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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. 

An escaped radial director profile in a nematic liquid crystal cell can be transformed into a pair of strength m = +1/2 surface defects (and their associated disclination lines) at a threshold electric field. Analogously, a half-integer defect pair can be transformed at a threshold electric field into a director profile that escapes into the third dimension. These transitions were demonstrated experimentally and numerically, and are discussed in terms of topologically discontinuous and continuous pathways that connect the two states. Additionally, we note that the pair of disclination lines associated with the m = +1/2 surface defects were observed to co-rotate around a common point for a sufficiently large electric field at a sufficiently low frequency.