592 research outputs found
Photon Channelling in Foams
Experiments by Gittings, Bandyopadhyay, and Durian [Europhys. Lett.\
\textbf{65}, 414 (2004)] demonstrate that light possesses a higher probability
to propagate in the liquid phase of a foam due to total reflection. The authors
term this observation photon channelling which we investigate in this article
theoretically. We first derive a central relation in the work of Gitting {\em
et al.} without any free parameters. It links the photon's path-length fraction
in the liquid phase to the liquid fraction . We then construct
two-dimensional Voronoi foams, replace the cell edges by channels to represent
the liquid films and simulate photon paths according to the laws of ray optics
using transmission and reflection coefficients from Fresnel's formulas. In an
exact honeycomb foam, the photons show superdiffusive behavior. It becomes
diffusive as soon as disorder is introduced into the foams. The dependence of
the diffusion constant on channel width and refractive index is explained by a
one-dimensional random-walk model. It contains a photon channelling state that
is crucial for the understanding of the numerical results. At the end, we
shortly comment on the observation that photon channelling only occurs in a
finite range of .Comment: 9 pages, minor change
Flux Expulsion - Field Evolution in Neutron Stars
Models for the evolution of magnetic fields of neutron stars are constructed,
assuming the field is embedded in the proton superconducting core of the star.
The rate of expulsion of the magnetic flux out of the core, or equivalently the
velocity of outward motion of flux-carrying proton-vortices is determined from
a solution of the Magnus equation of motion for these vortices. A force due to
the pinning interaction between the proton-vortices and the neutron-superfluid
vortices is also taken into account in addition to the other more conventional
forces acting on the proton-vortices. Alternative models for the field
evolution are considered based on the different possibilities discussed for the
effective values of the various forces. The coupled spin and magnetic evolution
of single pulsars as well as those processed in low-mass binary systems are
computed, for each of the models. The predicted lifetimes of active pulsars,
field strengths of the very old neutron stars, and distribution of the magnetic
fields versus orbital periods in low-mass binary pulsars are used to test the
adopted field decay models. Contrary to the earlier claims, the buoyancy is
argued to be the dominant driving cause of the flux expulsion, for the single
as well as the binary neutron stars. However, the pinning is also found to play
a crucial role which is necessary to account for the observed low field binary
and millisecond pulsars.Comment: 23 pages, + 7 figures, accepted for publication in Ap
Superfluid Spin-down, with Random Unpinning of the Vortices
The so-called ``creeping'' motion of the pinned vortices in a rotating
superfluid involves ``random unpinning'' and ``vortex motion'' as two
physically separate processes. We argue that such a creeping motion of the
vortices need not be (biased) in the direction of an existing radial Magnus
force, nor should a constant microscopic radial velocity be assigned to the
vortex motion, in contradiction with the basic assumptions of the ``vortex
creep'' model. We point out internal inconsistencies in the predictions of this
model which arise due to this unjustified foundation that ignores the role of
the actual torque on the superfluid. The proper spin-down rate of a pinned
superfluid is then calculated and turns out to be much less than that suggested
in the vortex creep model, hence being of even less observational significance
for its possible application in explaining the post-glitch relaxations of the
radio pulsars.Comment: To be published in J. Low Temp. Phys., Vol. 139, May 2005 [Eqs 11,
15-17 here, have been revised and, may be substituted for the corresponding
ones in that paper
Effect of the Heterogeneity of Metamaterials on Casimir-Lifshitz Interaction
The Casimir-Lifshitz interaction between metamaterials is studied using a
model that takes into account the structural heterogeneity of the dielectric
and magnetic properties of the bodies. A recently developed perturbation theory
for the Casimir-Lifshitz interaction between arbitrary material bodies is
generalized to include non-uniform magnetic permeability profiles, and used to
study the interaction between the magneto-dielectric heterostructures within
the leading order. The metamaterials are modeled as two dimensional arrays of
domains with varying permittivity and permeability. In the case of two
semi-infinite bodies with flat boundaries, the patterned structure of the
material properties is found to cause the normal Casimir-Lifshitz force to
develop an oscillatory behavior when the distance between the two bodies is
comparable to the wavelength of the patterned features in the metamaterials.
