9,661 research outputs found
Rheology of fractal networks
We model the cytoskeleton as a fractal network by identifying each segment
with a simple Kelvin-Voigt element, with a well defined equilibrium length. The
final structure retains the elastic characteristics of a solid or a gel, which
may support stress, without relaxing. By considering a very simple regular
self-similar structure of segments in series and in parallel, in 1, 2 or 3
dimensions, we are able to express the viscoelasticity of the network as an
effective generalised Kelvin-Voigt model with a power law spectrum of
retardation times, . We relate the parameter
with the fractal dimension of the gel. In some regimes (), we
recover the weak power law behaviours of the elastic and viscous moduli with
the angular frequencies, , that occur in a variety of
soft materials, including living cells. In other regimes, we find different
power laws for and .Comment: 5 pages, 3 figure
Electromagnetism and multiple-valued loop-dependent wave functionals
We quantize the Maxwell theory in the presence of a electric charge in a
"dual" Loop Representation, i.e. a geometric representation of magnetic
Faraday's lines. It is found that the theory can be seen as a theory without
sources, except by the fact that the wave functional becomes multivalued. This
can be seen as the dual counterpart of what occurs in Maxwell theory with a
magnetic pole, when it is quantized in the ordinary Loop Representation. The
multivaluedness can be seen as a result of the multiply-connectedness of the
configuration space of the quantum theory.Comment: 5 page
Loop representation of charged particles interacting with Maxwell and Chern-Simons fields
The loop representation formulation of non-relativistic particles coupled
with abelian gauge fields is studied. Both Maxwell and Chern-Simons
interactions are separately considered. It is found that the loop-space
formulations of these models share significant similarities, although in the
Chern-Simons case there exists an unitary transformation that allows to remove
the degrees of freedom associated with the paths. The existence of this
transformation, which allows to make contact with the anyonic interpretation of
the model, is subjected to the fact that the charge of the particles be
quantized. On the other hand, in the Maxwell case, we find that charge
quantization is necessary in order to the geometric representation be
consistent.Comment: 6 pages, improved versio
Spatially resolved physical and chemical properties of the planetary nebula NGC 3242
Optical integral-field spectroscopy was used to investigate the planetary
nebula NGC 3242. We analysed the main morphological components of this source,
including its knots, but not the halo. In addition to revealing the properties
ofthe physical and chemical nature of this nebula, we also provided reliable
spatially resolved constraints that can be used for future photoionisation
modelling of the nebula. The latter is ultimately necessary to obtain a fully
self-consistent 3D picture of the physical and chemical properties of the
object. The observations were obtained with the VIMOS instrument attached to
VLT-UT3. Maps and values for specific morphological zones for the detected
emission-lines were obtained and analysed with routines developed by the
authors to derive physical and chemical conditions of the ionised gas in a 2D
fashion. We obtained spatially resolved maps and mean values of the electron
densities, temperatures, and chemical abundances, for specific morphological
structures in NGC 3242. These results show the pixel-to-pixel variations of the
the small- and large-scale structures of the source. These diagnostic maps
provide information free from the biases introduced by traditional single
long-slit observations. In general, our results are consistent with a uniform
abundance distribution for the object, whether we look at abundance maps or
integrated fluxes from specified morphological structures. The results indicate
that special care should be taken with the calibration of the data and that
only data with extremely good signal-to-noise ratio and spectral coverage
should be used to ensure the detection of possible spatial variations.Comment: 11 pages, 8 figures accepted for publication in Astronomy &
Astrophysic
Living bacteria rheology: population growth, aggregation patterns and cooperative behaviour under different shear flows
The activity of growing living bacteria was investigated using real-time and
in situ rheology -- in stationary and oscillatory shear. Two different strains
of the human pathogen Staphylococcus aureus -- strain COL and its isogenic cell
wall autolysis mutant -- were considered in this work. For low bacteria
density, strain COL forms small clusters, while the mutant, presenting
deficient cell separation, forms irregular larger aggregates. In the early
stages of growth, when subjected to a stationary shear, the viscosity of both
strains increases with the population of cells. As the bacteria reach the
exponential phase of growth, the viscosity of the two strains follow different
and rich behaviours, with no counterpart in the optical density or in the
population's colony forming units measurements. While the viscosity of strain
COL keeps increasing during the exponential phase and returns close to its
initial value for the late phase of growth, where the population stabilizes,
the viscosity of the mutant strain decreases steeply, still in the exponential
phase, remains constant for some time and increases again, reaching a constant
plateau at a maximum value for the late phase of growth. These complex
viscoelastic behaviours, which were observed to be shear stress dependent, are
a consequence of two coupled effects: the cell density continuous increase and
its changing interacting properties. The viscous and elastic moduli of strain
COL, obtained with oscillatory shear, exhibit power-law behaviours whose
exponent are dependent on the bacteria growth stage. The viscous and elastic
moduli of the mutant have complex behaviours, emerging from the different
relaxation times that are associated with the large molecules of the medium and
the self-organized structures of bacteria. These behaviours reflect
nevertheless the bacteria growth stage.Comment: 9 pages, 10 figure
Market Solutions to Public Recreation Finance: Creating User-Supported Parks
11 pages.
Contains 1 page of references
Market Solutions to Public Recreation Finance: Creating User-Supported Parks
11 pages.
Contains 1 page of references
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