08 Fakultät Mathematik und Physik
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/9
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Item Open Access Physics of inhomogeneous nematic liquid crystals : colloidal dispersions and multiple scattering of light(1999) Stark, Holger; Trebin, Hans-Rainer (Prof. Dr.)The habilitation thesis deals with two interesting aspects of nematic liquid crystals with an inhomogeneous orientational order induced either by dispersed particles or by thermal director fluctuations. In the first part, the phenomenological description of the nematic phase and its topological defects are reviewed. The second part addresses the physics of nematic colloidal dispersions as a novel challenging type of soft matter. We first investigate the nematic environment of one particle with homeotropic boundary condition. Three possible structures are identified and discussed in detail; the dipole, the Saturn-ring and the surface-ring configuration. Secondly, we treat dipolar and quadrupolar two-particle interactions with the help of a phenomenological theory. Thirdly, we calculate the anisotropic Stokes drag of a particle in a nematic environment which determines its Brownian motion. We then turn our interest towards colloidal dispersions in complex geometries where we identify the dipolar configuration and study its formation. Finally, we demonstrate that surface-induced nematic order above the nematic-isotropic phase transition results in a strongly attractive but short-range two-particle interaction. Its strength can be controlled by temperature and thereby induce flocculation in an otherwise stabilized dispersion. In the third part we study multiple scattering of light from thermal fluctuations of the director. We use this scattering mechanism to test our generalized theory for the diffuse transport of light and its temporal correlations in random anisotropic media. Diffusing light constitutes a successful regime for accessing multiply scattered light. In diffusing-wave spectroscopy it is used to monitor the dynamics of turbid systems. We first provide a review of all the fascinating facets of multiply scattered light, and we introduce the basic theory of diffuse light transport in isotropic systems.Item Open Access Atomistic simulation of shock waves : from simple crystals to complex quasicrystals(2005) Roth, Johannes Werner; Trebin, Hans-Rainer (Prof. Dr.)This habilitation thesis describes molecular dynamics simulations of solids. The impact of shock waves on a number of solids is studied: In the first part binary icosahedral quasicrystals and Laves crystals are treated, in the second part monatomic dodecagonal quasicrystals and body centered cubic crystals are dealt with. The third part contains studies of intermediate phases and solitons which show up in the body centered cubic crystals if shocked along a three-fold axis. In all cases three ranges of different behavior are observed: if the shock waves are weak, elastic deformations occur, in a medium range elastic and plastic waves or phase transitions are observed. If the shock waves are strong, the initial structures are completely destroyed. In this work we are concerned especially with the range of medium strong shock waves. For the binary crystal structure fragmentation occurs. The emerging crystallites are rotated with respect to each other and separated by boundary layers which are several atomic distances thick. The main difference between crystal and quasicrystal are phason-like defects which lead to a continuous transition between the range of weak and medium shock waves. For the monatomic crystal structures the Dzugutov potential has been applied to stabilize the structures. Here we find in the range of medium shock waves phase transitions from quasicrystals and approximants to the body centered cubic structure. Depending on the orientation and strength of the shock waves the transition takes places within a few atomic layers or spread out across many layers. In the quasicrystal and the approximants atomic flips are observed in the elastically compress region. Body centered cubic crystals possess an inherent instability along the three-fold axes. In many materials, this leads to a phase transition to the so-called omega-phase. In our case the omega-phase is stable only in a small range of compression, thus a forth- and back-transformation from body centered cubic to the omega-phase takes place, as long as the phase transition front moves slower than the speed of sound. If this is no longer the case, solitons shock up which contain in their interior. In summary several differences could be observed between crystals and quasicrystals. The results obtained for the Dzugutov potentials are comparable to the outcome of simulation of shock waves in iron with materials-specific interactions.