Study of lyotropic chromonic liquid crystals with additives, shear, and drying

Abstract

Water-loving and often biocompatible lyotropic chromonic liquid crystals (LCLCs) attract great attention because they may offer novel opportunities for applications such as biosensors. Besides, LCLCs are an interesting system for studying the physics of LCs because they exhibit different elastic properties from other thermotropic LCs. However, compared to well-known thermotropic LCs, the fundamental science and application potentials of LCLCs are far less understood. In this dissertation, I study LCLCs with additives, shear, and drying. Chapter 2 investigates how chiral dopants affect the chiral symmetry breaking of LCLCs, focusing on the double-twist (DT) director configurations of LCLCs in a cylindrical capillary. LCLCs of unusual elastic properties tend to exhibit chiral director configurations under confinement despite the absence of intrinsic chirality. The DT director configuration in a cylindrical cavity with a degenerate planar anchoring, resulting from the large saddle-splay-to-twist elastic modulus ratio, is a representative example. Here, I start by reexamining the DT configuration of nematic disodium cromoglycate (DSCG) in a cylindrical capillary and estimate the ratio of saddle-splay to bend modulus K24/K3 = 0.5 ± 0.1. I also study the DT configurations of the chiral nematic LCLCs with chiral dopants. The DT configuration becomes homochiral when the dopant concentration surpasses the critical concentration, which can be explained successfully by a theoretical model of their energetics. Finally, I observe the enantiomeric excess of chiral dopants determines the director configuration when dopants of two different handednesses are mixed. Chapter 3’s study of chiral nematic LCLCs confined in a cylinder manifest the topological properties of the layer structure of cholesteric LCs. I investigate experimentally and theoretically how cholesteric LCLC's DT director configuration exhibits a discontinuous layering transition upon increasing the concentration of a chiral dopant. The discontinuous transition results from local minima in the energy landscape and the selection of the new lowest energy configuration according to the dopant concentration. Domains of different twist angles, corresponding to each local minimum, can coexist, and topological defects form between them. I discover that any traditional topological invariants do not distinguish the metastable domains and accompanying defects; they are topologically protected chiral configurations characterized by the topological layer number invariant, unique to cholesterics. Chapter 4 presents the effect of purification and impurities on the self-assembly and phase behavior of the LCLC. LCLC molecules in water form aggregates by stacking, and the elongated nano-aggregates align to make liquid crystalline phases. Utilizing multiple experimental techniques, I unveil impurities in commercial Sunset Yellow FCF (SSY), the representative LCLC, and how the precipitation-based purification promotes the formation of the aggregates and mesophase. I further explore the roles of intrinsic impurities, i.e., byproducts of the SSY synthesis, whose molecular structures are almost identical to the SSY's but differ only in the number and position of sulfonates groups. Combining quantum chemical calculations of molecular structures and experimental investigation of aggregate structures and phase behavior, I propose that the impurities of the planar shapes behave as the planar SSY, i.e., participating in the aggregate formation, whereas the non-planar one disrupts the nematic phase. Chapter 5 discusses how nematic SSY aligns under the shear; SSY exhibits shear-dependent aligning behavior according to the concentration and the shear rate. Our optical observation reveals that nematic SSY at a low concentration aligns perpendicular to the shear direction, whereas nematic SSY of high concentrations shows a parallel alignment along with the shear. Additionally, by utilizing a wide-angle X-ray diffraction (WAXD) experiment, I confirm that the alignment direction depends on the shear rates when SSY concentrations are low. At the low shear rate, low concentration SSY has a perpendicular direction along the shear direction, but the alignment direction follows the shear direction upon the increase of the shear rate. To our interest, the SSY alignment direction comes back to the former direction, i.e., perpendicular to the shear, even after the flow stops. In contrast, the alignment direction with nematic SSY at a high concentration remains parallel along the shear direction regardless of the shear rate. We presume that this intriguing alignment behavior results from different values of multiple viscosities, so-called "Miezowicz coefficients." Namely, SSY with low concentration show ηa < ηb while the high concentration SSY has ηa > ηb. In the last chapter, I present the X-ray imaging results of how LCLCs sessile droplets on a hydrophobic substrate evaporate and leave deposits. For hydrophobic substrates, I adopt two different substrates: polycarbonate and aquapel-coated glass. As the solvent starts to evaporate, internal flows such as Marangoni and capillary flow appear, and the solute/dispersion distribution becomes non-uniform. These lead to a skin formation at the droplet surface and the caviation inside the droplet; the cavity grows to make the dried SSY droplet dome-shaped.