Use this URL to cite or link to this record in EThOS: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.489221
Title: Recovery and analysis of director profiles in liquid crystal cells
Author: Cornford, Stephen Leslie
Awarding Body: University of Exeter
Current Institution: University of Exeter
Date of Award: 2008
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Abstract:
This thesis is concerned with the determination of the director profile within a nematic liquid crystal cell from optical experiments. The larger part of the work details the de- velopment of computational methods which can be used to find the director profile, and the application of these tools to the fully leaky guided mode experiment. In a second part, a simple conoscopic device is built, and then used to undertake a novel viscodynamic experiment. In essence, the fully leaky guided mode experiments and its relatives measure the de- pendence of the transmission and reflection coefficients of a liquid crystal cell upon incident angle. It is simple enough to calculate these coefficients if the director profile is known, but experimentalists actually need to achieve the opposite. That is, having measured the transmission and reflection coefficients, they must determine the director profile. However, this turns out to be an ill-posed problem, and so some additional information about the director profile is required. There is indeed an appropriate source of additional information - the continuum theory of nematic liquid crystals - and it is exploited here to develop two computational tools. In the first, it is used to adapt a mathematical technique, Tikhonov regularization, to both steady-state and time-dependent situations, so that director profiles can be recov- ered having made only weak assumptions about their behaviour. A second tool makes stronger assumptions and can be deployed after the first to estimate some of the unknown parameters which appear in the continuum theory. These tools are use in the first instance to analyze data drawn from two fully leaky guided mode experiments. In the first experiment, a hybrid aligned cell was measured during AC switching, and from its data director profiles and several phenomenological parameters including four viscosities are determined. Following that, the DC switching of the same cell is studied, which turns out to be critically affected by the motion of tiny concentrations of charged impurities. Then, having noted that only limited information about the director profile can be recovered from even the most elaborate optical experi- ment, a conoscopy experiment is designed to recover it quickly. Following this approach, a previously unknown flow-induced transition between topologically distinct states in a homoetropically aligned cell is observed.
Supervisor: Sambles, J. Roy Sponsor: EPRSC ; HP Labs, Bristol
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.489221  DOI: Not available
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