Spontaneous emergence of twist and the formation of a monodomain in lyotropic chromonic liquid crystals confined to capillaries

Having a well-aligned monodomain is critical to performing fundamental studies on liquid crystals as well as exploiting them for technological applications. Unlike conventional nematics, lyotropic chromonic liquid crystals are notoriously hard to align. Here, we report on the homogeneous planar alignment of Sunset Yellow in a flat rectangular capillary. Remarkably, the in-plane director aligns perpendicular to the long axis of the rectangular capillary resulting in a counterintuitive configuration. We rationalize the evolution of this configuration from a metastable twisted configuration by considering the coupling of the curvature of the edges of the rectangle to the Frank free energy via the saddle-splay contribution. In contrast, for a square capillary, the doubly-twisted configuration is the final ground state. Using a simple scaling argument, we show that the aspect ratio of the confining geometry determines the ground state. Trapping biosensitive liquid crystals inside a rectangular capillary cell induces spontaneous alignments useful for device purposes. Lyotropic chromonic liquid crystals (LCLCs) undergo distinct optical changes when their core, a rod-like assembly of polyaromatic molecules, is disrupted by compounds such as antibody–antigen complexes. A team led by Mohan Srinivasarao of the Georgia Institute of Technology, USA, reports that differently shaped confinement chambers can manipulate the initial alignment of an LCLC called Sunset Yellow. A flickering pattern observed by polarized light microscopy revealed that flat rectangular capillaries cause Sunset Yellow to twist into a metastable configuration and then arrange into a well-ordered layer perpendicular to the rectangle's long axis. In square capillaries, however, the LCLCs remained in permanently twisted – a strong indication that surface-generated energy distortions can be controlled by geometric aspect ratios. Having a well aligned monodomain is critical to performing fundamental studies on liquid crystals as well as exploiting them for technological applications. Unlike conventional nematics, lyotropic chromonic liquid crystals are notoriously hard to align. Here, we report on the homogeneous planar alignment of Sunset Yellow in a flat rectangular capillary. Counterintuitively, the in-plane director aligns perpendicular to the long axis of the rectangular capillary. We rationalize the evolution of this configuration from a metastable twisted configuration by considering the coupling of the curvature of the edges of the rectangle to the Frank free energy via the saddle-splay contribution.