The non-uniformity also leads to the emergence of lateral Casimir-Lifshitz
forces, which tend to strengthen as the gap size becomes smaller. Our results
suggest that the recent studies on Casimir-Lifshitz forces between
metamaterials, which have been performed with the aim of examining the
possibility of observing the repulsive force, should be revisited to include
the effect of the patterned structure at the wavelength of several hundred
nanometers that coincides with the relevant gap size in the experiments.Comment: 9 pages, 13 figures. Rewriting equations (10) and (12) and increasing
the size of the lettering/numeral in figure
Diffusive transport of light in two-dimensional granular materials
We study photon diffusion in a two-dimensional random packing of monodisperse
disks as a simple model of granular material. We apply ray optics approximation
to set up a persistent random walk for the photons. We employ Fresnel's
intensity reflectance with its rich dependence on the incidence angle and
polarization state of the light. We present an analytic expression for the
transport-mean-free path in terms of the refractive indices of grains and host
medium, grain radius, and packing fraction. We perform numerical simulations to
examine our analytical result.Comment: 9 pages, 3 figure
Diffusive transport of light in three-dimensional disordered Voronoi structures
The origin of diffusive transport of light in dry foams is still under
debate. In this paper, we consider the random walks of photons as they are
reflected or transmitted by liquid films according to the rules of ray optics.
The foams are approximately modeled by three-dimensional Voronoi tessellations
with varying degree of disorder. We study two cases: a constant intensity
reflectance and the reflectance of thin films. Especially in the second case,
we find that in the experimentally important regime for the film thicknesses,
the transport-mean-free path does not significantly depend on the topological
and geometrical disorder of the Voronoi foams including the periodic Kelvin
foam. This may indicate that the detailed structure of foams is not crucial for
understanding the diffusive transport of light. Furthermore, our theoretical
values for transport-mean-free path fall in the same range as the experimental
values observed in dry foams. One can therefore argue that liquid films
contribute substantially to the diffusive transport of light in {dry} foams.Comment: 8 pages, 8 figure
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Analysing the perceptions of pedestrians and drivers to shared space
Shared space is an approach to improving streets and places where both pedestrians and vehicles are present, with layouts related more to the pedestrian scale and with features encouraging drivers to assume priority having been reduced or removed. It creates a more pedestrian-friendly environment than conventional street layouts, which are based on greater segregation between pedestrians and vehicles, while at the same time introducing uncertainty, which makes drivers engage more fully with their surroundings, leading to lower vehicle speeds and improved safety. This paper investigates the importance of certain person-, context- and design-specific factors affecting the perceptions of pedestrians and drivers to shared space. Using two web-based stated-preference surveys, two sets of responses are collected from pedestrians and drivers, who are presented with different combinations of binary factors forming scenarios. Regression analysis is carried out with logit models for each survey. The results suggest that pedestrians feel most comfortable in shared space under conditions which ensure their presence is clear to other road users â these conditions include low vehicular traffic, high pedestrian traffic, good lighting and pedestrian-only facilities. Conversely, the presence of many pedestrians and, in particular, children and elderly, makes drivers feel uneasy and, therefore, enhances their alertness
A two-parameter random walk with approximate exponential probability distribution
We study a non-Markovian random walk in dimension 1. It depends on two
parameters eps_r and eps_l, the probabilities to go straight on when walking to
the right, respectively to the left. The position x of the walk after n steps
and the number of reversals of direction k are used to estimate eps_r and
eps_l. We calculate the joint probability distribution p_n(x,k) in closed form
and show that, approximately, it belongs to the exponential family.Comment: 12 pages, updated reference to companion paper cond-mat/060126
Review on the strategies to improve the mechanical strength of highly porous bone bioceramic scaffolds
Persistent random walk on a one-dimensional lattice with random asymmetric transmittances
We study the persistent random walk of photons on a one-dimensional lattice
of random asymmetric transmittances. Each site is characterized by its
intensity transmittance t (t') for photons moving to the right (left)
direction. Transmittances at different sites are assumed independent,
distributed according to a given probability density Distribution. We use the
effective medium approximation and identify two classes of probability density
distribution of transmittances which lead to the normal diffusion of photons.
Monte Carlo simulations confirm our predictions.Comment: 7 pages, submitted to Phys. Rev.
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