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Interactions and Confinement of Particles in Liquid Crystals: Novel Particles and Defects

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Interactions and Confinement of Particles in Liquid Crystals: Novel Particles and Defects Interactions and Confinement of Particles in Liquid Crystals: Novel Particles and Defects Anne Helen Macaskill Submitted in accordance with the requirements for the degree of Doctor of Philosophy The University of Leeds School of Physics and Astronomy July 2019 Declaration The candidate confirms that the work submitted is her own and that appropriate credit has been given where reference has been made to the work of others. This copy has been supplied on the understanding that it is copyright material and that no quotation from the thesis may be published without proper acknowledgement. ©2019 The University of Leeds and Anne Helen Macaskill i Acknowledgements UOM: University of Manchester UOL: University of Leeds SMP: Soft Matter Physics LC: Liquid Crystals The original research proposal and funding for this project were secured by J. Cliff Jones (then UOM, currently UOL) and Ingo Dierking (UOM). At this time both were members of the Soft Matter Research Group, University of Manchester. Both Jones and Dierking oversaw, supervised and oversaw many details of this project, especially early on, during chapter 3. Later chapters of the project, from chapter 4 onwards, were primarily supervised by Helen F. Gleeson (originally UOM, now UOL), with exceptions given below. In Chapter 3, the synthetic particles were produced by Sean Butterworth of the Organic Materials Innovation Centre, School of Chemistry, UOM. Other members of the research group also gave advice on, for example, choosing a solvent for the particles (notably Steve Yeates and Joshua Moore). The group also allowed the use of their labs and equipment. From chapter 4 onwards, the focus of the project began to move to the interaction of particles with defects and particle tracking (a suggestion originally made by Gleeson). Much advice (on practical use of equipment and data analysis) was given by David Wei, UOM. Wei also personally supervised the first experiments on particle tracking on the Leica laser tweezing system in chapter 6, which was based in the Photon Science Institute, UOM. We also credit M. Dickinson for granting access to labs and equipment. The Nikon tweezing system was based in labs of the Molecular and Nanoscale Physics group, UOL, although the equipment is half-owned by the SMP group. Some assistance on setting up the tweezing system (for ii example, aligning the laser and attaching the colour camera) was given by Ben Johnson (MNP group). On the method of treating particles with DMOAP I was advised by M Nikkhou, who recently joined the SMP group, UOL. The micrometre scale Janus particles in chapter 7 were manufactured by Andrew Leeson of the School of Chemical and Process Engineering, UOL. Discussions with Leeson and Olivier Cayre helped to inform the further work section on Janus particles. In the further work section we mention a project conducted at the CLF, Oxfordshire. All work was conducted under Jones and Mamatha Nagaraj (also SMP group, UOL), who performed the use of the high resolution microscopes and all data analysis. Some of the further work section (chapter 8) was inspired by discussion with Gleeson, but also Shajeth Srigengan, who also provided me with the raw data for the elastic constants of compound 1 and compound 2. Finally, I am indebted to all the members of the research groups (originally the LC group, School of Physics and Astronomy, UOM, later the SMP group, School of Physics and Astronomy, UOL). Many standard techniques of experimental liquid crystal physics were taught to me by other members of the group . For example, cell construction was originally taught to me by James Bailey. As is the nature of a research group, many members helped my project with day-to-day discussions, and general practical advice. Abstract The motivating topic for this project was to work towards building a new type of electronic paper. Current electronic paper technologies are not able to perform at frame rates high enough to display video. The project was inspired by a prototype device based on the ‘Janus particle’ (particles with two halves of different properties, for example, colour), and to try to reduce the power consumption. The starting point for the project was an investigation of 500nm Janus particles and spheres. The question of whether the addition of particles affected physical properties of the bulk liquid crystal was investigated. Above concentrations up to 1% wt/wt, aggregations formed quickly. At this weight percentage, no significant changes in order parameter, refractive indices or elastic constants of the LC could be seen. It is known qualitatively that topological defects in liquid crystals can attract or repel particles. Importantly the strength of interaction of particles with defects has been quantified in this work. A passive method of microrheology was implemented to quantify the confinement strength. Depending on the system, confinement strengths ranged between 10 and 10,000pN/µm. Particles treated for strong surface anchoring were found to be more strongly confined than particles with weak surface anchoring. Further, particles in liquid crystals with higher elastic constants were found to have higher confinement strengths than in particles in liquid crystals with lower elastic constants. Particle size was not found to affect confinement strength significantly in the size range studied. Finally, the topic of Janus particles was readdressed. In the size regime ~5- 10µm Janus particles show evidence of hybrid alignment and rotation in an electric field. In conclusion, the idea of using Janus particles in a device appears promising: we hope this work is continued in future. iv Table of Contents Declaration .................................................................................................... i Acknowledgements ..................................................................................... ii Abstract ....................................................................................................... iv Table of Contents ........................................................................................ v List of Tables ............................................................................................... x List of Figures ........................................................................................... xii List of Abbreviations .............................................................................. xxiii List of Symbols ...................................................................................... xxvi Chapter 1 Display Devices and Electronic Paper ................................ 1 1.1 Introduction ...................................................................................... 1 1.2 Electronic Paper .............................................................................. 1 1.2.1 Approaches to Reduce Power Consumption .......................... 2 1.3 Liquid Crystal Displays .................................................................... 3 1.3.1 LCDs adapted for low-power applications .............................. 3 1.4 Electrophoretic Displays .................................................................. 5 1.4.1 Janus Particle-based device: The Gyricon ............................. 6 1.4.2 Other low-powered display types ........................................... 8 1.5 Summary of Low-Powered Display Devices .................................... 8 1.6 Displays based on Liquid Crystal Colloids ..................................... 10 1.7 Conclusion ..................................................................................... 10 Chapter 2 Introduction to Liquid Crystals, Colloids and Liquid Crystal Colloids ................................................................................ 17 2.1 Introduction .................................................................................... 17 2.2 Optics of Birefringent Materials ...................................................... 17 2.2.1 Refractive Index ................................................................... 17 2.2.2 Birefringence ........................................................................ 18 2.2.3 Wave Plates and Wave Plate Conditions ............................. 20 2.2.4 Full Wave Plate .................................................................... 21 2.2.5 Half Wave Plate .................................................................... 21 2.2.6 Quarter Wave Plate .............................................................. 23 2.2.7 Crossed Polarisers and Polarising Microscopy .................... 23 v 2.2.8 Polarising Microscopy with Wave Plates .............................. 28 2.3 Liquid Crystals ............................................................................... 32 2.3.1 Nematic Liquid Crystals ........................................................ 33 2.3.2 The Director .......................................................................... 33 2.3.3 Order Parameter[5] ................................................................ 33 2.3.4 Viscosity ............................................................................... 34 2.3.5 Elastic Constants .................................................................. 35 2.3.6 Anchoring Conditions[17] ....................................................... 36 2.3.7 Freedericks Transition .......................................................... 37 2.3.8 Liquid Crystal Displays ......................................................... 39 2.3.9 Defects in Liquid Crystals
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