Ringlike cores of cylindrically confined nematic point defects

Abstract

Nematic liquid crystals confined in a cylindrical capillary and subjected to strong homeotropic anchoring conditions is a long-studied fundamental problem that uniquely incorporates nonlinearity, topological stability, defects, and texture physics. The observed and predicted textures that continue to be investigated include escape radial, radial with a line defect, planar polar with two line defects, and periodic array of point defects. This paper presents theory and multiscale simulations of global and fine scale textures of nematic point defects, based on the Landau-de Gennes tensor order parameter equations. The aim of this paper is to further investigate the ringlike nature of point defect cores and its importance on texture transformation mechanisms and stability. The paper shows that the ringlike cores can be oriented either along the cylinder axis or along the radial direction. Axial rings can partially expand but are constrained by the capillary sidewalls. Radial rings can deform into elliptical structures whose major axis is along the capillary axis. The transformation between several families of textures under capillary confinement as well as their stability is discussed in terms of defect ring distortions. A unified view of nematic textures found in the cylindrical cavities is provided.

Experimental Insights into the Nanostructure of the Cores of Topological Defects in Liquid Crystals

“…Third, in contrast to the defects of positive strength, our model predicts the -1 point defect to be stable relative to a -1/2 loop disclination (∆F > 0), both in the absence and presence of amphiphiles (blue curves in Fig. 4; see detailed discussion in SM) [27,28]. This result is consistent with our above-stated experimental observation of the absence of molecular assemblies within apparent -1 point defects, and hints that the cores of different defects have distinct influences on processes of molecular self-assembly.…”

Liquid crystal models of biological materials and silk spinning

“…Because the points defects are loops, two topological processes are possible: (a) a disclinations loop can expand and break into a pair of disclination lines, and (b) a loop can react with another loop to form a ring whose charge is the sum of the initial two loops. 91 These two topological …”

“…16,23,90,91 For rod-like nematics in capillaries and homeotropic anchoring, a periodic array of alternating þ1 and 21 point defects along the axis is usually observed; the defect-defect separation distance is of the order of the capillary radius.…”

“…Given that point defect are disclination loops, the behavior of these loops under capillary confinement has implications to fiber textures and to topological transformations. 16,28,91 Consider the capillary confinement of a LC such as that shown in Figure 14. Because the points defects are loops, two topological processes are possible: (a) a disclinations loop can expand and break into a pair of disclination lines, and (b) a loop can react with another loop to form a ring whose charge is the sum of the initial two loops.…”

“…The original disclination loop is stable at U 5 6 only for capillary radius R [ 21.5 ' i . 90,91 The originally circular disclination loop is oriented with its unit normal along the radial direction of the cylindrical capillary and the loop expansion axis is the axial z-coordinate. Once the lines form, they drift apart due to repulsive forces and settle at distance slightly larger than the fiber radius.…”