Douglas J. Cleaver

Computer simulation of liquid crystals

A review is presented of molecular and mesoscopic computer simulations of liquid crystalline systems. Molecular simulation approaches applied to such systems are described, and the key findings for bulk phase behaviour are reported. Following this, recently developed lattice Boltzmann approaches to the mesoscale modelling of nemato-dynamics are reviewed. This paper concludes with a discussion of possible areas for future development in this field.

A lattice spring model of heterogeneous materials with plasticity

A three-dimensional lattice spring model of a heterogeneous material is presented. For small deformations, the model is shown to recover the governing equations for an isotropic elastic medium. The model gives reasonable agreement with theoretical predictions for the elastic fields generated by a spherical inclusion, although for small particle sizes the discretization of the underlying lattice causes some departures from the predicted values. Plasticity is introduced by decreasing the elastic moduli locally whilst maintaining stress continuity. Results are presented for a spherical inclusion in a plastic matrix and are found to be in good agreement with the predictions of Wilner (1988 J. Mech. Phys. Solids 36 141-65).

Bicontinuous minimal surface nanostructures for polymer blend solar cells

This paper presents the first examination of the potential for bicontinuous structures such as the gyroid structure to produce high efficiency solar cells based on conjugated polymers. The solar cell characteristics are predicted by a simulation model that shows how the morphology influences device performance through integration of all the processes occurring in organic photocells in a specified morphology. In bicontinuous phases, the surface defining the interface between the electron and hole transporting phases divides the volume into two disjoint subvolumes. Exciton loss is reduced because the interface at which charge separation occurs permeates the device so excitons have only a short distance to reach the interface. As each of the component phases is connected, charges will be able to reach the electrodes more easily. In simulations of the current-voltage characteristics of organic cells with gyroid, disordered blend and vertical rod (rods normal to the electrodes) morphologies, we find that gyroids have a lower than anticipated performance advantage over disordered blends, and that vertical rods are superior. These results are explored thoroughly, with geminate recombination, i.e. recombination of charges originating from the same exciton, identified as the primary source of loss. Thus, if an appropriate materials choice could reduce geminate recombination, gyroids show great promise for future research and applications.

Ordering of hard particles between hard walls

The structure of a fluid of hard Gaussian overlap particles of elongation κ = 5, confined between two hard walls, has been calculated from density-functional theory and Monte Carlo simulations. By using the exact expression for the excluded volume kernel (Velasco E and Mederos L 1998 J. Chem. Phys. 109 2361) and solving the appropriate Euler-Lagrange equation entirely numerically, we have been able to extend our theoretical predictions into the nematic phase, which had up till now remained relatively unexplored due to the high computational cost. Simulation reveals a rich adsorption behaviour with increasing bulk density, which is described semi-quantitatively by the theory without any adjustable parameters.

Computer modelling of the structure of 4-n-octyl-4′-cyanobiphenyl adsorbed on graphite

We have developed a united atom model for the mesogenic molecule 8-CB and have performed energy minimizations of single and multiple molecule systems in the presence of a planar substrate. The largest isosteric energy of adsorption obtained with this model for an independent molecule on a smooth substrate is -183·2 kJ mol-1. For this system, we have found eight stable configurations for pairs of roughly antiparallel molecules. A fifty-molecule strip based on two of these stable pairs is found to develop small, periodic fractures when minimized, consistent with scanning tunnelling microscopy studies of these systems. When a graphite substate potential is used, the largest single molecule energy of adsorption differs by only ⋍ 1% from the smooth substrate value. Although this suggests a weak coupling between the registry of the adsorbed monolayer of 8-CB and the underlying graphite, the adsorbed strip does appear to be commensurate with the surface. The strip normal is aligned with a lattice vector of the g...

Computer simulations of adsorbed liquid crystal films

The structures adopted by adsorbed thin films of Gay-Beme particles in the presence of a coexisting vapour phase are investigated by molecular dynamics simulation. The films are adsorbed at a flat substrate which favours planar anchoring, whereas the nematic-vapour interface favours normal alignment. On cooling, a system with a high molecule-substrate interaction strength exhibits substrate-induced planar orientational ordering and considerable stratification is observed in the density profiles. In contrast, a system with weak molecule-substrate coupling adopts a director orientation orthogonal to the substrate plane, owing to the increased influence of the nematic-vapour interface. There are significant differences between the structures adopted at the two interfaces, in contrast with the predictions of density functional treatments of such systems.

The role of attractive interactions in rod-sphere mixtures

We present a computer simulation study of binary mixtures of prolate Gay-Berne particles and Lennard-Jones spheres. Results are presented for three such rod-sphere systems which differ from each other only in the interaction between unlike particles. Both the mixing-demixing behavior and the transitions between the isotropic and any liquid crystalline phases are studied for each system, as a function of temperature and concentration ratio. For systems which show macroscopic demixing, the rod-sphere interaction is shown to give direct control over interfacial anchoring properties, giving rise to the possibility of micellar phase formation in the case of homeotropic anchoring. Additionally, it is shown that on incorporating high concentrations of spheres into a system of rods with weak demixing properties, microphase-separated structures can be induced, including bicontinuous and lamellar arrangements.

Discontinuous structural transition in a thin hybrid liquid crystal film

We show, by Monte Carlo simulation of the hard Gaussian overlap model, that the structure of a molecularly-thin liquid crystal film subject to hybrid anchoring conditions can change discontinuously if the anchoring coefficients at the two walls are made sufficiently different. We discuss our finding in the context of equivalent simulations of systems with symmetrical anchoring conditions and of recent theoretical work on pseudo-Casimir forces in hybrid liquid crystal films.

Computer modelling of the 4-n-alkyl-4′-cyanobiphenyls adsorbed on graphite: energy minimizations and molecular dynamics of periodic systems

The structures adopted within adsorbed monolayers of 4-n-octyl-4′-cyanobiphenyl (8-CB) molecules have been investigated using energy minimizations and molecular dynamics simulations of periodic systems. Using a smooth substrate potential, the most favourable energy of adsorption is found for a system with an eight-molecule unit-cell structure. This result is entirely consistent with scanning tunnelling microscopy studies of such systems, and differs from previous results using simulations of short strips which suggested a four-molecule unit cell. Molecular dynamics simulations of this 8-CB monolayer show that while the system exhibits smectic ordering at 150 K, the detailed eight-molecule unit-cell structure is lost. Simulations performed on a bilayer system indicate that the presence of a second molecular layer stabilizes the unit cell structure, except in the regions where there is partial penetration by the second layer molecules into the first layer. A third set of molecular dynamics simulations where...

Nematic liquid crystal alignment on chemical patterns

Patterned Self‐Assembled Monolayers (SAMs) promoting both homeotropic and planar degenerate alignment of 6CB and 9CB in their nematic phase were created using microcontact printing of functionalized organothiols on gold films. The effects of a range of different pattern geometries and sizes were investigated, including stripes, circles and checkerboards. Evanescent wave ellipsometry was used to study the orientation of the liquid crystal (LC) on these patterned surfaces during the isotropic‐nematic phase transition. Pretransitional growth of a homeotropic layer was observed on 1 µm homeotropic aligning stripes, followed by a homeotropic monodomain state prior to the bulk phase transition. Accompanying Monte Carlo simulations of LCs aligned on nanoscale‐patterned surfaces were also performed. These simulations also showed the presence of the homeotropic monodomain state prior to the transition.