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Computer simulations of hard pear-shaped particles We report results obtained from Monte Carlo simulations investigating mesophase formation in two model systems of hard pear-shaped particles. The first model considered is a hard variant of the truncated Stone-Expansion model previously shown to form nematic and smectic mesophases when embedded within a 12-6 Gay-Berne-like potential [R. Berardi, M. Ricci and Z. Zannoni. ChemPhysChem, 7:443, 2001]. When stripped of its attractive interactions, however, this system is found to lose its liquid crystalline phases. For particles of length to breadth ratio k = 3, glassy behaviour is seen at high pressures, whereas for k = 5 several bi-layer-like domains are seen, with high intradomain order but little interdomain orientational correlation. For the second model, which uses a parametric shape parameter based on the generalised Gay-Berne formalism, results are presented for particles with elongation k = 3, 4 and 5. Here, the systems with k = 3 and 4 fail to display orientationally ordered phases, but that with k = 5 shows isotropic, nematic and, unusually for a hard-particle model, interdigitated smectic A2 phases.
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Symmetric alignment of the nematic matrix between close penetrable colloidal particles A simple model is proposed for the liquid crystal matrix surrounding ‘soft’ colloidal particles whose separation is much smaller than their radii. We use our implementation of the Onsager approximation of density-functional theory [A. Chrzanowska, P. I. C. Teixeira, H. Ehrentraut and D. J. Cleaver, J. Phys.: Condens. Matter 13, 4715 (2001)] to calculate the structure of a nanometrically thin film of hard Gaussian overlap particles of elongations κ = 3 and κ = 5, confined between two solid walls. The penetrability of either substrate can be tuned independently to yield symmetric or hybrid alignment. Comparison with Monte Carlo simulations of the same system [D. J. Cleaver and P. I. C. Teixeira, Chem. Phys. Lett. 338, 1 (2001); F. Barmes and D. J. Cleaver (unpublished)] reveals good agreement in the symmetric case.
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Computer simulation of a liquid-crystal anchoring transition We present a study of the effects of confinement on a system of hard Gaussian overlap particles interacting with planar substrates through the hard-needle–wall potential. Using geometrical arguments to calculate the molecular volume absorbed at the substrates, we show that both planar and homeotropic arrangements can be obtained using this model. Monte Carlo simulations are then used to perform a systematic study of the model’s behaviour as a function of the system density and the hard-needle–wall interaction parameter. As well as showing the homeotropic to planar anchoring transition, the anchoring phase diagrams computed from these simulations indicate regions of bistability. This bistable behaviour is examined further through the explicit simulation of field-induced two-way switching between the two arrangements.
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Using particle shape to induce tilted and bistable liquid crystal anchoring We use Monte Carlo simulations of hard Gaussian overlap (HGO) particles symmetrically confined in slab geometry to investigate the role of particle-substrate interactions on liquid crystalline anchoring. Despite the restriction here to purely steric interactions and smooth substrates, a range of behaviours are captured, including tilted anchoring and homeotropic-planar bistability. These macroscopic behaviours are all achieved through appropriate tuning of the microscopics of the HGO-substrate interaction, based upon non-additive descriptions for the HGO-substrate shape parameter. Download this article
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Computer simulation of bistable switching in a nematic device containing pear-shaped particles. We study the microscopic basis of bistable switching of a confined liquid crystal via Monte Carlo simulations of hard pear-shaped particles. Using both dielectric and dipolar field couplings to this intrinsically flexoelectric fluid, it is shown that pulsed fields of opposing polarity can be used to switch between the vertical and hybrid aligned states. Further, it is shown that the field susceptibility of the surface polarisation, rather than the bulk flexoelectricity, is the main driver of this switching behaviour.
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Molecular dynamics of shock-wave induced structural changes in silica glasses We seek to model the shock wave induced structural changes in silicate glass at the atomic scale. We use both direct shock propagation with non-equilibrium molecular dynamics (NEMD) and bulk simulations in the Hugoniot ensemble to characterize the structure and topology of the shocked glass. Despite the lack of long-range interactions in our model, the close agreement between our structures and those obtained by experimental and simulation studies alike, underlines the importance of the role played by first neighbor interactions on the structure of silicate glass. The results obtained from this study show that, in agreement with experimental work, the structure and topology of the shock-induced densified phase is unique in its structure as can be revealed by medium-range order measurements. The modifications include a reduction of the average tetrahedra size and an increase in the proportion of 3-4 and 8-10 membered Si-rings. Application of a Hugoniostat method based on constraint dynamics shows near-perfect agreement with the NEMD results. Besides validating the former method, this opens the prospect of studying shock-induced effects at a fraction of the cost required to run large scale shock simulations while using much more complicated potentials and setups.
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Entropy-driven formation of the gyroid cubic phase We show, by computer simulation, that tapered or pear-shaped particles, interacting through purely repulsive interactions, can freely self-assemble to form the three-dimensionally periodic, gyroid cubic phase. The Ia3d gyroid cubic phase is formed by these particles both on compression of an isotropic configuration and on expansion of a smectic A bilayer arrangement. For the latter case, it is possible identify the steps by which the topological transformation from non-intersecting planes to fully interpenetrating, periodic networks takes place.
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Simulation and theory of hybrid aligned liquid crystal films We present a study of the effects of nanoconfinement on a system of hard Gaussian overlap particles interacting with planar substrates through the hard-needle-wall potential, extending earlier work by two of us [D. J. Cleaver and P. I. C. Teixeira, Chem. Phys. Lett. 338, 1 (2001)]. Here, we consider the case of hybrid films, where one of the substrates induces strongly homeotropic anchoring, while the other favors either weakly homeotropic or planar anchoring. These systems are investigated using both Monte Carlo simulation and density-functional theory, the latter implemented at the level of Onsager’s second-virial approximation with Parsons-Lee rescaling. The orientational structure is found to change either continuously or discontinuously depending on substrate separation, in agreement with earlier predictions by others. The theory is seen to perform well in spite of its simplicity, predicting the positional and orientational structure seen in simulations even for small particle elongations.
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