Molecular dynamics simulation of a liquid crystalline mixture

Phase ordering kinetics in uniaxial nematic liquid crystals with second- and fourth-rank interactions

We present comprehensive results of the Monte Carlo (MC) simulations of the phase ordering dynamics in d = 2 nematic liquid crystals. We study a system of size N (2)(N = 512) with molecules confined to a 512×512 square lattice and report the results for two LC Hamiltonians: generalized Lebwohl-Lasher (GLL) model and r(-6) dependent anisotropic dispersion interaction potential. In these Hamiltonians a fourth-rank Legendre polynomial P4 interaction term is added to the usual second-rank P2 term. We find that in both the cases the presence of the P4 interaction term significantly influences the nematic domains morphology. Our numerical data show a diffusive growth law with a logarithmic correction: L(t) ∼ (t/ln t)(1/2).

Dynamical and structural properties of charged and uncharged lidocaine in a lipid bilayer

Molecular dynamics computer simulations have been performed to investigate dynamical and structural properties of a lidocaine local anesthetic. Both charged and uncharged forms of the lidocaine molecule were investigated. Properties such as membrane area per lipid, diffusion, mass density, bilayer penetration and order parameters have been examined. An analysis of the lidocaine interaction with the lipid surrounding according to a simple mean field theory has also been performed. Almost all examined properties were found to depend on which of the two forms of lidocaine, charged or uncharged, is studied. The overall picture is a rather static behavior determined by the lipids for the charged molecules and more mobile situation of the uncharged form with higher diffusion and lower orientational and positional order.

Microscopic structure and dynamics of a partial bilayer smectic liquid crystal

Cyanobiphenyls (nCB's) represent a useful and intensively studied class of mesogens. Many of the peculiar properties of nCB's (e.g., the occurrence of the partial bilayer smectic-A(d) phase) are thought to be a manifestation of short-range antiparallel association of neighboring molecules, resulting from strong dipole-dipole interactions between cyano groups. To test and extend existing models of microscopic ordering in nCB's, we carry out large-scale atomistic simulation studies of the microscopic structure and dynamics of the Sm-A(d) phase of 4-octyl-4'-cyanobiphenyl (8CB). We compute a variety of thermodynamic, structural, and dynamical properties for this material, and make a detailed comparison of our results with experimental measurements in order to validate our molecular model. Semiquantitative agreement with experiment is found: the smectic layer spacing and mass density are well reproduced, translational diffusion constants are similar to experiment, but the orientational ordering of alkyl chains is overestimated. This simulation provides a detailed picture of molecular conformation, smectic layer structure, and intermolecular correlations in Sm-A(d) 8CB, and demonstrates that pronounced short-range antiparallel association of molecules arising from dipole-dipole interactions plays a dominant role in determining the molecular-scale structure of 8CB.

Dielectric and elastic properties of liquid crystals

The structural properties, the static and relaxation dielectric coefficients [epsilon(j) and epsilon(j)(omega) (j= ||, perpendicular)], the rotational diffusion constants D( perpendicular) and D( ||), the orientational correlation times tau(1)(i0) (i=0,1), and the bulk elastic constants K(i) (i=1,2,3) are investigated for polar liquid crystals, such as 4-n-pentyl-4(')-cyanobiphenyl (5CB). epsilon(j) are calculated by a combination of the existing molecular theory and statistical-mechanical approach (SMA) that takes into account translational and orientational correlations as well as their coupling, whereas epsilon(j)(omega) are calculated by combining SMA and nuclear magnetic resonance relaxation theory, both based on a rotational diffusion model in which the reorientation of an individual molecule is assumed as stochastic Brownian motion in a potential of mean torque. Reasonable agreement between the calculated and experimental values of epsilon(j) and epsilon(j)(omega) for 5CB is obtained. The bulk Frank elastic constants K(i) (i=1,2,3), for splay, twist, and bend distortion modes, as well as their ratios K(3)/K(1) and K(2)/K(1) are also obtained.

Statistical-mechanical study of the pair correlations for the dipolar Gay-Berne model

A statistical-mechanical theory based upon the method of conditional distribution function has been applied to calculations of the nearest-neighbor (NN) and next NN correlators as well as order parameters. The method takes into account translational and orientational correlations as well as their coupling. Using the dipolar Gay-Berne interaction potential, calculations have been carried out for temperatures and densities corresponding to a nematic phase. The results in orientational distribution functions indicate a significant effect of the dipole-dipole interactions on the orientational order of the liquid crystal. It was found that NN particles tend to be mutually antiparallel, whereas a reversed tendency is observed for next NN particles. These results are in agreement with computer simulations and with interpretations of experimental data.

Rotational viscosity in a nematic liquid crystal: a theoretical treatment and molecular dynamics simulation

The rotational viscosity coefficient gamma(1) of 4-n-pentyl-4(')-cyanobiphenyl in the nematic phase is investigated by combination of existing statistical-mechanical approaches (SMAs), based on a rotational diffusion model and computer simulation technique. The SMAs rest on a model in which it is assumed that the reorientation of an individual molecule is a stochastic Brownian motion in a certain potential of mean torque. According to the SMAs, gamma(1) is found to be a function of temperature, density, rotational diffusion coefficient, and a number of order parameters (OPs). The diffusion coefficient and the OPs were obtained from an analysis of a trajectory generated in a molecular dynamics simulation using realistic atom-atom interactions. In addition, a set of experimentally determined diffusion coefficients and OPs was used for evaluation of gamma(1). Reasonable agreement between calculated and experimental values of gamma(1) is obtained. It is shown that near the clearing point gamma(1) is proportional to (-)P22, where (-)P2 is the second-rank OP. This limiting value of gamma(1) is in agreement with mean-field theory.