257 research outputs found
Characteristics of the local distribution of the Y139C resistance gene in Norway rats (Rattus norvegicus
Klemann, N., Esther, A., Endepols, S
Automatic and permanent rodent-monitoring - a proper method to evaluate rodenticide effects?
Fuelling, O., Klemann, N., Endepols, S
Elasticity-mediated self-organization and colloidal interactions of solid spheres with tangential anchoring in a nematic liquid crystal
Using laser tweezers and fluorescence confocal polarizing microscopy, we
study colloidal interactions of solid microspheres in the nematic bulk caused
by elastic distortions around the particles with strong tangential surface
anchoring. The particles aggregate into chains directed at about 30 degrees to
the far field director and, at higher concentrations, form complex kinetically
trapped structures. We characterize the distance and angular dependencies of
the colloidal interaction forces.Comment: 6 pages, 5 figure
Disclinations, dislocations and continuous defects: a reappraisal
Disclinations, first observed in mesomorphic phases, are relevant to a number
of ill-ordered condensed matter media, with continuous symmetries or frustrated
order. They also appear in polycrystals at the edges of grain boundaries. They
are of limited interest in solid single crystals, where, owing to their large
elastic stresses, they mostly appear in close pairs of opposite signs. The
relaxation mechanisms associated with a disclination in its creation, motion,
change of shape, involve an interplay with continuous or quantized dislocations
and/or continuous disclinations. These are attached to the disclinations or are
akin to Nye's dislocation densities, well suited here. The notion of 'extended
Volterra process' takes these relaxation processes into account and covers
different situations where this interplay takes place. These concepts are
illustrated by applications in amorphous solids, mesomorphic phases and
frustrated media in their curved habit space. The powerful topological theory
of line defects only considers defects stable against relaxation processes
compatible with the structure considered. It can be seen as a simplified case
of the approach considered here, well suited for media of high plasticity
or/and complex structures. Topological stability cannot guarantee energetic
stability and sometimes cannot distinguish finer details of structure of
defects.Comment: 72 pages, 36 figure
Annihilation of edge dislocations in smectic A liquid crystals
This paper presents a theoretical study of the annihilation of edge dislocations in the same smectic plane in a bulk smectic-A phase. We use a time-dependent Landau-Ginzburg approach where the smectic ordering is described by the complex order parameter psi( r--> ,t) =eta e(iphi) . This quantity allows both the degree of layering and the position of the layers to be monitored. We are able to follow both precollision and postcollision regimes, and distinguish different early and late behaviors within these regimes. The early precollision regime is driven by changes in the phi ( r--> ) configuration. The relative velocity of the defects is approximately inversely proportional to the interdefect separation distance. In the late precollision regime the symmetry changes within the cores of defects also become influential. Following the defect collision, in the early postcollision stage, bulk layer order is approached exponentially in time. At very late times, however, there seems to be a long-time power-law tail in the order parameter fluctuation relaxation
Elliptic Phases: A Study of the Nonlinear Elasticity of Twist-Grain Boundaries
We develop an explicit and tractable representation of a twist-grain-boundary
phase of a smectic A liquid crystal. This allows us to calculate the
interaction energy between grain boundaries and the relative contributions from
the bending and compression deformations. We discuss the special stability of
the 90 degree grain boundaries and discuss the relation of this structure to
the Schwarz D surface.Comment: 4 pages, 2 figure
A stochastic derivation of the geodesic rule
We argue that the geodesic rule, for global defects, is a consequence of the
randomness of the values of the Goldstone field in each causally
connected volume. As these volumes collide and coalescence, evolves by
performing a random walk on the vacuum manifold . We derive a
Fokker-Planck equation that describes the continuum limit of this process. Its
fundamental solution is the heat kernel on , whose leading
asymptotic behavior establishes the geodesic rule.Comment: 12 pages, No figures. To be published in Int. Jour. Mod. Phys.
Defect kinetics and dynamics of pattern coarsening in a two-dimensional smectic-A system
Two-dimensional simulations of the coarsening process of the
isotropic/smectic-A phase transition are presented using a high-order Landau-de
Gennes type free energy model. Defect annihilation laws for smectic
disclinations, elementary dislocations, and total dislocation content are
determined. The computed evolution of the orientational correlation length and
disclination density is found to be in agreement with previous experimental
observations showing that disclination interactions dominate the coarsening
process. The mechanism of smectic disclination movement, limited by the
absorption and emission of elementary dislocations, is found to be facilitated
by curvature walls connecting interacting disclinations. At intermediate times
in the coarsening process, split-core dislocation formation and interactions
displaying an effective disclination quadrupole configuration are observed.
This work provides the framework for further understanding of the formation and
dynamics of the diverse set of curvature defects observed in smectic liquid
crystals and other layered material systems
Properties of the Broad-Range Nematic Phase of a Laterally Linked H-Shaped Liquid Crystal Dimer
In search for novel nematic materials, a laterally linked H-shaped liquid
crystal dimer have been synthesized and characterized. The distinct feature of
the material is a very broad temperature range (about 50 oC) of the nematic
phase, which is in contrast with other reported H-dimers that show
predominantly smectic phases. The material exhibits interesting textural
features at the scale of nanometers (presence of smectic clusters) and at the
macroscopic scales. Namely, at a certain temperature, the flat samples of the
material show occurrence of domain walls. These domain walls are caused by the
surface anchoring transition and separate regions with differently tilted
director. Both above and below this transition temperature the material
represents a uniaxial nematic, as confirmed by the studies of defects in flat
samples and samples with colloidal inclusions, freely suspended drops, X-ray
diffraction and transmission electron microscopy.Comment: 30 pages (including Supplementary Information), 7 Figure
Kramers rate theory of ionization and dissociation of bound states
Calculating the microscopic dissociation rate of a bound state, such as a
classical diatomic molecule, has been difficult so far. The problem was that
standard theories require an energy barrier over which the bound particle (or
state) escapes into the preferred low-energy state. This is not the case when
the long-range repulsion responsible for the barrier is either absent or
screened (as in Cooper pairs, ionized plasma, or biomolecular complexes). We
solve this classical problem by accounting for entropic memory at the
microscopic level. The theory predicts dissociation rates for arbitrary
potentials and is successfully tested on the example of plasma, where it yields
an estimate of ionization in the core of Sun in excellent agreement with
experiments. In biology, the new theory accounts for crowding in
receptor-ligand kinetics and protein aggregation
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