Chiral structures from achiral liquid crystals in cylindrical capillaries

Significance Nematic liquid crystals (LCs) are arguably the simplest examples of partially ordered condensed matter, and they are core materials in many commercial products. Our experiments explore fundamental questions about how chiral configurations of LCs can arise from achiral building blocks. Left- and right-handed chiral structures are produced by a delicate balance of LC bulk elasticity and surface conditions in confinement. The key experimental ingredients are biocompatible aqueous lyotropic chromonic LCs that twist easily. Combined with the new constraints, this class of achiral LC exhibits chiral structures and a rich assortment of defects, which hint at applications in sensing and optics. We study chiral symmetry-broken configurations of nematic liquid crystals (LCs) confined to cylindrical capillaries with homeotropic anchoring on the cylinder walls (i.e., perpendicular surface alignment). Interestingly, achiral nematic LCs with comparatively small twist elastic moduli relieve bend and splay deformations by introducing twist deformations. In the resulting twisted and escaped radial (TER) configuration, LC directors are parallel to the cylindrical axis near the center, but to attain radial orientation near the capillary wall, they escape along the radius through bend and twist distortions. Chiral symmetry-breaking experiments in polymer-coated capillaries are carried out using Sunset Yellow FCF, a lyotropic chromonic LC with a small twist elastic constant. Its director configurations are investigated by polarized optical microscopy and explained theoretically with numerical calculations. A rich phenomenology of defects also arises from the degenerate bend/twist deformations of the TER configuration, including a nonsingular domain wall separating domains of opposite twist handedness but the same escape direction and singular point defects (hedgehogs) separating domains of opposite escape direction. We show the energetic preference for singular defects separating domains of opposite twist handedness compared with those of the same handedness, and we report remarkable chiral configurations with a double helix of disclination lines along the cylindrical axis. These findings show archetypally how simple boundary conditions and elastic anisotropy of confined materials lead to multiple symmetry breaking and how these broken symmetries combine to create a variety of defects.