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Biaxial nematic liquid crystals: fact or fiction?
No full textMesogenic molecules invariably deviate from the cylindrical symmetry usually assumed for them. One consequence of this is that, in addition to the ubiquitous uniaxial nematic phase, there should also be a biaxial nematic phase. For this there are three different principal components. of a second rank tensorial property, such as the magnetic susceptibility, as opposed to two for a uniaxial phase. Each of these components is associated with a separate director; the three directors correspond to the directions about which three orthogonal axes set in the molecule tend to be aligned. Although the existence of the biaxial nematic phase was predicted over 30 years ago it was not until 10 years later that the first claim to have found this phase appeared. This system was, in fact, a lyotropic liquid crystal. Surprisingly, the search for a thermotropic biaxial nematic has proved to be especially challenging. This review is concerned with the thermotropic materials for which a biaxial nematic have now been claimed. Of particular importance is the technique used to establish the symmetry of the nematic phase and it is suggested that deuterium NMR spectroscopy is a powerful method with which to determine this. However, the biaxial nematics claimed to be formed by certain compounds are shown by NMR to have uniaxial symmetry. The reasons why the biaxial nematic phase proves to be so elusive are explored using molecular field theory and used to inform the design of thermotropic mesogens which are likely to form this phase
On the creation of director disorder in nematic liquid crystals
No full textOne of the striking features of a nematic liquid crystal is the ease with which the random director distribution, characteristic of an unperturbed system, can be converted into a state of uniform alignment with weak magnetic fields. Here we are concerned with how this order can be destroyed not only because new liquid crystal physics may be involved in the process but because there are uses for the disordered state. The torque responsible for creating the disorder, which must compete with the uniform magnetic field, should not only exceed some threshold value but must also be random. These conditions can be achieved using surface and elastic torques produced by a suspension of colloidal particles and by the network of a gelator. The hydrodynamics produced by sample spinning also competes with the magnetic field but in an apparently coherent manner. However, a random element is introduced during the spinning and this produces a random distribution of the director in the plane orthogonal to the spinning axis. Pressure waves seem to have little influence on the director distribution but at the onset of cavitation the director alignment is destroyed, presumably as a result of the implosion of bubbles created in the low-pressure regions. Our studies have benefited from the use of ESR spectroscopy to determine the extent of director disorder and the basis of this powerful technique is described here
A computer simulation investigation of the Freedericksz transition for the nematic phase of a Gay-Berne mesogen
No full textWe present the first computer simulation study of the Freederichsz transition in a nematic phase using a Gay-Berne model mesogen, in which the constituent molecules can both rotate and translate freely. The behaviour and structure of the nematic obtained from the simulation for the Freederichsz transition are compared with those for real nematogens. The twist elastic constant has been estimated directly from the threshold field and found to be significantly larger than for real systems although comparable to another Gay-Berne nematogen. The molecules in the middle of the cell are observed to have a biaxial order in the vicinity of the Freederichsz transition. Those molecules adjacent to the surface are found to be unpinned in a discontinuous transition at a field significantly larger than the threshold field for the Freederichsz transition
Computer simulation of the field-induced alignment of the smectic A phase of the Gay-Berne mesogen GB(4.4,20.0,1,1)
No full textAs an aid to understanding the mechanism for the field-induced alignment of the smectic A phase we have attempted to simulate this process for the Gay-Berne model mesogen GB(4.4,20.0,1,1). The results for the quasi-dynamical behaviour of the alignment process obtained from a constant pressure-constant temperature Monte Carlo simulation are discussed. It. is found that the molecules realign parallel to the field without destroying the layer structure and, more or less, as a monodomain in the simulations. This is qualitatively in agreement with the results obtained from recent deuterium NMR and small angle X-ray experiments. In addition, the relationship between the box shape and structures formed during the alignment process is discussed. The choice of box is found to be important for the simulation of the field-induced alignment process
Electric-field driven director oscillations in nematic liquid crystals
No full textWe have used deuterium NMR spectroscopy to investigate the director dynamics and equilibrium behaviour in nematic liquid crystals (4-pentyl and 4-octyl-4?-cyanobiphenyl (5CB and 8CB) both specifically deuteriated) when subject to magnetic and ac electric fields. The angle between the magnetic and electric fields can be varied between 0 and 90° and the most common geometry we have used is for an angle of about 45°. For 5CB and 8CB (with positive and ) the director orientation was measured using time-resolved NMR both when the electric field is applied and when it is turned off. In all cases it was found that the director alignment was uniform and the director relaxation follows closely the predictions of the torque-balance equation given by the Leslie-Ericksen theory. In all these experiments we have employed a 10kHz electric field; at such a relatively high frequency the director experiences an effectively constant value of the electric field.
