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Results are presented from molecular dynamics simulations of binary liquid crystal mixtures using a generalisation of a well established Gay-Berne intermolecular potential. The simulations are undertaken in both the microcanonical (NVE) and the isoenthalpic-isobaric (NPH) ensembles. Firstly a 50:50 mixed system is simulated in the NVE ensemble containing generalised Gay-Berne (GGB) rod-like molecules with length to breadth axial ratios of 3.5:1 (molecules A) and 3:1 (molecules B). The molecules in this system differs only slightly from the well-characterised Gay-Berne (GB) potential with length to breadth ratio of 3:1. It is shown that the system exhibits isotropic (/), nematic (N) and smectic-B (SmB) phases with both the I-N and N-SmB phase boundaries not clearly defined. Competition between two density waves parallel to the director of the same wavelength but different phase lead to a pre-smectic ordering preceding the N-SmB phase transition. The longer molecules are shown to have a consistently higher order parameter the difference being greatest in the nematic phase and decrease with lowering temperature. Although a degree of local ordering is shown within each smectic layer the smectic phase is fully commensurate. Secondly, phase behaviour diagrams are presented from a series of constant-NPH simulations over a range of pressures and concentrations. The binary mixtures exhibit a rich phase behaviour, displaying isotropic, nematic, smectic-A (SmA), induced smectic-A and smectic-B phases depending on the choice of pressure and concentration. It is shown that the temperature range over which the nematic phase is stable can be extended greater than either homogeneous system by elevating the system pressure and/or by choice of concentration, agreeing with experimental results. The mixture exhibits a stable SmA island at a mole fraction of xa = 0.50 depending on the choice of pressure and a narrow induced SmA phase at xa = 0.25.

Abstract In this study, we employed the Monte Carlo simulation method to investigate the q-state coupled XY model based on the Landau free energy of couple hexatic order and herringbone order proposed by Bruinsma and Aeppli. On two-dimensional triangular lattices simulation results reveal that the q-state coupled XY model will generate a q-state clock phase transition and a XY transition. The unique generated q-state clock phase transition and XY transition will couple in some coupling parameter domain. The novel coupled transitions behavior agree with the phase transition of some kinds of liquid crystal. For example, the three-state Potts phase transition generated by a 3-state coupled XY model and the Sm-A ¡÷Hex-B transition of free ¡V standing two layers liquid crystal are matched. Their heat capacity anomaly is similar and the heat capacity exponent is both closed to £\¡Ü0.3. We also investigated the system of coupled ferromagnetic order and antiferromagnetic order. Adapted the positive coupling parameter on the Hamiltonian of 3-state coupled XY model, the simulation results reveal that the system generate an antiferromagnetic three-state Potts transition. In some parameter domain the antiferromagnetic three-state Potts transition and XY transition are coupled, and become a novel transition. The novel transition may explain the Sm-A ¡÷Hex-B transition of some kinds of liquid crystal which lack herringbone order.

This thesis presents the investigation of the liquid crystal (LC) - particle suspensions. Particles from nano- to micro-size, spherical to two-dimensional shapes, with different functionality are dispersed into nematic and smectic phases. The aim is to create ordered nanoparticle (NP) assemblies and thereby modify the common properties of the liquid crystal, such as dielectric anisotropy and electro-optical, revealing any interaction between particles and LC properties. It is found that for concentrations (>0.5vol%), the ferroelectric NPs have increased the sensitivity of the nematic liquid crystal to the electric field through electro-optical responses, which is seen by an enhancement in the dielectric anisotropy. This could be induced by the coupling of the electrical dipole moments in the spherical NPs with the LC director field. The electro-optical properties of the chiral smectic (SmC*) phase (tilt angle Θ, switching time τ_s and spontaneous polarisation P_s) are found to be independent of the concentration and sizes of the doped NPs. The relaxation frequency f_R of the Goldstone mode is faster in the ferroelectric NPs suspensions of 2.0vol% compared to the paraelectric NPs. In the graphene oxide (GO) - nematic LC (5CB) suspensions, the small GO sizes of mean size 560 nm are more easily dispersible than larger flakes of 2.8 micro metre mean size. As the GO concentration is increased, each of the threshold voltage and splay elastic constant dramatically increases, reaching saturation at ≈1.0wt%. The field driven switching-on time is practically not affected, while the purely elastically driven switching-off time is strongly sped-up. Interestingly, thermotropic and lyotropic LC phases are exhibited in the GO-5CB suspensions when heating the thermotropic liquid crystal into its isotropic phase. The isotropic phase of 5CB acts as a solvent for the GO particles, forming a lyotropic nematic phase with largely reduced birefringence. It is found that the nematic to isotropic phase transition is shifted toward higher temperature for the GO-5CB system compared to the BaTiO3-5CB system. Dispersions of different sizes of GO flakes are prepared in isotropic and nematic fluid media. The dielectric relaxation behaviour of GO-dispersions was examined for a wide temperature range (25-60 ℃) and frequency range (100 Hz-2 MHz). The mixtures containing GO flakes were found to exhibit varying dielectric relaxation processes, depending on the size of the flakes and the elastic properties of the dispersant fluid. The relaxation frequencies in the isotropic media were lower compared to the nematic medium. Relaxation frequencies (~10 kHz) are observed in the GO-isotropic media, which are reduced as the size of the GO flakes are decreased, are anticipated to be inherited from GO flakes. However, the fast relaxations (~100 kHz) that are observed in the nematic suspensions could imply strongly slowed down molecular relaxation modes of the nematogenic molecules. Finally, the phase diagram of lyotropic LC as a function of the lateral dimensions of the GO flakes, their concentration, geometrical confinement configuration and solvent polarity was investigated. Polarising optical microscopy was used to determine isotropic-biphasic-nematic phase evolution. The confinement volume and geometry of the sample relative to the GO size are shown to be vital to the observation of the lyotropic phase. GO LCs have the potential for a range of applications from display technologies to conductive fibres. The confinement related LC phase transition is critical toward their applications. It is also found that the stability of the LC phase is higher for the solvent of higher dielectric constant.

