Two-step mechanism of rotational relaxation in lamellar phases of rods: accelerating effect of the addition of spheres.

By using computer simulation on a model colloidal rod-sphere mixture in its lamellar phase, the mechanism responsible for the rod rotational relaxation has been definitely identified and characterized. It consists of two steps: first, a rod, parallel to the director, has to escape from the layer in which it is located and go into the interlayer region mostly populated by spheres, perpendicular to the director; then, it has to insinuate again into one of the adjacent layers, with 50% of probability of resulting antiparallel to the director. While this mechanism is also operating in a pure smectic phase of rods, the presence of spheres increases notably its efficacy, thus promoting the rotational relaxation and facilitating its observation.

Chemical Detail Force Fields for Mesogenic Molecules.

Intra- and intermolecular potential energy surfaces of the 4,4'-di-n-heptyl azoxybenzene molecule have been sampled by ab initio calculations and represented through a force field suitable for classical bulk simulations. The parametrization of the molecular internal flexibility has been performed by a fitting procedure based on single molecule Hessian, gradients and torsional energies, computed using density functional theory. The intermolecular part of the force field has been derived as a pure pair potential, by fitting the dimer potential energy surface sampled by the Fragmentation Reconstruction Method.

Diffusivity of wormlike particles in isotropic melts and the influence of local nematization.

Systems of wormlike particles are studied by molecular dynamics computer simulations in the isotropic phase. The particles are modeled as a chain of nine soft repulsive, partially overlapping, spherical sites. These particles are characterized by a varying degree of internal flexibility: from very stiff and rodlike to very deformable and stringlike.

Mechanism of diffusion in the smectic- A phase of wormlike rods studied by computer simulation.

Molecular dynamics computer simulations have been carried out in the smectic- A phase of stiff wormlike rods. The analysis of the long trajectories generated has allowed for a detailed insight into that diffusion mechanism which is operative in the above-mentioned liquid-crystalline phase, as recently visualized in a system of colloidal virus rods. Fast particles, i.

Phase behavior of wormlike rods.

By employing molecular dynamics computer simulations, the phase behavior of systems of rodlike particles with varying degree of internal flexibility has been traced from the perfectly rigid rod limit till very flexible particles, and from the high density region till the isotropic phase. From the perfectly rigid rod limit and enhancing the internal flexibility, the range of the smectic- A phase is squeezed out by the concomitant action of the scarcely affected crystalline phase at higher density and the nematic phase at lower density, until it disappears. These results confirm the supposition, drawn from previous theoretical, simulational and experimental studies, that the smectic- A phase is destabilized by introducing and enhancing the degree of particle internal flexibility.

Atomistic computer simulation and experimental study on the dynamics of the n-cyanobiphenyls mesogenic series.

Several dynamic properties of the 4-n-alkyl-4'-cyanobiphenyls series ( nCB) with n=5, 6, 7, 8 have been studied by atomistic molecular dynamics (MD) simulations in the NVE ensemble adopting an ab initio derived force field (J. Phys. Chem.

Subdiffusive dynamics of a liquid crystal in the isotropic phase.

The isotropic phase dynamics of a system of 4-n-hexyl-4'-cyano-biphenyl (6CB) molecules has been studied by molecular dynamics computer simulations. We have explored the range of 275-330 K keeping the system isotropic, although supercooled under its nematic transition temperature. The weak rototranslational coupling allowed us to separately evaluate translational (TDOF) and orientational degrees of freedom (ODOF).

Anomalous diffusion and cage effects in the isotropic phase of a liquid crystal.

The translational motion of 4-n-hexyl-4'-cyanobiphenyl (6CB) in its isotropic phase has been studied by atomistic molecular dynamics simulation from 280 to 330 K. The mean square displacement shows evidence of a subdiffusive dynamics, with a plateau that becomes very apparent at the lowest temperatures. A three-time self-intermediate scattering function reveals that this plateau is connected with a homogeneous dynamics that, at longer times, becomes heterogeneous and finally exponential.

Sixfold bond orientational properties of a model liquid crystal in the dimensional crossover of B phases: a computer simulation study.

A wide range of NPT simulations of a bead necklace liquid crystal model in the crystal B, smectic B, smectic A, and nematic phases have been performed. Systems with up to 21 600 molecules have been studied to observe the behavior of slowly decaying spatial correlation functions. The pair correlation function and its in-plane restriction are consistent with a crystalline phase made of independent two-dimensional crystalline layers.

Liquid crystal properties of the n-alkyl-cyanobiphenyl series from atomistic simulations with ab initio derived force fields.

Lengthy molecular dynamics (MD) simulations were performed at constant atmospheric pressure and different temperatures for the series of the 4-n-alkyl-4'-cyanobiphenyls (nCB) with n = 6, 7, and 8. The accurate atomistic force field (Bizzarri, M.; Cacelli, I.

Smectic order parameters from diffusion data.

Microcanonical molecular dynamics simulations have been performed in the smectic A phase of an elementary liquid-crystal model. Smectic order parameters and diffusion coefficients along directions parallel and perpendicular to the director have been calculated during the same trajectory for a number of state points. This has permitted the satisfactory testing of a procedure, adopted in the analysis of experimental self-diffusion coefficients, leading to an estimate of the temperature dependence of the smectic order parameters.

Modeling a liquid crystal dynamics by atomistic simulation with an ab initio derived force field.

Atomistic molecular dynamics (MD) simulations of 4-n-pentyl 4'-cyano-biphenyl (5CB) have been performed, adopting a specific ab initio derived force field. Two state points in the nematic phase and three in the isotropic phase, as determined in a previous work, have been considered. At each state point, at least 10 ns have been produced, allowing us to accurately calculate single-molecule properties.

Orientational dynamics in the isotropic phase of a calamitic liquid-crystal model.

We report a molecular dynamics simulation study on the isotropic phase of an idealized calamitic liquid crystal model with a length-to-width ratio of approximately 5-6. The study focuses on the characterization of single-particle and collective orientational dynamics on approaching the phase transition to the nematic phase. Recent experimental and simulation works have suggested that a power law behavior exists at relatively short times in the decay of the time derivative of the orientational correlation functions.

Diffusion and viscosity of a calamitic liquid crystal model studied by computer simulation.

We report a molecular dynamics simulation study on an ensemble of rod-like particles, each composed of nine soft spheres held rigidly along a line. We have calculated translational mean square displacements and velocity autocorrelation functions in the fluid phases exhibited by the model, i.e.

Numerical study of a calamitic liquid-crystal model: phase behavior and structure.

We have studied an idealized calamitic liquid-crystal model, consisting of a linear rigid array of nine soft repulsive spheres, employing both theory and molecular dynamics simulation. The phase behavior (which includes crystalline, smectic, nematic, and isotropic phases) and structure of a collection of these rodlike particles have been determined by molecular dynamics simulation in an isothermal-isobaric ensemble. The liquid crystalline part of the phase diagram has been compared to that emerging from an Onsager-type density-functional theory.