Topological Defects in Nematic Liquid Crystals and the Behavior Induced by Adding Polystyrene Microspheres to 5CB

Joseph Smith

2015

Polystyrene spheres with diameters of 1 μm were added to the nematic liquid crystal 5CB (4-cyano-4'-pentylbiphenyl) in varied concentrations and the resulting formation of topological defects was analyzed. The topological concept of fundamental groups was used to determine the types of defects that are stable in nematic liquid crystals. This research used traditionally theoretical concepts in topology to describe experimental systems in soft matter physics. Liquid crystal mixtures with varying concentrations of untreated polystyrene spheres were put into nonrubbed 10 μm thick cells. The cells were placed on the heating stage of a polarizing microscope and quenched from a liquid state to a liquid crystal state. When samples were repeatedly quenched from 40 degrees C to 30 degrees C, we found that the resulting locations and strengths of the topological defects had little variance. This suggests that the inner surfaces of the cells treated with indium tin oxide and polyimide PI 2555 play a major role in the alignment of the molecules. The novel technique of using bouquets of circles to describe the topological defects in a Schlieren texture was applied to regions of our samples. The consistency over repeated quench cycles suggests that using bouquets of circles is a promising method for describing these textures. The addition of the polystyrene spheres did not cause a significant increase of ±1 strength defects, which suggests that the untreated spheres do not promote radial alignment. We also observed that the addition of the spheres lowered the transition temperature between the nematic liquid crystal phase and the isotropic liquid phase, although additional work is needed to isolate the cause and quantify this effect. This thesis served as a new exploration into the area of topological defects both experimentally and theoretically and it has provided multiple directions for future study.