Chiral configurations from achiral lyotropic chromonic liquid crystals under confinements

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Chang, Rui
Srinivasarao, Mohan
Reichmanis, Elsa
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Chirality has fascinated scientists for the past two centuries from structural color to origins of life. Intuitively, chiral structures can result from the chirality of constituent molecules. But achiral molecules may also lead to macroscopic chiral configurations. In this study, we documented the emergence of chiral structures from achiral lyotropic chromonic liquid crystals (LCLCs) confined to curved geometries. When LCLCs were confined to cylindrical capillaries with degenerate planar anchoring, we surprisingly found a highly twisted configuration that replaced the usual axial configuration with no deformation. The stability of this chiral configuration was attributed to the saddle-splay elasticity which significantly lowers the elastic free energy. Additionally, the emergence of the chiral configuration enabled us to estimate of the saddle-splay elastic constant for LCLCs as a function of temperature and concentration. Due to the small twist elastic constant, two chiral configurations were observed for LCLCs in cylindrical capillaries with homeotropic anchoring. The study of these chiral configurations revealed the roles of aggregation behavior and surface anchoring on the director field. Using lyotropic polymer liquid crystals, we further generalized the emergence of chirality to other achiral lyotropics confined to cylindrical geometry. We suggested that the spontaneous chirality should be a universal configurational behavior for various achiral lyotropic systems with small twist elastic constant, which is independent from their chemical components. Moreover, besides the usual achiral bipolar configuration, we observed two chiral configurations with LCLCs confined to the tactoidal geometry, which is a natural confinement of nematics formed during the phase transition. Remarkably, one of these chiral configurations has long been proposed theoretically, but for first time it was realized in our experiments. At different temperatures and concentrations, the configurational transformation of LCLC tactoids was rationalized by the shifts of the average aggregate length. By exploiting the configurations of tactoids, we then studied the aggregation behavior of LCLCs with the addition of neutral polymer and ionic salts additives. Finally, the effects of different monovalent salts were further investigated on the transition temperatures and the orientational order of LCLCs. Our study reveals the physical insights of the chiral configurations from achiral lyotropics, which changes the common expectations of nematic materials based on the studies of thermotropic liquid crystals. The spontaneous emergence of chirality from achiral liquid crystals improves the general understanding of the symmetry-breaking phenomena in nature.
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