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See detailTopological Defect-Guided Regular Stacking of Focal Conic Domains in Hybrid-Aligned Smectic Liquid Crystal Shells
Noh, Junghyun; Lagerwall, Jan UL

in Crystals (2021), 11(8), 913

We study liquid crystal (LC) shells in hybrid configuration (director tangential to the inside but normal to the outside) as they slowly undergo a transition from a nematic (N) to a smectic-A (SmA) phase ... [more ▼]

We study liquid crystal (LC) shells in hybrid configuration (director tangential to the inside but normal to the outside) as they slowly undergo a transition from a nematic (N) to a smectic-A (SmA) phase. Every shell has two antipodal +1 topological defects, at the thinnest and thickest points, respectively. On cooling from N to SmA, the symmetry axis connecting the defects gradually reorients from blackalong gravity to perpendicular to it, reversibly and continuously, if the LC and aqueous phase are density matched at the N-SmA transition. This suggests reduced density near the defects---reflecting a local reduction in order---under the strong confinement with antagonistic boundary conditions. In the SmA phase, a regular array of focal conic domains (FCDs) develops, templated in position and orientation by the +1 defect at the thinnest point. Around this defect, a single complete toroidal FCD always develops, surrounded by incomplete FCDs. In contrast to similar FCD arrangements on flat aqueous interfaces, this is a stable situation, since the two +1 defects are required by the spherical topology. Our results demonstrate how the topological defects of LC shells can be used to template complex self-organized structures. blackWith a suitable adaption of the LC chemistry, shells might serve as a basis for producing solid particles with complex yet highly regular morphologies. [less ▲]

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See detailMeasuring the Anisotropy in Interfacial Tension of Nematic Liquid Crystals
Honaker, Lawrence; Sharma, Anjali UL; Schanen, Andy et al

in Crystals (2021), 11(6), 687

<jats:p>Liquid crystal (LC) phases typically show anisotropic alignment-dependent properties, such as viscosity and dielectric permittivity, so it stands to reason that LCs also have anisotropic ... [more ▼]

<jats:p>Liquid crystal (LC) phases typically show anisotropic alignment-dependent properties, such as viscosity and dielectric permittivity, so it stands to reason that LCs also have anisotropic interfacial tensions. Measuring the interfacial tension ? of an LC with conventional methods, such as pendant drops, can be challenging, however, especially when we need to know ? for different LC aligning conditions, as is the case when we seek ??, the interfacial tension anisotropy. Here, we present measurements of ?? of the common synthetic nematic LC compound 5CB against water using a microfluidic droplet aspiration technique. To ensure tangential and normal alignment, respectively, we add poly(vinyl alcohol) (PVA) and sodium dodecylsulfate (SDS), respectively, as a stabilizer and measure ? for different concentrations of stabilizer. By fitting the Szyszkowski equation to the data, we can extrapolate to zero-stabilizer concentration, obtaining the ? of 5CB to pure water for each alignment. For normal alignment, we find ??=31.9?0.8 mN?m-1, on the order of 1 mN?m-1 greater than ?$_$=30.8?5 mN?m-1 for tangential alignment. This resonates with the empirical knowledge that 5CB aligns tangentially to an interface with pure water. The main uncertainty arises from the use of polymeric PVA as tangential-promoting stabilizer. Future improvements in accuracy may be expected if PVA can be replaced by a low molar mass stabilizer that ensures tangential alignment.</jats:p [less ▲]

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See detailTime dependent lyotropic chromonic textures in PDMS-based microfluidic confinements
Sharma, Anshul; Ong, Irvine UL; Sengupta, Anupam UL

in Crystals (2021), 11(1),

Nematic and columnar phases of lyotropic chromonic liquid crystals (LCLCs) have been long studiedfor their fundamental and applied prospects in material science and medical diagnostics. LCLC ... [more ▼]

