Chiral structures and motions under confinement: Liquid crystals and bacteria

Abstract

Chirality, representing the breaking of mirror symmetry, is a phenomenon observed ubiquitously in nature. Despite its prevalence, many questions about chirality remain unanswered, such as the origins of homochirality in nature. This doctoral thesis explores the chiral structures and motions, focusing on the relationship between chirality and confinement using liquid crystals (LCs) and bacteria as experimental models. The research aims to address whether an achiral system can become chiral due to confinement, whether a chiral system can become homochiral or achiral, and if the natural chiral direction can be reversed.

The thesis examines these questions through three case studies. In the first case, the study investigates how achiral LCs exhibit chiral structures and the possibility of homochiral configuration within and around sessile droplet confinement. Using polarized optical microscopy, the research observes LC structures and proposes director field models to minimize elastic free energy. The study finds that small twist elastic properties of LCs induce chiral symmetry breaking in achiral LCs and metastable structures with opposite handedness remain in the chiral system due to confinement-induced boundary conditions.

The second case explores whole and local chiral inversion of chiral LCs confined in cylindrical confinement. By applying calculus of variations, the research solves the Euler-Lagrange equation derived from the Oseen-Frank free energy equation to find energy-minimum solutions. The findings confirm that whole and local chiral inversion, corresponding to overall and partial changes in natural twist direction, occur only in local minima. The third case demonstrates the transformation of chiral motion into achiral motion of bacteria by controlling the confinement thickness. The study shows that near-surface chiral swimming of bacteria changes to achiral motion when the gap between two substrates becomes thin enough for bacteria to sense both substrates simultaneously. By analyzing trajectories and adopting theoretical models of run-and-tumble motions, various motility parameters are quantified, including speed and turning angle distributions, to infer model parameters like tumbling rate and time. These parameters are used to investigate changes in bacterial motility under external fields, such as temperature.

This study aims to reveal the complex interplay between chirality and confinement, offering insights into fundamental physical phenomena. It is expected to extend into various applications by enhancing the understanding of the chiral nature of both LCs and living bacteria.