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Field-induced alignment of the directors in the smectic A phase. Experiment and simulation
No full textIn contrast to nematics far less is known about the alignment of the directors in a smectic A phase by an external field. To characterise this process, which necessarily involves the flow of the molecules, we have undertaken an NMR investigation of the smectic A phase of 4-octyl-4'-cyanobiphenyl. The results of these experiments have revealed that, as expected, the alignment of the directors is a complex process strongly influenced by the nature of the containing surface for the smectic A phase. To assist in understanding the alignment process we have used computer simulations to study the field induced alignment of the directors in the smectic A phase formed by the Gay-Berne mesogen GB(4.4,20.0,1.1)
Computer simulation studies of anisotropic systems; IV: the effect of translational freedom
No full textComputer simulation studies of anisotropic systems; II: uniaxial and biaxial nematics formed by non-cylindrically symmetric molecules
No full textComputer simulation studies of anisotropic systems; IX: the Mayer-Saupe theory for nematic liquid crystals and the molecular-field approximation
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Computer simulation studies of anisotropic systems. XXXII. Field-induction of a smectic A phase in a Gay-Berne mesogen
No full textMolecular field theory predicts the induction of a smectic A phase by the application of a field, either magnetic or electric, to a nematic phase. This intriguing behavior results from an enhancement of the orientational order which is coupled to the translational order and so shifts the smectic A-nematic transition. To test this prediction we have investigated a system of Gay-Berne mesogenic molecules subject to an applied field of second rank using isothermal-isobaric Monte Carlo simulations. The results of our calculations are compared with the Kventsel-Luckhurst-Zewdie molecular field theory of smectogens, modified to include the effect of an external field. We have also used the simulations to explore the possibility of inducing more ordered smectic phases with stronger fields
A deuterium nuclear magnetic resonance investigation of the director distribution in a thin nematic liquid crystal slab
No full textThe director distribution in a thin nematic liquid crystal (NLC), 4-pentyl-d(2)-4'-cyanobiphenyl (5CB-d(2)) deuteriated in the ex-position of the pentyl chain, confined between two glass plates, with untreated and treated anchoring conditions, has been investigated using a deuterium nuclear magnetic resonance (NMR). The NMR spectra have been measured as a function of the applied electric field. In the absence of surface forces it is found that the director aligns parallel to the magnetic field at relatively low values of the electric field as Delta is positive for 5CB. In the presence of surface forces with increasing electric field the quadrupolar splitting decreases, passes through zero and then increases again to a value which is essentially half of that at zero electric field. That is, the director orientation changes more or less continuously from being parallel to the magnetic field to being orthogonal to it, as the electric field grows
Computer simulation studies of anisotropic systems. Part XXXI. The continuous generic model for liquid crystal dimers
No full textThe essential molecular features responsible for the unusual nematic behaviour of liquid crystal dimers appear to be captured by the continuous version of the generic model. In this, two cylindrically symmetric mesogenic groups are linked by a virtual group to give a butane-like structure where the torsional potential controls the relative amounts of linear and bent conformers. The transitional behaviour of this continuous generic model has been predicted, using molecular field theory, as a function of the energy separation between the cis and trans conformers. The predictions are compared with the behaviour of real liquid crystal dimers. In addition the properties of the model have been evaluated for one parameterisation of the torsional potential, which favours the bent conformers, using a Monte Carlo simulation. Included in the properties determined as a function of temperature are the orientational order parameters for the mesogenic groups, the local orientational order parameter and the conformational order parameters. These results are compared with the predictions of the molecular field theory. The continuous generic model can also be used to describe the behaviour of other systems which include mesogens such as the azobenzenes capable of undergoing photo-induced cis trans isomerisation. We have, therefore, used our model to undertake a preliminary investigation of the quasi-dynamics of the nematic isotropic phase transition resulting from a conformational change. In these and other studies the behaviour of the continuous generic model is seen to depend critically on the synergy between the orientational and conformational order
Magnetic field induced alignment of the director in a smectic A phase: surface effects
No full textDeuterium NMR studies of the alignment by a magnetic field of the director in the smectic A phase of 4-octyl-4'-cyanobiphenyl have shown this to be a complex process. To explore the influence of the interaction between the smectic A phase and the surface of its container on the pathways available for director alignment we have investigated a droplet of this phase suspended in glycerol. The field induced alignment of the director in the droplet is found to be dramatically different, both quantitatively and qualitatively, to that found for the smectic A phase confined to a glass tube
The alignment of the smectic a phase of 4-octyl-4'-cyanobiphenyl induced by an electric field. A time-resolved deuterium NMR study
No full textStudies of the field-induced alignment of the SmA phase using deuterium NMR spectroscopy have revealed a complex pattern of behaviour when the director is initially orthogonal to the aligning field. Here we report the electric field-induced alignment of the SmA director using time-resolved deuterium NMR when the aligning electric field E is at an angle with the magnetic field B of the spectrometer which is considerably smaller than 90degrees; here the director is initially aligned parallel to the magnetic field. The dynamics of the electric field-induced alignment of the director for the smectic phase of 4-alpha,alpha-d(2)-octyl-4'-cyanobiphenyl (8CB-d(2)) was investigated at two angles between B and E of roughly 45degrees and 54.5degrees the so-called magic angle, at different electric field strengths and also at two temperatures, 305.1 K and 302.6 K The dynamics of the SmA director alignment of 8CB-d(2) was monitored by measuring the deuterium NMR spectrum as a function of time. The results for the 45degrees and 54.5degrees geometries revealed, in contrast to the complex dynamics of alignment of the SmA director observed for the 90degrees geometry, a much simpler pattern of relaxation. Here the director appeared to be aligned almost as a monodomain. In general, for either of the two geometries employed here, the ultimate angle of alignment the director achieves relative to E depends on the electric field strength. Lowering the temperature by just 2.5degreesC from 305.1 K to 302.6 K has a dramatic effect on the rate of director relaxation presumably because of the large increase in the combined rotational viscosity and the elastic energy effects of the SmA phase with decreasing temperature. Furthermore for the 45 geometry at 302.6 K, the SmA sample separates on relaxation ultimately into two domains with different alignment angles
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