Date of Award
Doctor of Philosophy (PhD)
Physics & Astronomy
Arjun G. Yodh
This dissertation describes experiments which explore assembly and self-shaping behavior of liquid crystals (LCs) in confined environments, and characterize colloidal crystals made via LC-mediated defect interactions. Specifically, we report on: (1) LC textures that arise in the spherical-cap like confinement of a Janus drop; (2) assembly and dynamics of two-dimensional colloidal crystals residing atop of a LC surface, and (3) shape transitions of micro-emulsion drops containing LC oligomers.
The first experiments investigated nematic and smectic LCs confined in the spherical-cap like cavities of Janus droplets. We employed solvent-induced phase separation to make the Janus droplets, and we systematically characterized droplet morphologies, which were shown to be sensitive to the compartment volume ratio and the type/concentration of surfactants in the aqueous background phase. A variety of stable and metastable LC textures were found, including focal conic domains, dislocation rings, and layer undulations. Polarized optical microscopy was employed to analyze director configurations and follow their evolution. The lowest free energy state appears to be a lamellar layered structure with dislocation rings relaxed by smectic layer undulations. Novel drop functionalities such as directional self-propelled motion were also reported.
The second set of experiments investigated two-dimensional crystalline packings of colloidal particles residing at the interface between air and a nematic LC; the interparticle interactions were mediated by defects that extended from the particles on the surface into the bulk LC. The assembly and dynamics of the colloidal lattices were then studied by video microscopy. Specifically, particle motions including short-time diffusion and crystal phonon modes were measured and demonstrated to depend on the underlying LC viscosity and elastic constants. Displacement correlations enabled characterization of transverse and longitudinal sound velocities of the crystal packings, as well as quantification of the particle interactions produced by the LC defects.
The third set of experiments discovered and probed reversible shape transitions of micro-emulsion drops containing nematic liquid crystal oligomers (NLCOs). We reported spontaneous shape transitions of the drop morphology from spheres to roughened spheres, flowers, and filamentous structures. The control parameters dictating final morphologies included temperature, mean oligomer chain length, and surfactant concentration in the aqueous background. Surprisingly, our experiments and modeling suggested that molecular heterogeneity is a key feature leading to chain-length-dependent oligomer segregation within the micro-droplet that drives the observed transitions. Thus polydispersity, in this case, does not destroy order, rather it drives the transitions. In different sets of experiment with the same system, we permanently locked the resultant NLCO structures into liquid crystal elastomers (LCEs) by UV curing. With this process, and especially using the filamentous structures, we made non-woven, free-standing mats and well-aligned yarns. Currently, work is directed toward exploration of photonic and rheological properties of the self-assembled filamentous nematic networks.
Wei, Wei-Shao, "Novel Liquid Crystal Self-Assembly, Driven Assembly, And Self-Shaping In Confined Environments" (2020). Publicly Accessible Penn Dissertations. 5095.