals 2020, 10(5), 393; https://doi.org/10.3390/cryst10050393 - 14 May 2020

Cited by 3 | Viewed by 3471

Abstract

In liquid crystal applications, boundary conditions are essential to ensuring suitable bulk molecular orientation and a deterministic response to external fields. Be it confinement to a droplet or a shell, a glass plate, or an interface with air or another liquid, proper surface [...] Read more.

In liquid crystal applications, boundary conditions are essential to ensuring suitable bulk molecular orientation and a deterministic response to external fields. Be it confinement to a droplet or a shell, a glass plate, or an interface with air or another liquid, proper surface alignment must be ensured—mechanically by rubbing, by chemical treatment that adds a layer of aligning molecules, by using photoalignment or even by leaving the surface untreated, using the intrinsic properties of the substrate itself. The anchoring can be classified as unidirectional (perpendicular homeotropic, or at oblique angles), or degenerate (planar or pre-tilted). However, if both substances at the interface are anisotropic, more diverse behaviour is expected. Here, we present a numerical simulation of a nematic droplet in a nematic host, and investigate behaviour of the director field and defects at the interface for different interfacial couplings. Finally, we compare the simulations to experimental images of discotic droplets in a calamitic nematic host. Full article

(This article belongs to the Special Issue Early Career Stars of the Decade)

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14 pages, 4436 KiB

Open AccessArticle

Toward Programmed Complex Stress-Induced Mechanical Deformations of Liquid Crystal Elastomers

by Devesh Mistry and Helen F. Gleeson

Crystals 2020, 10(4), 315; https://doi.org/10.3390/cryst10040315 - 18 Apr 2020

Cited by 3 | Viewed by 3362

Abstract

We prepare a liquid crystal elastomer (LCE) with a spatially patterned liquid crystal director field from an all-acrylate LCE. Mechanical deformations of this material lead to a complex and spatially varying deformation with localised body rotations, shears and extensions. Together, these dictate the [...] Read more.

We prepare a liquid crystal elastomer (LCE) with a spatially patterned liquid crystal director field from an all-acrylate LCE. Mechanical deformations of this material lead to a complex and spatially varying deformation with localised body rotations, shears and extensions. Together, these dictate the evolved shape of the deformed film. Using polarising microscopy, we map the local rotation of the liquid crystal director in Eulerian and Lagrangian frames and use these to determine rules for programming complex, stress-induced mechanical shape deformations of LCEs. Moreover, by applying a recently developed empirical model for the mechanical behaviour of our LCE, we predict the non-uniform stress distributions in our material. These results show the promise of empirical approaches to modelling the anisotropic and nonlinear mechanical responses of LCEs which will be important as the community moves toward realising real-world, LCE-based devices. Full article

(This article belongs to the Special Issue Early Career Stars of the Decade)

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12 pages, 5599 KiB

Open AccessArticle

Topological Defect Arrays in Nematic Liquid Crystals Assisted by Polymeric Pillar Arrays: Effect of the Geometry of Pillars

by MinSu Kim and Francesca Serra

Crystals 2020, 10(4), 314; https://doi.org/10.3390/cryst10040314 - 18 Apr 2020

Cited by 13 | Viewed by 4659

Abstract

Topological defects that spontaneously occur in condensed matter and structured fluids such as liquid crystals are useful for their elastic and optical properties, but often the applicability of defect arrays to optics and photonic devices relies on the regularity and tunability of the [...] Read more.

Topological defects that spontaneously occur in condensed matter and structured fluids such as liquid crystals are useful for their elastic and optical properties, but often the applicability of defect arrays to optics and photonic devices relies on the regularity and tunability of the system. In our recent work [Adv. Opt. Mater. 8, 1900991 (2020)], we showed the formation of regular, reconfigurable, and scalable patterns by exploiting the elastic response of a defect array in liquid crystals in the presence of a polymeric pillar array. In this work, we experimentally investigate the role of size and shape of the pillars on the defect array. We find that the pillar size and geometry provide additional means to regulate the response time, the threshold voltage for the defects’ formation, and the spatial arrangement of the defects. Full article

(This article belongs to the Special Issue Early Career Stars of the Decade)

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16 pages, 8182 KiB

Open AccessArticle

Twist-Bend Nematogenic Supramolecular Dimers and Trimers Formed by Hydrogen Bonding

by Rebecca Walker, Damian Pociecha, Alfonso Martinez-Felipe, John MD Storey, Ewa Gorecka and Corrie T Imrie

Crystals 2020, 10(3), 175; https://doi.org/10.3390/cryst10030175 - 6 Mar 2020

Cited by 32 | Viewed by 4182

Abstract

A selection of novel non-symmetric supramolecular liquid crystal dimers and trimers formed by hydrogen-bonding have been prepared and their phase behaviour characterised by polarised optical microscopy, X-ray diffraction and temperature-dependent Fourier-transform infrared spectroscopy. We mix the bent twist-bend nematogen 4-[6-(4’-cyanobiphenyl-4-yl) hexyloxy]-benzoic acid (CB6OBA) [...] Read more.

