1,721,057 research outputs found
Computer simulation studies of nematic liquid crystal tactoids
No full textMonte Carlo simulations are used to study the equilibrium shapes of unconstrained nematic liquid crystal droplets or tactoids in an isotropic medium. The tactoids are approximately spherical when the mesogen is in its isotropic phase. On cooling into the nematic, the tactoid shape becomes anisotropic, with the symmetry axis aligned along the director. The molecular organisation inside the tactoids is investigated
Phase behavior and free interfaces of a lattice-gas nematic-liquid-crystal model
No full textThe phase behavior of a mesogenic lattice-gas model consisting of molecules located at the sites of a three-dimensional cubic lattice has been studied using grand canonical Monte Carlo simulations. When two neighboring sites are occupied, the molecules interact via a potential composed of an isotropic lattice-gas (LG) term and an anisotropic Humphries-Luckhurst-Romano (HLR) term [Mol. Phys. 42, 1205 (1981)]. The LGHLR model is shown to exhibit either nematic-isotropic, nematic-vapor (NV), and isotropic-vapor (IV) coexistence or just nematic-isotropic fluid coexistence, depending on the strength of the isotropic term. The liquid-vapor (i.e., NV and IV) interfaces were studied using canonical Monte Carlo simulations. By controlling the strength of the term that governs the anisotropy in the attractive forces, either planar or homeotropic anchoring is observed at the NV interface. The temperature dependencies of the density and order parameter profiles across the interfaces are determined for these two anchoring geometries
Computer simulation of liquid crystals
No full textIn this thesis, the results of a series of computer simulation studies into the structure and dynamics of the phases exhibited by anisotropic molecules are presented. The initial two chapters give an introduction to the major topics of the thesis; the first chapter contains a brief description of the chemical and physical properties of liquid crystals and the second introduces the techniques of computer simulation. Particular attention is paid to the microscopic description of liquid crystals and to the models used to represent mesogens. The next three chapters are concerned with the structure and dynamics of rod shaped or calamitic molecules. In Chapter 3, we explore the phase behaviour of a system of such molecules interacting via the Gay-Berne potential, primarily at constant pressure.The structure of the observed phases is discussed in terms of single and pair distribution functions. We also use computer graphics techniques to view the arrangement of the molecules in the various phases. The translational and rotational dynamics of this model system are studied in Chapter 4. In Chapter 5, we return to the structure of the mesophases, but here we concentrate on the calculation of the X-ray diffraction patterns of configurations taken from computer simulations and the information which they contain. The final two chapters are concerned with disc shaped molecules. Chapter 6 describes a simulation study of the phase behaviour of a system of discotic liquid crystals at constant pressure and, in Chapter 7, we investigate the role of quadrupolar interactions in the phase behaviour of binary mixtures of disc shaped molecules.</p
Modelling the chemically induced liquid crystalline phases in mixtures of disc-shaped mesogens: the phase diagram of quadrupolar Gay-Berne discs revisited
No full textWe investigate the phase behaviour of pure systems and mixtures of quadrupolar Gay-Berne discs. The interaction potential mimics that of disc-shaped molecules which exhibit chemically induced phases with a structure based on columns of alternately stacking species. The phase diagram of the pure system is determined over a range of pressures, and the phase sequence is shown to include tilted columnar and nematic phases. The mixtures, in which the species have equal but opposite quadrupole moments, are investigated over a range of pressures using semigrand canonical simulations, such that the composition of the system is allowed to change. The fact that the composition is not fixed is especially important at the phase transitions, where the compositions of the coexisting phases may not necessarily be the same. In this situation, preparation of a system in the 'biphasic region' will lead to phase separation into the two distinct phases of differing compositions. The resulting phase diagram obtained using semigrand canonical simulations indicates that the columnar nematic phase observed in previous fixed composition simulations of this model [Liq. Cryst., 24, 229 (1998)] is not stable with respect to phase separation into an isotropic phase rich in a single component and a hexagonal columnar phase composed of roughly equal quantities of the two components. The structure of the columnar phase for the mixture is shown to be based on the alternate stacking of the different species. The relative concentrations of the different species in this phase may deviate up to approximately 60 : 40 mol %, after which any further material added will separate into the pure isotropic state
Computer simulation study of the phase behavior of a nematogenic lattice-gas model
No full textThe phase behavior of a mesogenic lattice-gas model consisting of freely rotating spins located at the sites of a three-dimensional cubic lattice has been studied using grand canonical Monte Carlo simulations. When two neighboring sites are occupied, the spin vectors interact via the extensively studied anisotropic Lebwohl-Lasher potential, plus an isotropic term of variable strength. The interaction between occupied and empty sites and two empty sites is taken to be zero. If the parameter governing the strength of the isotropic term is zero, the model exhibits an isotropic fluid-nematic transition, which becomes increasingly stronger as the temperature is lowered. The additional isotropic term is found to be important if the model is to reproduce experimental phase behavior, that is, to exhibit both nematic-vapor coexistence at low temperature and isotropic-vapor coexistence at higher temperatures
Coarse grained models for flexible liquid crystals: parameterization of the bond fluctuation model
