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UBC Theses and Dissertations

Liquid crystalline tactoids in microscopic ordered-disordered interfaces : emergence of self-assembly and topological defects Wang, Pei-xi


Liquid crystalline tactoids are discrete anisotropic microdroplets coexisting with continuous disordered phases. In this thesis, an in-situ photopolymerization method was designed to rapidly capture and solidify liquid crystalline tactoids in a crosslinked polymer matrix, which facilitated the direct observation of these fluid ordered microdroplets by scanning electron microscopy with the resolution of individual liquid crystal mesogens. Different stages of the evolution of tactoids were captured and examined, where the emergence of small-sized tactoids in initially disordered phases, the coalescence of multiple tactoids, the generation of topological defects in coalescence, and the sedimentation of tactoids were directly observed by electron microscopy. The in-situ photopolymerization method was then extended to inverse emulsions, where the structure and evolution of chiral nematic liquid crystalline tactoids in geometrical confinement of microspheres were investigated by both optical and electron microscopy. This study revealed the microstructures of topological defects of frustrated chiral nematic order in spherical confinement. Moreover, polymer and mesoporous silica microspheres with helical structures were obtained. The behavior of tactoids in the presence of colloidal doping nanoparticles was examined by electron microscopy at the resolution of individual particles, which showed that liquid crystalline tactoids have size-selective exclusion effects on foreign nanoparticles. This principle was applied to the separation of polymer nanospheres, gold nanoparticles, and paramagnetic nanoparticles by size. These results suggest an approach to size-selectively separate nanoparticles using lyotropic liquid crystals, where nanoparticles smaller than a threshold size will be selectively collected into the liquid crystalline tactoids and thus transferred from the disordered phase to the ordered phase during phase separation. The phase separation of liquid crystals in the presence of paramagnetic doping nanoparticles and gradient magnetic fields was studied. In this case, the disordered phases have higher volume magnetic susceptibility than liquid crystalline tactoids due to the exclusion effects of tactoids on paramagnetic nanoparticles. Thus, the movement and orientation of tactoids could be controlled by gradient magnetic fields as weak as several hundred Gauss/cm. This approach enables control of the phase separation rate and configuration, as well as the orientation of director fields in both discrete tactoids and continuous macroscopic ordered phases.

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