Dissertation/Thesis Abstract

Microparticles as a New Analytical Method to Study Liquid Crystal Colloids
by Zhang, Ke, Ph.D., Kent State University, 2006, 164; 10817885
Abstract (Summary)

The research described in this dissertation was conducted in a special manner: analyzing the properties of liquid crystals from the observation of microparticle behaviors. The sizes of the particles are ideal as they are large enough to be visible by microscopy (visible, IR and Raman) and are small enough to sense the motion of surrounding liquid crystal molecules. The shape and surface properties of the particles determine their interactions with the surrounding liquid crystal molecules, including surface anchoring, defects generation and etc. The behavior of individual microparticle is the result of orientational and translational motions of neighboring liquid crystal molecules and is closely related to the external field (eg. temperature gradient or electric field) acting on the liquid crystal host. Based on this strategy, a series of experiments were designed to study microparticle behaviors in a moving NI interface with/without patterned electric field. As a result, particle drag, attraction and pumping effects were observed for the first time. The analysis of these effects lead to the discovery that the moving NI interface has a meniscus shape and nonuniform director distribution. The minimum of free energy defines the preferable position of the particle is at the vertex of the curved interface, which is the origin of interesting particle drag and attraction effects. When a patterned electric field is applied, the NI interface is greatly deformed and strong hydrodynamic flows are generated. The polymer microparticles follow the hydrodynamic flow around the deformed NI interface and are pumped into the nematic phase. While these fascinating microparticle behaviors led us to explore the nature of liquid crystals, they also can be transferred to novel methods to fabricate and modulate guest phase structures in liquid crystals. It was found that varying interface velocities, electric field geometry and amplitude, and particle nature allow us to delicately control the particle structures or polymer morphology in liquid crystal medium. Further rheological study suggests the viscosity of liquid crystal colloids can be tuned by adjusting the particles structures. Analyzing how the nature and structures of the guest phases affects the macroscopic properties of the host liquid crystal is significant for the generation of novel functional liquid crystal materials. As the ability to control the morphologies of liquid crystal composites expands, it is possible construct new generation optical devices for processing, recording, and display of information and liquid crystal type of smart materials such as artificial muscles, responsive membranes and biological sensors.

Indexing (document details)
Advisor: West, John
School: Kent State University
Department: Chemistry
School Location: United States -- Ohio
Source: DAI-B 79/08(E), Dissertation Abstracts International
Subjects: Molecular physics
Keywords: Dielectrophoresis, Drag effect, Liquid crystal colloids, Microparticle, Nematic isotropic interface, Raman mapping
Publication Number: 10817885
ISBN: 978-0-355-84472-6
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