Visualization of Electrostatic Dipoles in Molecular Dynamics of Metal Oxides

Metal oxides are important for many technical applications. For example alumina (aluminum oxide) is the most commonly-used ceramic in microelectronic devices thanks to its excellent properties. Experimental studies of these materials are increasingly supplemented with computer simulations. Molecular dynamics (MD) simulations can reproduce the material behavior very well and are now reaching time scales relevant for interesting processes like crack propagation. In this work we focus on the visualization of induced electric dipole moments on oxygen atoms in crack propagation simulations. The straightforward visualization using glyphs for the individual atoms, simple shapes like spheres or arrows, is insufficient for providing information about the data set as a whole. As our contribution we show for the first time that fractional anisotropy values computed from the local neighborhood of individual atoms of MD simulation data depict important information about relevant properties of the field of induced electric dipole moments. Iso surfaces in the field of fractional anisotropy as well as adjustments of the glyph representation allow the user to identify regions of correlated orientation. We present novel and relevant findings for the application domain resulting from these visualizations, like the influence of mechanical forces on the electrostatic properties.

Correlations in the isotropic phases of chiral liquid crystals: the role of helicity modes

The phenomenological theory of chiral liquid crystals is further developed by generalizing the model of self-consistent correlations [J. Englert, L. Longa, H. Stark, and H.-R. Trebin, Phys. Rev. Lett. 81, 1457 (1998)]. In the present approach, not only a leading helicity mode of the tensor order parameter is retained but also the remaining four modes. By considering a full fluctuating spectrum of the order parameter, the role of correlations between helicity modes in the isotropic phases is studied. Additionally, an exact form of the two-point correlation function in real space is derived and its properties are thoroughly discussed. It is shown that for chiral isotropic liquids purely chiral modes could be identified that do not exist for an ordinary liquid. Detailed results of the numerical calculations are compared with those obtained from the earlier model and these show regions where the coupling between the modes becomes important, in agreement with the available experimental data. Though the analysis up to first-order cumulant expansion does not predict a direct phase transition between the blue phase III and the isotropic phase, it is fairly easy to identify two differently correlated regions in a temperature-chirality plane. Various structural quantities, such as optical activity and specific heat, also reveal a behavior characteristic of two isotropic phases with different correlation lengths.

Motion, creation, and annihilation of disclinations in multidomain structured nematic liquid crystal cells

We study dynamical processes in a multidomain (MD) structured nematic liquid crystal cell with a particular emphasis on the motion, creation, and annihilation of disclinations. In the MD cell right- and left-handed director helices alternate due to a special choice of the director pretilt angles at the surfaces. As a result, a net of twist disclinations occurs. We have implemented a numerical algorithm based on a pure rotational dynamics of the director field to monitor the motion of the defect lines during the switching process, i.e., when an electric voltage is applied to or removed from the cell. We demonstrate that the total light transmission vs time is not affected by the presence of the defects compared to a conventional twisted nematic cell. If the pretilt angles at the surfaces are sufficiently small, the twisting sense of one species of helices is reversed and a configuration free of defects occurs. On the other hand, for an applied voltage twist disclinations close to the surface have to exist. Therefore, defect lines are created or they annihilate during the switching process. We investigate these situations in detail and reveal the underlying mechanisms.