Gegenstand der vorliegenden Arbeit ist die Herstellung und Charakterisierung neuer sternförmiger Mesogene. Bei den aufgeführten Sternverbindungen, handelt es sich um Oligobenzoate, bestehend aus aromatischen Hydroxy- oder Aminocarbonsäuren, die durch Kupplungsreaktionen mit Dicylohexycarbodiimid, in einer konvergenten Synthesestrategie verknüpft wurden. Das besondere Augenmerk der Arbeit richtete sich auf die Charakterisierung der von den neuen Substanzen gebildeten Mesophasen, die mit Hilfe von Polarisationsmikrokopie, dynamischer Differenzialkalorimetrie und Röntgenstreuung erfolgte. Zur Aufklärung spezieller dreidimensionaler Strukturen wurden als zusätzliche Methoden die Rasterkraftmikroskopie angewandt und der Zirkulardichroismus untersucht.

Ces travaux concernent l’étude expérimentale des textures se développant dans les ménisques de films smectiques de cristaux liquides thermotropes. Ces fluides complexes nous servent de système modèle pour l’étude des couplages élasto-capillaires qui se manifestent dans certaines conditions par des déformations périodiques de l’interface cristal liquide-air. Dans un premier temps, à partir de l’étude détaillée de la topographie de l’interface par une méthode interférométrique, nous caractérisons les différents types de structures, puis, identifions les principaux paramètres impliqués dans le processus d'apparition des défauts dans le ménisque de films libres suspendus. Ces résultats nous servent de base pour discuter des mécanismes physiques qui génèrent les différents types de défauts observés. Nous regardons en particulier, comment la diminution d’épaisseur de couche associée à une transition de phase va induire une instabilité mécanique responsable de l’apparition des ondulations de l’interface. Dans un deuxième temps, afin d’aller plus loin dans la compréhension des mécanismes, nous avons utilisé des films minces de cristaux liquides déposés sur des substrats solides microstructurés par des plots fabriqués par des techniques photolitographiques. Une telle géométrie permet de générer un ménisque autour de chaque plot et surtout de faire varier un plus grand nombre de paramètres comme par exemple l’ancrage sur le substrat via un traitement chimique de surface. L’ensemble de nos résultats apportent un éclairage nouveau sur la caractérisation et la compréhension des déformations spécifiques aux ménisques de fluides complexes. / The present work is an experimental study of the textures that appear in the meniscus of free standing smectic films with thermotropic liquid crystals. These complex fluids serve as model systems to investigate elasto-capillary phenomena which, under certain conditions, manifest themselves through periodic deformations of the liquid crystalair interface. In the first part of the thesis, we focus our attention on meniscus structures whose interfacial topographies are thoroughly characterized thanks to an in-house optical interferometry technique. Our study allows us to identify the main parameters involved in the development of meniscus structures and to discuss the physical mechanisms that are likely to be responsible for their formation. In particular, we show how a phase transition-induced layer shrinkage triggers a mechanical instability leading to interfacial undulations of the smectic free surface. In the second part of the manuscript, we address the case of thin liquid crystal films deposited on solid patterned solid substrates. The latter consist of regular arrays of microposts fabricated through photolithographic techniques. Such a geometry allows a meniscus to be formed around each micropost and makes it possible to examine the influence of other parameters such as the anchoring conditions on the solid substrate. The results gathered so far are able to shed some light on the characterization and the understanding of the specific deformations and textures that appear in the menisci of complex fluids.

Gegenstand der vorliegenden Arbeit ist die Herstellung und Charakterisierung neuer sternförmiger Mesogene. Bei den aufgeführten Sternverbindungen, handelt es sich um Oligobenzoate, bestehend aus aromatischen Hydroxy- oder Aminocarbonsäuren, die durch Kupplungsreaktionen mit Dicylohexycarbodiimid, in einer konvergenten Synthesestrategie verknüpft wurden. Das besondere Augenmerk der Arbeit richtete sich auf die Charakterisierung der von den neuen Substanzen gebildeten Mesophasen, die mit Hilfe von Polarisationsmikrokopie, dynamischer Differenzialkalorimetrie und Röntgenstreuung erfolgte. Zur Aufklärung spezieller dreidimensionaler Strukturen wurden als zusätzliche Methoden die Rasterkraftmikroskopie angewandt und der Zirkulardichroismus untersucht.