Electro-Optic Effects in Thermotropic Liquid Crystals

Hristo Petkov Hinov
Institute of Solid State Physics, Bulgarian Academy of Sciences, Laboratory of Liquid Crystals, Sofia, Bulgaria

Series: Materials Science and Technologies
BISAC: SCI053000

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Since the discovery of the first liquid crystal in 1888 by the Austrian biologist Friedrich Reinitzer with the considerable assistance of the German physicist Otto Lehman, who in that time had been universally recognized authority in the field of microscope technique, thousands and thousands of liquid crystals have been synthesized among them nematics, short-pitch cholesterics and various kinds of smectics such as smectic-A and smectic-C.

The smectic-A or smectic-C phases grow from a nematic or unwinded large-pitch cholesteric oriented in the middle part of the cells and from strongly-deformed nematic or unwinded large-pitch cholesteric layer in the boundary regions of the liquid crystal cells. These have two advantages. First, it shows the texture fitting between the surface and bulk alignment of the smectics, sometimes unknown, and permits in some cases the approximate determination of the surface energy of smectics and second, it is useful for application in the laser-addressed liquid crystal devices.

On the other hand, the experimental results obtained with the smectic-C permit the evaluation of certain material parameters. This book includes the electro-optic behavior of the large-pitch cholesteric at a new, strong-weak or weak-weak anchoring as well as the hysteretic behavior leading to the appearance or disappearance of various large-pitch textures. One of the most interesting electro-optic effects, discovered in the large-pitch cholesterics, is the observation of polar unidirectional spirals under a DC voltage application.

Finally, the book draws attention to the discovery of a new chiral liquid crystal with unusual electro-optic behavior composed from a mixture of a nematic and a ferroelectric smectic-C* liquid crystals. The authors of this book have experimentally studied the electro-optic dynamic hysteretic behavior of weakly-anchored twisted nematic layer as well. The flexoelectric deformations were identified except by additional application of a high-frequency orienting electric field and, qualitatively, by the simultaneous application of DC and low-frequency (several hertz) electric fields. In this way, for the first time, unusual amplification was observed of the low-frequency flexoelectric oscillations. The “gradient flexoeffect” can be realized not only by the accumulation of ions in the nematic layer, but also by inhomogeneity of the electric field, created by the geometry of the liquid crystal cell.

Finally, the authors focus on the polar flexoeffect, without taking into account the second-order elasticity. The pulsations of the light transmitted through a nematic layer with a homeotropic anchoring and a free-slipping boundary are again considered, and new ideas are developed. The material parameters are of a great importance for the theory and application of the liquid crystals. Due to this reason, the authors have suggested new methods for their measurement, some cases being illustrated by examples. Finally, the authors proposed the Becke’s method for measurement of the optical index of various solids to be combined with the use of a monodomain liquid crystal in its nematic or isotropic phase. (Imprint: Nova)

Preface

Abstract

Introduction Brief Historical Notes on the Discovery and Study of the Liquid Crystals

Chapter 1 - Formation of the Smectic-A Liquid Crystal from Strongly-Deformed in the Boundary Regions Nematic (pp. 1-46)

Chapter 2 - Formation of Smectic-C Liquid Crystal from a Non-Deformed, Weakly-Deformed and Strongly-Deformed Nematic Layer. Possibility for the Determination of Certain Smectic-C Elastic Constants (pp. 47-68)

Chapter 3 - Electro-Optic Behavior and Textures in Large-pitch Cholesterics (pp. 69-110)

Chapter 4 - Electro-Optic Effects in Nematics: The Fréedericksz Transition in Weakly-Anchored and Initially Deformed Nematic Films (pp. 111-130)

Chapter 5 - Flexoelectricity in Nematic Liquid Crystals. Polar Flexoelect Flexoelectricity in Non-Planar Geometries. The Flexoeffect in Simultaneous Applied DC and AC (< 5 Hz) Electric Fields (pp. 131-150) Chapter 6 - DC Voltage-Controlled Pulsations of a Homeotropic Nematic Layer with a Free-Slipping Liquid Crystal-Polar Liquid Boundary (pp. 151-162) Chapter 7 - Orienting Action of an AC Electric Field with a Frequency above the Relaxational 4/ of the Ions on a Nematic Layer. New Methods for Measurement of the Material Parameters of Nematics and Their Application (pp. 163-176) Conclusion Index

This book is written for both researchers in the science and practical applications of the liquid crystals.

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