Nematic and columnar phases of lyotropic chromonic liquid crystals (LCLCs) have been long studiedfor their fundamental and applied prospects in material science and medical diagnostics. LCLC phasesrepresent different self-assembled states of disc-shaped molecules, held together by noncovalentinteractions that lead to highly sensitive concentration and temperature dependent properties. Yet,microscale insights into confined LCLCs, specifically in the context of confinement geometry andsurface properties, are lacking. Here, we report the emergence of time dependent textures in staticdisodium chromoglycate (DSCG) solutions, confined in PDMS-based microfluidic devices. We use acombination of soft lithography, surface characterization and polarized optical imaging to generateand analyze the confinement-induced LCLC textures, and demonstrate that over time, herringboneand 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 confinementgeometry, anchoring conditions and the initial DSCG concentration, we can systematically tune thetemporal dynamics of the N to M-phase transition and textural behaviour of the confined LCLC. Sincestatic molecular states register the initial conditions for LC flows, the time dependent boundary andbulk conditions reported here suggest that the local surface-mediated dynamics could be central inunderstanding LCLC flows, and in turn, the associated transport properties of this versatile material. [less ▲]

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See detailTime dependent lyotropic chromonic textures in PDMS-based microfluidic confinements
Sharma, Anshul; Ong, Irvine UL; Sengupta, Anupam UL

in Crystals (2021), 11(1), 35

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 ... [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 materia [less ▲]

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See detailFrom Equilibrium Liquid Crystal Formation and Kinetic Arrest to Photonic Bandgap Films Using Suspensions of Cellulose Nanocrystals
Schütz, Christina; Bruckner, Johanna R.; Honorato-Rios, Camila et al

in Crystals (2020), 10(3), 199

The lyotropic cholesteric liquid crystal phase developed by suspensions of cellulose nanocrystals (CNCs) has come increasingly into focus from numerous directions over the last few years. In part, this is ... [more ▼]

The lyotropic cholesteric liquid crystal phase developed by suspensions of cellulose nanocrystals (CNCs) has come increasingly into focus from numerous directions over the last few years. In part, this is because CNC suspensions are sustainably produced aqueous suspensions of a fully bio-derived nanomaterial with attractive properties. Equally important is the interesting and useful behavior exhibited by solid CNC films, created by drying a cholesteric-forming suspension. However, the pathway along which these films are realized, starting from a CNC suspension that may have low enough concentration to be fully isotropic, is more complex than often appreciated, leading to reproducibility problems and confusion. Addressing a broad audience of physicists, chemists, materials scientists and engineers, this Review focuses primarily on the physics and physical chemistry of CNC suspensions and the process of drying them. The ambition is to explain rather than to repeat, hence we spend more time than usual on the meanings and relevance of the key colloid and liquid crystal science concepts that must be mastered in order to understand the behavior of CNC suspensions, and we present some interesting analyses, arguments and data for the first time. We go through the development of cholesteric nuclei (tactoids) from the isotropic phase and their potential impact on the final dry films; the spontaneous CNC fractionation that takes place in the phase coexistence window; the kinetic arrest that sets in when the CNC mass fraction reaches ~10 wt.\%, preserving the cholesteric helical order until the film has dried; the 'coffee-ring effect' active prior to kinetic arrest, often ruining the uniformity in the produced films; and the compression of the helix during the final water evaporation, giving rise to visible structural color in the films. [less ▲]

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See detailCoherent Magnetization Rotation of a Layered System Observed by Polarized Neutron Scattering under Grazing Incidence Geometry
Maruyama, Ryuji; Bigault, Thierry; Saerbeck, Thomas et al

in Crystals (2019), 9(8), 383

The in-plane magnetic structure of a layered system composed of polycrystalline grains smaller than the ferromagnetic exchange length was studied to elucidate the mechanism controlling the magnetic ... [more ▼]

The in-plane magnetic structure of a layered system composed of polycrystalline grains smaller than the ferromagnetic exchange length was studied to elucidate the mechanism controlling the magnetic properties considerably different from the bulk using polarized neutron scattering under grazing incidence geometry. The measured result, together with quantitative analysis based on the distorted wave Born approximation, showed that the in-plane length of the area with a uniform orientation of moments ranging from 0.5–1.1 μ m was not significantly varied during the process of demagnetization followed by remagnetization. The obtained behavior of moments is in good agreement with the two-dimensional random anisotropy model where coherent magnetization rotation is dominant. [less ▲]

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