No full textWe extend the bond fluctuation model, originally devised to investigate polymer systems, to contain anisotropic interactions suitable for the simulation of large flexible molecules such as liquid crystalline polymers and dendrimers. This extended model coarse grains the interaction between the flexible chains at a similar level of detail to the mesogenic units. Suitable interaction parameters are obtained by performing trial simulations on a low molar mass liquid crystalline system. The phase diagram of this system is determined as a function of the molecular stiffness. The nematic to isotropic transition temperature is found to increase with increasing stiffness
Biaxial nematic phases and V-shaped molecules: a Monte Carlo simulation study
No full textInspired by recent claims that compounds composed of V-shaped molecules can exhibit the elusive biaxial nematic phase, we have developed a generic simulation model for such systems. This contains the features of the molecule that are essential to its liquid crystal behavior, namely the anisotropies of the two arms and the angle between them. The behavior of the model has been investigated using Monte Carlo simulations for a wide range of these structural parameters. This allows us to establish the relationship between the V-shaped molecule and its ability to form a biaxial nematic phase. Of particular importance are the criteria of geometry and the relative anisotropy necessary for the system to exhibit a Landau point, at which the biaxial nematic is formed directly from the isotropic phase. The simulations have also been used to determine the orientational order parameters for a selection of molecular axes. These are especially important because they reveal the phase symmetry and are connected to the experimental determination of this. The simulation results show that, whereas some positions are extremely sensitive to the phase biaxiality, others are totally blind to this
Biaxial nematics: computer simulation studies of a generic rod-disc dimer model
No full textOne possible route to the elusive biaxial nematic phase is through rod–disc dimers in which the rod and disc mesogenic units are linked via a flexible spacer. We have developed a continuous generic model of such rod–disc dimers in which neighbouring like groups tend to align parallel to each other while unlike groups tend to be orthogonal. A torsional potential controls the relative orientations of the groups within a single dimer; depending on the strength of the torsional potential, the groups may be orthogonal or parallel in the conformational ground state. Monte Carlo simulations show that a rigid rod–disc dimer is most likely to form a biaxial nematic phase if the anisotropies of the two groups are the same. Introduction of flexibility is found to have little effect on the qualitative behaviour of the dimer as the relative anisotropy of the two mesogenic groups is changed. However, when the torsional potential strongly favours the alignment of the rod and disc within a single molecule with their symmetry axes parallel there is a dramatic change. The system then exhibits a strong hysteresis in the molecular shape and biaxiality and the biaxial nematic–isotropic transition becomes strongly first order, in marked contrast to the second-order character usually found for this transition. This first-order transition is observed to occur for a range of relative anisotropies of the two groups rather than at a single point
Determination of the Maier-Saupe strength parameter from dielectric relaxation experiments: a molecular dynamics simulation study
No full textMolecular dynamics simulations have been performed to investigate the rotational motion in the nematic and isotropic phases of a model mesogenic system in which the interactions between the molecules are represented by the Gay-Berne potential. First-rank end-over-end rotational relaxation times, analogous to those measured using dielectric relaxation spectroscopy for real mesogens with a longitudinal electric dipole, have been determined as a function of temperature and density. The relaxation times at temperatures throughout the nematic region are found to be larger than the values extrapolated from the isotropic phase to the same temperature. The simulation results are compared with the extended Debye theory for dielectric relaxation in the nematic phase. This relates the reduction in the relaxation rate to the retardation factor which depends on the Maier-Saupe strength parameter, and in turn is defined uniquely by the second-rank orientational order parameter. The simulations indicate that the retardation factor at constant strength parameter is density dependent, a feature neglected in the relaxation theory. We compare the simulation results where possible with experiment
Palaeolithic geoarchaeology: palaeolandscape modelling and scales of investigation
No full textThe present day landscape context of Palaeolithic artefacts is very different to that of their original use and deposition. Reconstructing the contemporary landscapes that were associated with even the best known sites is often unusually fraught with problems. This paper argues that a landscape-based approach is essential to contextualise our limited but growing knowledge of early human activity in Britain. Further, it argues through a series of case studies at macro, meso and micro level that the use of landscape concepts at a variety of spatial and temporal scales can increase our understanding of the Palaeolithic beyond individual sites. Most importantly, a landscape framework starts to give added meaning to the disturbed and transported material which predominates in fluvial contexts, but which has often been regarded as of insufficient significance to merit research protection or research. Indeed, researching such data in a landscape context - treating it as a time- and space-averaged sample - might give a more representative view of the Palaeolithic world, for example explaining general patterns of material cultural change, and of Palaeolithic peoples' landscape perceptions, than even highly preserved evidence from a few square metres representing one afternoon in the distant past
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