Elasticity Theory And Topological Defects In Nematic Liquid Crystals PDF Download

Author: Cheng Long
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Languages : en
Pages : 0

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Book Description
Liquid crystal is a state of matter where constituents show orientational order, despite lack of translational order. For regular nematic liquid crystals, the ground state of orientational distribution of mesogens is described by a single axis, known as the director. Due to effects such as surface anchoring or chiral nature of added liquid crystal molecules, the uniformity in an orientational order field can be broken. The short-range spatial correlation persisting in the orientational order field, as well as topological defects enabled by the uniaxial symmetry manifested from the local orientational order of a nematic liquid crystal, often gives rise to abundant intriguing and sophisticated pattern formation in nematic liquid crystals. Studying the pattern formation and the topological defects in those orientational order fields is essential for understanding rheological and optical properties of nematic liquid crystals. Employing analytical and numerical tools, this dissertation explores the implications of elasticity theory which is commonly used to characterize the deformation of a uniform orientational order field, and the motion of different topological defects in nematic liquid crystals. In the conventional Oseen-Frank elasticity theory, a uniform ground state is protected by the elastic constants satisfying Ericksen inequalities. To examine the scope of the elasticity theory beyond the Ericksen inequalities, we revisit the Oseen-Frank elasticity theory for nematic liquid crystals from the perspective of a reformulated form and find a new set of necessary inequalities for Frank elastic constants to ensure the existence of stable solutions, which is weaker than the Ericksen inequalities. We therefore identify a regime where the Ericksen inequalities are violated but the system is still stable. Remarkably, lyotropic chromonic liquid crystals are in that regime. We investigate the nonuniform structure of the director field in that regime, show that it depends sensitively on system geometry, and discuss the implications for lyotropic chromonic liquid crystals. Applying the same reformulated elasticity theory, we prove that geometric frustration exists in cholesteric liquid crystals. We explicitly demonstrate influences of geometric frustration in two models. First, we consider a chiral liquid crystal confined in a long cylinder with free boundaries. When the radius of the tube is sufficiently small, the director field forms a double-twist configuration, which is the ideal local structure. However, when the radius becomes large enough, due to the geometric frustration, the director field transforms into either a cholesteric phase with single twist, or a set of double-twist regions separated by disclinations, depending on the ratio of disclination energy density to elastic energy density. Second, we study a cholesteric liquid crystal confined between two infinite parallel plates with free boundaries, and we find that geometric frustration induces buckled helical cholesteric structure close to the free boundaries, reminiscent of the Helfrich-Hurault instability. Inspired by the experimental observation that skyrmions in cholesteric liquid crystals can move like particles under applied electric fields, we propose a general theoretical methodology for studying the motion of localized topological objects in liquid crystals, based on collective coordinate method. In our method, the continuum field of a topological soliton is represented by a few macroscopic degrees of freedom, including the position of the excitation and the orientation of the background field, and the motion of the topological soliton is thus derived from the equations of motion for those macroscopic degrees of freedom. Using the coarse-grained method, we elucidate the mechanism of moving solitons and skyrmions in a toggling field. Finally, to understand disclinations, an important class of topological defects in liquid crystals, we build a simple nematic order tensor model for a disclination in a nematic liquid crystal clarifying an analytical relation between the properties of the tensor field close to a disclination and the rotation axis of the nematic orientation around the disclination, which turns out to be an important quantity for the behaviors of a disclination. Analogous to a dislocation in a solid, we find that a Peach-Koehler force can be induced to drive a disclination to move by applying an effective external stress, and that the force is closely related to the rotation axis of the nematic orientation. With the help of the Peach-Koehler force, we further develop a theoretical model for explaining the Frank- Read mechanism in nematic liquid crystals, where a pinned disclination can be multiplied under an effective external stress.