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Geoffrey R Luckhurst

Publications and source records attributed to Geoffrey R Luckhurst.

5 recordsLinked to original sources
PubMed2005

Biaxial nematic phases and V-shaped molecules: a Monte Carlo simulation study.

Inspired 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.

Journal Article↗
PubMed2005

Biaxial nematics: computer simulation studies of a generic rod-disc dimer model.

One 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.

Anisotropy↗
PubMed2004

Studies of translational diffusion in the smectic A phase of a Gay-Berne mesogen using molecular dynamics computer simulation.

Molecular dynamics computer simulations are used to determine the self-diffusion coefficients for a Gay-Berne model mesogen GB (4.4,20,1,1) in the isotropic, nematic and smectic A phases along two isobars. The values of the parallel and perpendicular diffusion coefficients, D(parallel) and D(perpendicular), are calculated and compared in the different phases. For the phase sequence isotropic-smectic A, D(perpendicular)*> or =D(parallel)* over the whole smectic A range with the ratio D(parallel)*/D(perpendicular)* decreasing with decreasing temperature. At a higher pressure, a nematic phase is observed between these two phases and we find that D(parallel)*>D(perpendicular)* throughout the nematic region and the inequality D(parallel)*>D(perpendicular)* remains on entering the smectic A phase. However, the ratio D(parallel)*/D(perpendicular)* decreases with decreasing temperature within the smectic A range and eventually this ratio inverts such that D(perpendicular)*>D(parallel)* at low temperatures. The temperature dependence of the parallel diffusion coefficient in the smectic A phase for this model mesogen is compared to that predicted by a theoretical model for diffusion subject to a cosine potential.

Journal Article↗