TOPOLOGICAL DEFECTS IN LYOTROPIC AND THERMOTROPIC NEMATICS A dissertation submitted to Kent State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy by Young-Ki Kim August 2015 © Copyright All rights reserved Except for previously published materials Dissertation written by Young-Ki Kim B. S., Korea University, Republic of Korea, 2007 M. Eng., Hanyang University, Republic of Korea, 2009 Ph.D., Kent State University, USA. 2015 Approved by , Chair, Doctoral Dissertation Committee Dr. Oleg D. Lavrentovich , Member, Doctoral Dissertation Committee Dr. Hiroshi Yokoyama , Member, Doctoral Dissertation Committee Dr. Liang-Chy Chien , Member, Outside Discipline Dr. Samuel Sprunt , Member, Graduate Faculty Representative Dr. Elizabeth Mann Accepted by , Director, Chemical Physics Interdisciplinary Program Dr. Hiroshi Yokoyama , Dean, College of Arts and Sciences Dr. James L. Blank ii TABLE OF CONTENTS LIST OF FIGURES AND TABLES ……………………………………………………vii ACKNOWLEDGEMENTS…………………………………………………………..xxiii CHAPTER 1. INTRODUCTION…………………………………………………………1 1.1. Liquid Crystal Phases..………………………………………………………..1 1.2. Topological Defects in Nematics..…………………………………………….5 1.3. Biaixial Nematic Liquid Crystal….………………………………………….12 1.4. Scope and Objectives of the Dissertation…………………………………….16 CHAPTER 2. MORPHOGENESIS OF DEFECTS AND TACTOIDS DURING ISOTROPIC-NEMATIC PHASE TRANSITION IN SELF-ASSEMBLED LYOTROPIC CHROMONIC LIQUID CRYSTALS…………………………………………………...19 2.1. Introduction………………………………………………………………….19 2.2. General Properties of LCLCs and Experimental Techniques……………….22 2.3. Topological Characteristics of Point Defects in 2D N: Disclinations and Boojums………………………………...………………………………………...27 iii 2.4. Early Stages of I-to-N Transition: Tactoids, Boojums and Disclinations…....31 2.5. Shape of Positive N Tactoids with Two Cusps……………………………….35 2.6. Late Stages of I-to-N Transition: I Tactoids as Disclination Cores…………37 2.7. Homogeneous N; Cores of Semi-integer Disclinations………………………42 2.8. N-to-I Transition: I Tactoids and Multiply Connected Tactoids……………..44 2.9. Shape of N and I Tactoids in Frozen Director Field…………………………50 2.10. Conclusions………………………………………………………………...56 CHAPTER 3. DOMAIN WALLS AND ANCHORING TRANSITIONS MIMICKING NEMATIC BIAXIALITY IN THE THERMOTROPIC OXADIAZOLE BENT-CORE LIQUID CRYSTAL C7………………………………………………………………….58 3.1. Introduction………………………………………………………………….58 3.2. Materials and Techniques……………………………………………………60 3.3. Alignment, Anchoring Transition and Domain Walls………………………..62 3.3.1. Polarizing Optical Microscopy………………………………………..62 3.3.2. Maps of Retardance and Director Field by LC-Polscope Observation…………………………………………………………………..64 3.3.3. Behavior of Textures in the Electric Field……………………………..67 iv 3.3.4. Fluorescence Confocal Polarizing Microscopy of the Surface Anchoring Transition…………………………………………………………………….70 3.3.5. Thickness-dependent Anchoring Transition and Electric Double Layers of Ions…...…………………………………………………………………...72 3.3.6. Influence of Thermal Degradation of C7 on the Alignment…………...77 3.4.Conclusions…………………………………………………………………...79 CHAPTER 4. SURFACE ALIGNMENT, ANCHORING TRANSITIONS, OPTICAL PROPERTIES, AND TOPOLOGICAL DEFECTS IN THE THERMOTROPIC NEMATIC PHASE OF AN ORGANO-SILOXANE TETRAPODES…………………..81 4.1. Introduction………………………………………………………………….81 4.2. Material and Techniques……………………………………………………..82 4.3. Search of Biaxiality in the Cell with a Homeotropic Alignment.…………….85 4.4. Topological Defects………………………………………………………….98 4.4.1. Escape of Director in a Round Capillary………………………………98 4.4.2. Point Defects in Droplets Suspended in Isotropic Fluid……………...101 4.4.3. Point Defects at Colloidal Spheres in the Tetrapode Material………..105 4.5.Conclusions………………………………………………………………….108 v CHAPTER 5. DIRECTOR REORIENTATION OF A NEMATIC LIQUID CRYSTAL BY THERMAL EXPANSION…………………………………………………………111 5.1. Introduction………………………………………………………………...111 5.2. Methods………………………………………………………………...…..113 5.2.1. NLC Materials, Capillaries, Alignment, Temperature Control………113 5.2.2. Fluorescent Tracers of Flow…………………………………………113 5.2.3. Fluorescent Anisometric Dye………………………………………..114 5.3. Contraction / Expansion, Flow, and Realignment in Flat Capillary………...115 5.4. Director Profile in Shear ( xz ) Plane………………………………………..118 5.5. Thermal Expansion Effects in Other Types of LCs…………………………122 5.5.1. Thermotropic and Lyotropic Chromonic LC………………………...122 5.5.2. Complex-Shaped LC, DT6Py6E6…………………………….……..123 5.6. Thermal Expansion Effects in Other Types of Geometries…………………136 5.7. Applications………………………………………………………………...140 5.8. Discussion and Conclusion…………………………………………………141 CHAPTER 6. SUMMARY…………………………………………………………….148 REFERENCES…………………………………………………………………………152 vi LIST OF FIGURES AND TABLES Figure 1.1. Schematic illustration of various nematic phase: (a) uniaxial nematic, (b) chiral nematic, and (c) twist-bend nematic...