Chiral symmetry breaking and surface faceting in chromonic liquid crystal droplets with giant elastic anisotropy

Abstract

Confined liquid crystals (LC) provide a unique platform for technological applications and for the study of LC properties, such as bulk elasticity, surface anchoring, and topological defects. In this work, lyotropic chromonic liquid crystals (LCLCs) are confined in spherical droplets, and their director configurations are investigated as a function of mesogen concentration using bright-field and polarized optical microscopy. Because of the unusually small twist elastic modulus of the nematic phase of LCLCs, droplets of this phase exhibit a twisted bipolar configuration with remarkably large chiral symmetry breaking. Further, the hexagonal ordering of columns and the resultant strong suppression of twist and splay but not bend deformation in the columnar phase, cause droplets of this phase to adopt a concentric director configuration around a central bend disclination line and, at sufficiently high mesogen concentration, to exhibit surface faceting. Observations of director configurations are consistent with Jones matrix calculations and are understood theoretically to be a result of the giant elastic anisotropy of LCLCs.emulsions | complex colloids T he director configurations of confined liquid crystals exhibit a rich phenomenology, the physics of which is determined by a delicate interplay of topology, elastic free energy, and anchoring conditions at the boundaries (1-12). Droplets present arguably the simplest and most symmetric confining container for liquid crystals. Droplets of thermotropic liquid crystals (TLCs) and the manipulation of their director configurations, for example, are actively studied, in part because of their demonstrated use as core materials in display technologies (3, 13) and their potential applications ranging from biosensors (14, 15) to microlasers (16). Indeed, significant fundamental and technological progress has been made with TLC droplets, because their bulk elasticity and surface anchoring phenomena are now well understood and easily controlled.Lyotropic chromonic liquid crystals (LCLCs) are composed of organic, charged, and plank-like mesogens that self-assemble in water into columnar aggregates via noncovalent electrostatic, excluded volume, hydrophobic, and pi-pi stacking interactions (17)(18)(19)(20). The aggregates, in turn, assemble into nematic or columnar phases, depending on temperature and concentration. A variety of organic molecules such as dyes, drugs, and biomolecules form LCLCs (17-28). However, far less is known about the fundamental science and applications potential of LCLCs than the more-studied TLCs. Indeed, basic properties of LCLCs, including aggregate size distribution and formation dynamics, bulk elasticity, and surface anchoring are neither fully characterized nor understood and are the subject of exciting ongoing research. Only recently, for example, have measurements been made of fundamental properties, such as the FrankOseen elastic constants (28, 29), of any LCLC, and they have revealed unusual concentration and temperature dependences of the splay ...

Chiral structures from achiral liquid crystals in cylindrical capillaries

“…Liquid crystals (LCs) are soft materials composed of anisotropic mesogens that provide remarkable examples of chiral symmetry breaking arising from elastic anisotropy (29)(30)(31)(32)(33)(34)(35)(36)(37)(38)(39)(40)(41)(42). In essence, an LC can minimize elastic free energy by organizing its achiral units into chiral structures, such as helices and chiral layers, that incorporate twist deformation (7,8).…”

“…In essence, an LC can minimize elastic free energy by organizing its achiral units into chiral structures, such as helices and chiral layers, that incorporate twist deformation (7,8). The elastic free energy describing nematic LC deformations depends on so-called splay, twist, bend, and saddle-splay elastic moduli, and when twist deformation is comparatively easy, twisting can relieve strong splay and/or bend deformation and lead to production of equilibrium chiral structures (29,32,(35)(36)(37)(38). Similarly, saddlesplay deformation can stabilize chiral structures (43)(44)(45)(46).…”

“…Elasticity-driven chiral symmetry breaking is perhaps most readily manifested in confined LCs (31)(32)(33)(34)(35)(36)(37)(38)(39)(40)(41)(42)(43), wherein surface anchoring imposes a preferred angle for LC mesogens at the interface of the confining boundary. Topological defects enforced by boundary conditions can play a key role in the symmetry breaking as well, because energetically costly deformations are often concentrated in the vicinity of the defects (35)(36)(37).…”

Confined chromonics and viral membranes

“…Spherical droplets with degenerate planar boundary conditions [1]: The normal expectation is that nematics under these conditions will adopt the bipolar configuration shown in Fig. 1(a).…”

“…This article, based on my plenary lecture at ILCC 2016 will review recent work done at the University of Pennsylvania on two separate topics: chromonic liquid crystals in confined geometries [1][2][3][4], carried out mostly in Arjun Yodh's lab by Joonwoo Park and Zoey Davidson with some theoretical assistance from me and my student Louis Kang, and a theory, which is mostly the work of Louis Kang [5,6], for multi-micron-sized two-dimensional membranes composed of filamentous viruses stabilized by depletants studied in exquisite detail by Zvonimir Dogic and collaborators [7,8].…”