We have now investigated the behaviour of the nematic director for the two liquid crystals at much lower frequencies of the electric fields: several Hz to about 1000Hz. As before, the director orientation was measured using time-resolved deuterium NMR spectroscopy. We have employed two geometries. In one, the electric and magnetic fields were inclined at ~ 50° We found that the director oscillates between two extreme orientations (determined by the frequency and the field strength) in a plane formed by the magnetic and electric fields. The oscillations were observed to continue for many cycles, indicating that the coherence in the director orientation was not lost during this motion. The director was found to remain uniformly aligned. The two extreme director orientations can also be determined from the NMR spectrum time-averaged over many thousands of cycles of oscillations. At low frequencies (several Hz) these limiting angles are essentially independent of frequency but as the frequency increases so the two angles approach each other and become equal at high frequencies.
More recently, we have used a geometry with the angle between the fields of ~90º. A threshold behaviour is observed in this geometry for the director orientation as a function of the applied voltage. The time-averaged spectra at low frequencies and at certain voltages showed unusual powder-like features. Time-resolved NMR measurements at 40Hz and different voltages near the threshold value were carried out to understand the oscillatory behaviour which was also simulated. Turn-on and turn-off dynamics at high frequency were conducted revealing intriguing differences between the two pathways for the field-induced relaxation. These results will be discussed and interpreted in terms of the torque-balance equation with a time dependent electric field
The Gay-Berne mesogen: a paradigm shift?
No full textA Commentary on the paper “Computer simulation studies of anisotropic systems XIX. Mesophases formed by the Gay-Berne model mesogen„, by G.R. Luckhurst, R.A. Stephens and R.W. Phippen. First published in Liquid Crystals, 8, 451-464 (1990)
Monte Carlo lattice simulations of the elastic behaviour of nematic liquid crystals
No full textA pairwise additive potential, which approximately reproduces the free energy density for the elastic deformations of a nematic liquid crystal, originally proposed by Gruhn and Hess, has been investigated by simulating the three Freedericksz transitions as well as that of the Schadt-Helfrich cell. The pair potential depends on the three elastic constants K-1, K-2 and K-3 for the splay, twist acid bend deformations, respectively. The results of the simulations are compared with the analytical solutions obtained from continuum theory in order to test the accuracy of the model potential at a quantitative level. This comparison is also made for different temperatures to explore the influence of director fluctuations on the elastic behaviour
Computer simulation studies of anisotropic systems XIX: mesophases formed by the Gay-Berne model mesogen
No full textWe report the results of a molecular dynamics computer simulation of particles interacting via the Gay-Berne potential with parameters selected to approximate those of mesogenic molecules. The system was found to form a variety of mesophases as the temperature was lowered. We have characterized these phases with the aid of computer graphics techniques to visualize the molecular organization within configurations taken from the production stage of the simulations. The phases have been identified, on the basis of such images, as isotropic, nematic, smectic A, smectic B and crystal
Shear-induced structural changes of a smectic-A phase: a computer simulation study
No full textWe have carried out a Monte Carlo simulation of a thin sample of the smectic-A phase of the Gay-Berne mesogen GB(4.4,20.0,1,1) sandwiched between two plates and subject to shear. The smectic layers are perpendicular to the confining plates and are pinned at the boundaries. The thickness of the samples studied ranges from about three to twenty molecules. The layers tilt progressively with increasing shear, but rearrange themselves at a critical shear. At this critical shear the layers melt near the center of the sample and reform with a reduced tilt consistent with the layer pinning at the walls. The pseudodynamics of this process as the smectic layers melt and are reformed have been followed during the simulation. The critical layer tilt at which slippage takes place tends to a constant value for thick samples, but for very thin samples the critical shear tends toward half a smectic layer, with a significantly reduced translational order near the sample center just before the critical shear. The simulation results are consistent with the predictions of the mean field theory of this phenomenon developed by Mottram et al
Simple model for biaxial smectic-A liquid-crystal phases
No full textWe have generalized the McMillan theory of liquid crystalline smectic order in uniaxial particle fluids to biaxial particles. Upon varying the control parameter, a uniaxial nematic phase may: (i) order biaxially first, then smectically; (ii) order smectically first, then biaxially; and (iii) simultaneously order biaxially and smectically. We investigate, in the limit of complete orientational order of the molecular major axes, which of these scenarios are realized for a simple model of particles with the symmetry of rectangular parallelepipeds. We also present a generic variational derivation of the theory based on the identification of the dominant order parameters for the most ordered phase
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