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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 m1,m2 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 θc1=3π4 in all cases studied. On the other hand, two qualitatively different scenarios are observed for {−1/2,1/2}. In the thinner film regime hhc, the colliding disclinations at θc2 reconfigure into boojum-like twist disclinations. 

A thin Smectic-A liquid crystal (LC) film is deposited on a polymer vinyl alcohol-coated substrate that had been scribed with a uniform easy axis pattern over a square of side length L ≤ 85 μm. The small size of the patterned region facilitates material distribution to form either a hill (for a thin film) or divot (for a thick film) above the scribed square and having an oily streak (OS) texture. Optical profilometry measurements vs. film thickness suggest that the OS structure aims to adopt a preferred thickness z 0 that depends on the nature of the molecule, the temperature, and the surface tension at the air interface. We present a phenomenological model that estimates the energy cost of the OS layer as its thickness deviates from z 0 . 

A thin Smectic-A liquid crystal (LC) film is deposited on a polymer vinyl alcohol-coated substrate that had been scribed with a uniform easy axis pattern over a small square of dimensions side L < 85 um. Because of the small size of the patterned region facilitates material distribution to form either a hill (for a thin film) or divot (for a thick film) above the scribed square, which that exhibits an oily streak (OS) texture. Optical profilometry measurements vs. film thickness suggest that the OS structure aims to adopt a preferred thickness z0 that depends on the nature of the molecule, the temperature, and the surface tension at the air interface. We present a phenomenological model that estimates the energy cost of the OS layer as its thickness deviates from z0. 

A wedge cell made of homeotropically treated glass plates is filled with a chirally doped nematic liquid crystal. When a sufficiently large magnetic field is applied in the cell plane, a bend-like distortion occurs above a Fréedericksz threshold field H th . H th is reduced from the achiral case because of a field-induced bend distortion that facilitates a chiral twist distortion. Measurements of H th vs sample thickness are reported and compared favorably with a theoretical model presented herein. A further theoretical comparison is made between H th and the electric-field-induced transition in a geometry, exhibiting a 2π azimuthal degeneracy. The results may have technological implications in, for example, in-plane switching devices. 

A wedge cell made of homeotropically treated glass plates is filled with a chirally doped nematic liquid crystal. When a sufficiently large magnetic field is applied in the cell plane, a bend-like distortion occurs above a Fréedericksz threshold field H th . H th is reduced from the achiral case because of a field-induced bend distortion that facilitates a chiral twist distortion. Measurements of H th vs sample thickness are reported and compared favorably with a theoretical model presented herein. A further theoretical comparison is made between H th and the electric-field-induced transition in a geometry, exhibiting a 2π azimuthal degeneracy. The results may have technological implications in, for example, in-plane switching devices. 

A patterned surface defect of strength m = +1 and its associated disclination lines can decompose into a pair of surface defects and disclination lines of strength m = +1/2. For a negative dielectric anisotropy liquid crystal subjected to an applied ac electric field E , these half-integer defects are observed to wobble azimuthally for E > than some threshold field and, for sufficiently large fields, to co-revolve antipodally around a central point approximately midway between the two defects. This behavior is elucidated experimentally as a function of applied field strength E and frequency ν , where the threshold field for full co-revolution scales as ν 1/2 . Concurrently, nematic electrohydrodynamic instabilities were investigated. A complete field vs. frequency “phase diagram” compellingly suggests that the induced fluctuations and eventual co-revolutions of the ordinarily static defects are coupled strongly to—and driven by—the presence of the hydrodynamic instability. The observed behaviour suggests a Lehmann-like mechanism that drives the co-revolution. 

Topological line defects are ubiquitous in nature and appear at all physical scales, including in condensed matter systems, nuclear physics, and cosmology. Particularly useful systems to study line defects are nematic liquid crystals (LCs), where they describe singular or nonsingular frustrations in orientational order and are referred to as disclinations. In nematic LCs, line defects could be relatively simply created, manipulated, and observed. We consider cases where disclinations are stabilized either topologically in plane-parallel confinements or by chirality. In the former case, we report on studies in which defect core transformations are investigated, the intriguing dynamics of strength disclinations in LCs exhibiting negative dielectric anisotropy, and stabilization and manipulation of assemblies of defects. For the case of chiral nematics, we consider nanoparticle-driven stabilization of defect lattices. The resulting line defect assemblies could pave the way to several applications in photonics, sensitive detectors, and information storage devices. These excitations, moreover, have numerous analogs in other branches of physics. Studying their universal properties in nematics could deepen understanding of several phenomena, which are still unresolved at the fundamental level. 

We study theoretically and numerically chirality and saddle-splay elastic constant (K_24) enabled stability of multiple-twist-like nematic liquid crystal (LC) structures in cylindrical confinement. We focus on the so-called radially-z-twisted (RZT) and radially-twisted (RT) configurations, which simultaneously exhibit twists in different spatial directions. We express free energies of the structures in terms of dimensionless wave vectors, which characterise the structures and play the role of order parameters. The impact of different confinement anchoring conditions is explored. A simple Landau-type analysis provides insight into how different model parameters influence the stability of structures. We determine conditions for which the structures are stable in chiral and also nonchiral LCs. In particular, we find that the RZT structure could exhibit macroscopic chirality inversion on varying the relevant parameters. This phenomenon could be exploited for measurements of K_24. 

We studied numerically external stimuli enforced annihilation of a pair of daughter nematic topological defect (TD) assemblies bearing a relatively strong topological charge |m|=3/2. A Landau- de Gennes phenomenological approach in terms of tensor nematic order parameter was used in an effectively two-dimensional Cartesian coordinate system, where spatial variations along the z-axis were neglected. A pair of {m=3/2,m=−3/2} was enforced by an appropriate surface anchoring field, mimicking an experimental sample realization using the atomic force microscope (AFM) scribing method. Furthermore, defects were confined within a rectangular boundary that imposes strong tangential anchoring. This setup enabled complex and counter-intuitive annihilation processes on varying relevant parameters. We present two qualitatively different annihilation paths, where we either gradually reduced the relative surface anchoring field importance or increased an external in-plane spatially homogeneous electric field E. The creation and depinning of additional defect pairs {12,−12} mediated the annihilation in such a geometry. Furthermore, we illustrate the absorption of TDs by sharp edges of the confining boundary, accompanied by m=±1/4↔∓1/4 winding reversal of edge singularities, and also E-driven zero-dimensional to one-dimensional defect core transformation.