UC Merced

Colloids are ubiquitous in nature and technology, playing vital roles in variousfields, from pharmaceuticals to materials science. Liquid crystal colloids, formed by dispersing colloidal particles in a liquid crystal host, present a unique system where the interplay between the orientational order of the liquid crystal and the colloidal particles leads to complex, anisotropic interactions not seen in conventional colloidal systems. These interactions are characterized by the formation of topological defects and elastic deformations in the nematic order. This thesis aims to give insights into the fundamental physics at the intersection of liquid crystal and colloidal science, which can potentially lead to the development of advanced materials. In the first part of the thesis, we investigate the aggregation behavior of colloidal particles in a nematic liquid crystal phase, which leads to the formation of hierarchical aggregate morphologies distinct from those in isotropic solvents. We developed a novel self-assembling colloidal system to study aggregation over large length scales involving thousands of colloidal particles. In this method hollow micron-scale colloids form in situ within the nematic phase and aggregate into fractal structures and colloidal gels. The morphology of these aggregates is determined by colloid concentration and temperature quench depth through the isotropic-tonematic phase transition point. Using fluorescence microscopy, we measure the aggregate structure across various length scales, analyze ageing mechanisms, and explore the driving forces behind aggregation. Our findings suggest that the aggregate dynamics are influenced by a combination of Frank elasticity relaxation, spontaneous defect line annihilation, and internal aggregate fracturing. The second part of the thesis focuses on the interactions between active colloids and the anisotropic liquid crystal environment. For this work, we used platinumcoated Janus colloids, which exhibit self-propelled motion in aqueous solutions via the catalytic decomposition of hydrogen peroxide. When placed in a uniformly aligned nematic phase of lyotropic chromonic liquid crystal, disodium cromoglycate (DSCG), these active Janus colloids demonstrate motion that is strongly dictated by the anisotropy of the liquid crystal – they tend to move parallel to the nematic director. Motion analysis over a range of timescales reveals a cross-over from ballistic to anomalous diffusive behavior on timescales below the relaxation time for liquid crystal elastic distortions. Notably, we discover a size-dependent effect: smaller particles exhibit rolling motion during ballistic motion, whereas larger particles do not. This behavior highlights the complexity of phoretically-driven particle motion in the anisotropic fluid environment. By investigating both the aggregation of passive colloidal particles and the motion of active Janus colloids, this thesis contributes to a deeper understanding of how anisotropic environments influence colloidal dynamics. These findings have the potential to help in the design of new materials and technologies that leverage the unique properties of liquid crystal colloids.