………………………………….……………………3 Figure 1.2. (a) Anisotropy of physical properties in the (a) uniaxial nematics ( Nu ) along and perpendicular to the main director nˆ and (b) the physical properties different along three mutually perpendicular directors nˆ , mˆ , and ˆl in biaxial nematics ( Nb )………..…3 Figure 1.3. Examples of three types of topological defects in Nu : (a) linear disclination of strength m 1/ 2 ; (b) a point defect in the bulk of LC droplet (hedgehog) caused by a homeotropic anchoring of nˆ at the LC droplet surface; (c) surface point defects (boojums) at the LC droplet surface with a tangential anchoring of .…..……………...6 Figure 1.4. (a) Nuclei of the thermotropic N mixture E7 emerging from the I phase as viewed in a polarizing microscope with crossed polarizers; the I phase appear black; (b) spherical droplets of a thermotropic nematic n-butoxyphenyl ester of nonyloxybenzoic acid dispersed in a glycerin-lecithin matrix, viewed in a polarizing microscope with a single polarizer (no analyzer). In both systems, each N droplet shows two surface point defects- boojums (some are marked by white arrows) and disclination loops (black arrows). The topological defects occur as a result of balance of surface anchoring and elastic distortion energy; (c) principal scheme of director distortions.……………………………………...10 vii Figure 1.5. Candidate materials for biaxial nematic phases with nontrivial molecular shapes: Oxadiazole bent-core LCs, (a) C7 and (b) C12; (c) Azo-substituted bent-core LC, A131; Bent-core LCs with four lateral flexible chains and shape-persistent derivatives of (d) thiadiazole, (e) benzodithiophene and (f) fluorenone; (g) Tetrapode shaped LC; (h) Cyclic (ring-like) LC.......…………………………………….…………………………..13 Figure. 2.1. (a) Molecular structure of DSCG. (b) Schematic structure of chromonic aggregates in I and N phases. (c) Phase diagram of DSCG dispersed in water as the function of temperature T and DSCG concentration. The numbers correspond to the sections that describe the scenarios of phase transition; the dots indicate the approximate location of the system on the phase diagram and the arrows indicate whether the system is cooled down or heated up; the dot with the number 2.7 and no arrow corresponds to the discussion of disclination cores in section 2.7. (d) T dependence of the area occupied by the N phase in biphasic region, for heating and cooling; the rate of temperature change is 1o C / min . Note the difference in the two curves, associated with the different type of director distortions in the system.………………...………………………………………22 Figure 2.2. FCPM images of the I-N biphasic region in DSCG water solution doped with fluorescent dye: (a) in-plane and (b) vertical cross-section of the cell. The tilt angle of I-N interface with respect to the bounding plate is about (77 7)o .…………………..….......25 viii Figure 2.3. Director configurations for (a-d) disclinations m 1/ 2, 1, (e) positive 1 1 boojum m inside a positive cusp, (f) negative boojum m associated 22π 22π with a negative cusp, (g) schematic definition of angular parameters.…………………....28 Figure 2.4. LC-PolScope textures of nucleating N tactoids during the I-to-N phase transition in DSCG. (a) The N tactoids feature two cusps with point defects-boojums and tangential orientation of the director at the I-N interface. (b-e) The tactoids grow and merge as temperature is reduced. Note that a merger of two differently oriented 2c tactoids in (c) produces a 3c tactoid with a disclination m 1/ 2 in the center (d,e).………..…..31 Figure 2.5. Anisotropic shape of a small 2c tactoid: (a) LC-PolScope texture and (b) reconstructed shape; (c) phase retardance measured along the long l and short u axes of tactoid…….…………………………………………………………………………....32 Figure 2.6. LC-PolScope textures of N tactoids with (a) 2 cusps, (b) 4 cusps, (c) 3 cusps and one disclination m 1/ 2 , (d-f) coalescence of two large tactoids into a single tactoid with a pair of oppositely charged disclinations m 1/ 2 in the bulk, 4 positive and 2 negative cusps at the I-N interface………………………………………………………..34 Figure 2.7. (a) LC-PolScope textures of a round I domain associated with a m 1 director field around it; (b, c) the I tactoid shrinks as T is reduced and then (d) splits into two disclinations of m 1/ 2. (e) T dependence of characteristic size A / π and area A of the I tactoid. (f, g) Retardance profile of the I tactoid shown in parts (b, c), respectively. (h) Retardance profile of the two disclinations in part (d) separating from each other…....37 ix Figure 2.8. (a) LC-PolScope textures of a 4c negative tactoid associated with a m 1 director field around it; (b, c) the I tactoid shrinks