A selection of novel non-symmetric supramolecular liquid crystal dimers and trimers formed by hydrogen-bonding have been prepared and their phase behaviour characterised by polarised optical microscopy, X-ray diffraction and temperature-dependent Fourier-transform infrared spectroscopy. We mix the bent twist-bend nematogen 4-[6-(4’-cyanobiphenyl-4-yl) hexyloxy]-benzoic acid (CB6OBA) with a series of small stilbazole-based compounds 4-[(E)-2-(n-alkoxyphenyl)ethenyl]pyridines (nOS) of varying terminal chain length (n) to obtain the CB6OBA:nOS complexes. Complexes with n ≤ 7 exhibit nematic and twist-bend nematic behaviour, followed on cooling by a smectic C phase for n = 4–7, and finally, a hexatic-type smectic X phase for n = 3–7. Mixtures with n = 8–10 exhibit a smectic A phase below the conventional nematic phase, and on further cooling, a biaxial smectic A_b phase and the same hexatic-type SmX phase. Supramolecular trimers, CB6OBA:CB6OS and CB6OBA:1OB6OS, formed between CB6OBA and dimeric stilbazoles [(E)-2-(4-{[6-(4’-methoxy[1,1’-biphenyl]-4-yl)hexyl]oxy}phenyl)ethenyl]pyridine (1OB6OS) or 4-[(E)-4’-(6-{4-[(E)-2-(pyridin-4-yl)ethenyl]phenoxy}hexyl)[1,1’-biphenyl]-4- carbonitrile (CB6OS), exhibit nematic and twist-bend nematic phases, and are the first hydrogen-bonded trimers consisting of unlike donor and acceptor fragments to do so. Full article

(This article belongs to the Special Issue Early Career Stars of the Decade)

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12 pages, 5736 KiB

Open AccessArticle

Dielectric and Electro-Optic Effects in a Nematic Liquid Crystal Doped with h-BN Flakes

by Rajratan Basu, Lukas J. Atwood and George W. Sterling

Crystals 2020, 10(2), 123; https://doi.org/10.3390/cryst10020123 - 16 Feb 2020

Cited by 15 | Viewed by 4739

Abstract

A small quantity of hexagonal boron nitride (h-BN) flakes is doped into a nematic liquid crystal (LC). The epitaxial interaction between the LC molecules and the h-BN flakes rising from the π−π electron stacking between the LC’s benzene [...] Read more.

A small quantity of hexagonal boron nitride (h-BN) flakes is doped into a nematic liquid crystal (LC). The epitaxial interaction between the LC molecules and the h-BN flakes rising from the π−π electron stacking between the LC’s benzene rings and the h-BN’s honeycomb structure stabilizes pseudo-nematic domains surrounding the h-BN flakes. Electric field-dependent dielectric studies reveal that the LC-jacketed h-BN flakes follow the nematic director reorientation upon increasing the applied electric field. These anisotropic pseudo-nematic domains exist in the isotropic phase of the LC+h-BN system as well, and interact with the external electric field, giving rise to a nonzero dielectric anisotropy in the isotropic phase. Further investigations reveal that the presence of the h-BN flakes at a low concentration in the nematic LC enhances the elastic constants, reduces the rotation viscosity, and lowers the pre-tilt angle of the LC. However, the Fréedericksz threshold voltage stays mostly unaffected in the presence of the h-BN flakes. Additional studies show that the presence of the h-BN flakes enhances the effective polar anchoring strength in the cell. The enhanced polar anchoring strength and the reduced rotational viscosity result in faster electro-optic switching in the h-BN-doped LC cell. Full article

(This article belongs to the Special Issue Early Career Stars of the Decade)

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Review

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18 pages, 5724 KiB

Open AccessReview

Liquid Crystals Templating

by Mamatha Nagaraj

Crystals 2020, 10(8), 648; https://doi.org/10.3390/cryst10080648 - 27 Jul 2020

Cited by 13 | Viewed by 8306

Abstract

Liquid crystal templating is a versatile technique to create novel organic and inorganic materials with nanoscale features. It exploits the self-assembled architectures of liquid crystal phases as scaffolds. This article focuses on some of the key developments in lyotropic and thermotropic liquid crystals [...] Read more.

Liquid crystal templating is a versatile technique to create novel organic and inorganic materials with nanoscale features. It exploits the self-assembled architectures of liquid crystal phases as scaffolds. This article focuses on some of the key developments in lyotropic and thermotropic liquid crystals templating. The procedures that were employed to create templated structures and the applications of these novel materials in various fields including mesoporous membranes, organic electronics, the synthesis of nanostructured materials and photonics, are described. Full article

(This article belongs to the Special Issue Early Career Stars of the Decade)

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