Molecular aggregation and chromonic liquid crystals

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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 ...

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confidence: 99%

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confidence: 99%

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confidence: 99%

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

Jeong

Davidson

Collings

et al. 2014

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133

“…We used DSCG, because it is not amphiphilic, and thus, we predicted that it would not disrupt the lipid bilayers of GUVs (in contrast, many surfactants that form lyotropic phases solubilize lipid bilayers). DSCG molecules stack into anisometric assemblies when dissolved in water (9)(10)(11) and form mesophases in a manner that depends on temperature and the concentration. We note here that the ordering of nematic DSCG and other chromonic LCs has been explored in confined spherical (12) and cylindrical geometries (13) as well as surrounding rigid spherical inclusions (14).…”

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confidence: 99%

Straining soft colloids in aqueous nematic liquid crystals

Mushenheim

Pendery

Weibel

et al. 2016

Liquid crystals (LCs), because of their long-range molecular ordering, are anisotropic, elastic fluids. Herein, we report that elastic stresses imparted by nematic LCs can dynamically shape soft colloids and tune their physical properties. Specifically, we use giant unilamellar vesicles (GUVs) as soft colloids and explore the interplay of mechanical strain when the GUVs are confined within aqueous chromonic LC phases. Accompanying thermal quenching from isotropic to LC phases, we observe the elasticity of the LC phases to transform initially spherical GUVs (diameters of 2-50 μm) into two distinct populations of GUVs with spindle-like shapes and aspect ratios as large as 10. Large GUVs are strained to a small extent (R/r < 1.54, where R and r are the major and minor radii, respectively), consistent with an LC elasticity-induced expansion of lipid membrane surface area of up to 3% and conservation of the internal GUV volume. Small GUVs, in contrast, form highly elongated spindles (1.54 < R/r < 10) that arise from an efflux of LCs from the GUVs during the shape transformation, consistent with LC-induced straining of the membrane leading to transient membrane pore formation. A thermodynamic analysis of both populations of GUVs reveals that the final shapes adopted by these soft colloids are dominated by a competition between the LC elasticity and an energy (∼0.01 mN/m) associated with the GUV-LC interface. Overall, these results provide insight into the coupling of strain in soft materials and suggest previously unidentified designs of LC-based responsive and reconfigurable materials.liquid crystals | soft colloids | vesicles | strain | elasticity T he majority of living materials are soft. This characteristic emerges from noncovalent interactions that lead to the formation of supramolecular structures that reorganize in response to subtle chemical and mechanical cues (1). The regulation of mechanical strain in particular and the engineering of responses to it across a hierarchy of spatial scales (from the molecular to the supramolecular to the cellular level) are increasingly understood to be one of the central sciences of living systems (2, 3).Inspired in part by the functionality of biological materials, a wide range of soft synthetic materials has been assembled by noncovalent interactions of molecular and macromolecular components (1, 4). In particular, liquid crystals (LCs) (Fig. 1A), which are phases that combine the molecular mobility of liquids with the long-range orientational ordering of crystalline solids, have provided the basis for a spectrum of responsive materials, including systems where electrical fields and mechanical strain compete to control electrooptical properties (5, 6). More recently, micro-and nanometer-sized colloidal particles dispersed in LCs have been used to form tunable self-assembled structures for photonic crystals and metamaterials (7). In the systems studied to date, however, the colloids have been "hard" compared with the LC, leading to mechanical straining of the LC but not the...

“…LCLCs are made up of plank-like molecules with a polyaromatic core and polar peripheral groups (15)(16)(17)(18). The π−π interactions result in the stacking of the constituent molecules (19).…”

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Using chiral tactoids as optical probes to study the aggregation behavior of chromonics

Nayani

Chang

et al. 2017

Tactoids are nuclei of an orientationally ordered nematic phase that emerge upon cooling the isotropic phase. In addition to providing a natural setting for exploring chromonics under confinement, we show that tactoids can also serve as optical probes to delineate the role of temperature and concentration in the aggregation behavior of chromonics. For high concentrations, we observe the commonly reported elongated bipolar tactoids. As the concentration is lowered, breaking of achiral symmetry in the director configuration is observed with a predominance of twisted bipolar tactoids. On further reduction of concentration, a remarkable transformation of the director configuration occurs, wherein it conforms to a unique splay-minimizing configuration. Based on a simple model, we arrive at an interesting result that lower concentrations have longer aggregates at the same reduced temperature. Hence, the splay deformation that scales linearly with the aggregate length becomes prohibitive for lower concentrations and is relieved via twist and bend deformations in this unique configuration. Raman scattering measurements of the order parameters independently verify the trend in aggregate lengths and provide a physical picture of the nematic-biphasic transition.chirality | self-assembly | chromonics | tactoids | phase transitions W hen an isotropic phase of a lyotropic system undergoes a phase transition to an ordered nematic phase, the pathway is usually mediated through spindle-shaped droplets called tactoids. Observation and analysis of tactoids have been an integral part of the investigations on liquid crystals, including some of the earliest experiments that motivated the seminal theory of Onsager (1-3). Tactoids also provide a natural setting to study nematics under confinement (4-6). Hence, they are an attractive setting as a testbed for fundamental research as well as being relevant to technological applications. This has driven the experimental investigation of tactoids in a host of materials, including dispersions of viruses (1, 3), proteins (7), inorganic platelets (8), and lyotropic chromonic liquid crystals (LCLCs) (4).The director configuration in the tactoids is dictated by the individual contribution of the elastic constants, splay (K11), twist (K22), and bend (K33) to the Frank free energy (4, 9). Historically, tactoids of lyotropic nematic liquid crystals have been found to have an elongated shape and adopt a bipolar configuration with the director following the meridional lines (1,8,9). Only recently has a twisted-bipolar director configuration been experimentally realized for a LCLC system when it was crowded with polyethylene glycol (PEG) (4). Twisted-bipolar tactoids of cellulose nanocrystals have also been demonstrated recently (10). The emergence of chirality in achiral liquid crystals has fueled the curiosity of scientists ever since their discovery (11, 12). There have been a host of technological applications that take advantage of chiral configurations of confined liquid crystals (13,14). In thi...