MS, Kent State University, 2019, College of Arts and Sciences / Department of Mathematical Sciences

The study presented here utilizes analytical and numerical methods to explore magnetic field induced Fr&eecute;edericksz transitions in a homeotropic cholesteric liquid crystal film with negative magnetic anisotropy. The director field is assumed to vary only across the gap thickness. The Oseen-Frank bulk energy and Rapini-Papoular surface energy are sought to model the equilibrium configurations of the director field, which is a compromise among torques from boundary anchoring, material elasticity, and magnetic field. In the strong anchoring case, the director field is uniform if the gap distance is sufficiently small in the absence of the field. As the magnetic field strength increases, a first-order transition to a distorted configuration occurs if the intrinsic pitch is small, and a second-order transition occurs if the intrinsic pitch is large. As the anchoring strength on the boundary weakens, the critical field strength for a distorted configuration shifts to a lower value, and a second-order transition becomes first-order. As the field strength further increases, there is a second transition to a simple twist configuration. Detailed analyses near the bifurcation points were conducted to gain insight into finding the globally stable solution. The full numerical bifurcation diagrams contain both first- and second-order transitions for both the strong and weak anchoring. We discover that there are certain attributes that can be varied to arrive at similar evolutions in transitions for the strong and weak boundary anchoring systems.

Committee: Xiaoyu Zheng PhD (Advisor); Ulrike Vorhauer PhD (Committee Member); Eugene C. Gartland Jr./PhD (Committee Member) Subjects: Applied Mathematics

PHD, Kent State University, 2015, College of Arts and Sciences / Chemical Physics

Topological defects plays an important role in many physical processes ranging from morphogenesis of phase transitions in condensed matter system to the response to surface confinement and application of external fields. In this dissertation, we investigate the topological defects both in lyotropic and thermotropic nematics in order to characterize the studied materials.

Committee: Oleg Lavrentovich (Advisor); Hiroshi Yokoyama (Committee Member); Liang-Chy Chien (Committee Member); Samuel Sprunt (Committee Member); Elizabeth Mann (Committee Member) Subjects: Engineering; Experiments; Materials Science; Optics; Physical Chemistry; Physics

PHD, Kent State University, 2006, College of Arts and Sciences / Chemical Physics

We have observed anchoring transitions of nematic liquid crystal mixtures on obliquely evaporated SiOx by varying the relative abundance of two components in the mixture. Of these two components, one has a longitudinal cyano group and another has lateral cyano groups. Also found is that both temperature and SiOx thickness variations are able to shift the anchoring transitions. We believe that the anchoring on SiOx has two origins: long range van der Waals potential and short range surface interactions. Since the two origins have opposite preference in liquid crystal director orientation the observed transition is caused by the change of their relative strength. We found that thermal absorption-desorption of molecules on SiOx surface is important in determining the strength of short range interactions while the layer thickness and optical properties of SiOx are important to the van der Waals potential. Based on previous work by de Gennes we proposed a model to describe the observed phenomenon.

Committee: Philip Bos (Advisor) Subjects: Physics, General