593 research outputs found
Physical Optics Analysis of the ALMA Band 5 Front End Optics
The Atacama Large Millimetre Array will be a
ground based millimetre to submillimetre band
interferometer. The instrument will be comprised of up to 50
high precision 12m Cassegrain antennas. Each antenna will
cover a frequency range from 30 to 950 GHz, which will be
split into 10 observing channels/bands. Each frequency
channel will have its own specifically designed front end
optics to couple radiation from the secondary reflector focal
plane to the accompanying receiver. We present a full
electromagnetic analysis of the band 5 front end optics
system using physical optics, which covers a range from 163
to 211 GHz. This band is being developed by the Group for
Advanced Receiver Development (GARD) at Chalmers
University, Gothenburg, Sweden. Two software packages are
utilised for this analysis; the industry standard reflector
antenna software package GRASP9 developed by TICRA [1]
and a new optical software package MODAL [2,3]
(Maynooth Optical Design Analysis Laboratory) developed
at NUI Maynooth, Ireland. Electromagnetic predictions of
beam patterns are presented at the Cassegrain focal plane
and at the subreflector vertex.
The basis of the analysis is primarily to determine optical
performance and efficiency and the effects of beam
truncation by the off-axis reflectors of the front end optics.
Three levels of beam truncation are modelled varying rim
diameter
Physical Optics Analysis of the ALMA Band 5 Front End Optics
The Atacama Large Millimetre Array will be a
ground based millimetre to submillimetre band
interferometer. The instrument will be comprised of up to 50
high precision 12m Cassegrain antennas. Each antenna will
cover a frequency range from 30 to 950 GHz, which will be
split into 10 observing channels/bands. Each frequency
channel will have its own specifically designed front end
optics to couple radiation from the secondary reflector focal
plane to the accompanying receiver. We present a full
electromagnetic analysis of the band 5 front end optics
system using physical optics, which covers a range from 163
to 211 GHz. This band is being developed by the Group for
Advanced Receiver Development (GARD) at Chalmers
University, Gothenburg, Sweden. Two software packages are
utilised for this analysis; the industry standard reflector
antenna software package GRASP9 developed by TICRA [1]
and a new optical software package MODAL [2,3]
(Maynooth Optical Design Analysis Laboratory) developed
at NUI Maynooth, Ireland. Electromagnetic predictions of
beam patterns are presented at the Cassegrain focal plane
and at the subreflector vertex.
The basis of the analysis is primarily to determine optical
performance and efficiency and the effects of beam
truncation by the off-axis reflectors of the front end optics.
Three levels of beam truncation are modelled varying rim
diameter
A spherical model with directional interactions: I. Static properties
We introduce a simple spherical model whose structural properties are similar
to the ones generated by models with directional interactions, by employing a
binary mixture of large and small hard spheres, with a square-well attraction
acting only between particles of different size. The small particles provide
the bonds between the large ones. With a proper choice of the interaction
parameters, as well as of the relative concentration of the two species, it is
possible to control the effective valence. Here we focus on a specific choice
of the parameters which favors tetrahedral ordering and study the equilibrium
static properties of the system in a large window of densities and
temperatures. Upon lowering the temperature we observe a progressive increase
in local order, accompanied by the formation of a four-coordinated network of
bonds. Three different density regions are observed: at low density the system
phase separates into a gas and a liquid phase; at intermediate densities a
network of fully bonded particles develops; at high densities -- due to the
competition between excluded volume and attractive interactions -- the system
forms a defective network. The very same behavior has been previously observed
in numerical studies of non-spherical models for molecular liquids, such as
water, and in models of patchy colloidal particles. Differently from these
models, theoretical treatments devised for spherical potentials, e.g. integral
equations and ideal mode coupling theory for the glass transition can be
applied in the present case, opening the way for a deeper understanding of the
thermodynamic and dynamic behavior of low valence molecules and particles.Comment: 11 pages, 11 figure
Investigation of -dependent dynamical heterogeneity in a colloidal gel by x-ray photon correlation spectroscopy
We use time-resolved X-Photon Correlation Spectroscopy to investigate the
slow dynamics of colloidal gels made of moderately attractive carbon black
particles. We show that the slow dynamics is temporally heterogeneous and
quantify its fluctuations by measuring the variance of the instantaneous
intensity correlation function. The amplitude of dynamical fluctuations has a
non-monotonic dependence on scattering vector , in stark contrast with
recent experiments on strongly attractive colloidal gels [Duri and Cipelletti,
\textit{Europhys. Lett.} \textbf{76}, 972 (2006)]. We propose a simple scaling
argument for the -dependence of fluctuations in glassy systems that
rationalizes these findings.Comment: Final version published in PR
Resolving long-range spatial correlations in jammed colloidal systems using photon correlation imaging
We introduce a new dynamic light scattering method, termed photon correlation
imaging, which enables us to resolve the dynamics of soft matter in space and
time. We demonstrate photon correlation imaging by investigating the slow
dynamics of a quasi two-dimensional coarsening foam made of highly packed,
deformable bubbles and a rigid gel network formed by dilute, attractive
colloidal particles. We find the dynamics of both systems to be determined by
intermittent rearrangement events. For the foam, the rearrangements extend over
a few bubbles, but a small dynamical correlation is observed up to macroscopic
length scales. For the gel, dynamical correlations extend up to the system
size. These results indicate that dynamical correlations can be extremely
long-ranged in jammed systems and point to the key role of mechanical
properties in determining their nature.Comment: Published version (Phys. Rev. Lett. 102, 085702 (2009)) The Dynamical
Activity Mapsprovided as Supplementary Online Material are also available on
http://w3.lcvn.univ-montp2.fr/~lucacip/dam/movies.ht
Shearing a Glassy Material: Numerical Tests of Nonequilibrium Mode-Coupling Approaches and Experimental Proposals
The predictions of a nonequilibrium schematic mode-coupling theory developed
to describe the nonlinear rheology of soft glassy materials have been
numerically challenged in a sheared binary Lennard-Jones mixture. The theory
gives an excellent description of the stress/temperature `jamming phase
diagram' of the system. In the present paper, we focus on the issue of an
effective temperature Teff for the slow modes of the fluid, as defined from a
generalized fluctuation-dissipation theorem. As predicted theoretically, many
different observables are found to lead to the same value of Teff, suggesting
several experimental procedures to measure Teff. New, simple experimental
protocols to access Teff from a generalized equipartition theorem are also
proposed, and one such experiment is numerically performed. These results give
strong support to the thermodynamic interpretation of Teff and make it
experimentally accessible in a very direct way.Comment: Version accepted for publication - Physical Review Letter
Jamming transition in emulsions and granular materials
We investigate the jamming transition in packings of emulsions and granular
materials via molecular dynamics simulations. The emulsion model is composed of
frictionless droplets interacting via nonlinear normal forces obtained using
experimental data acquired by confocal microscopy of compressed emulsions
systems. Granular materials are modeled by Hertz-Mindlin deformable spherical
grains with Coulomb friction. In both cases, we find power-law scaling for the
vanishing of pressure and excess number of contacts as the system approaches
the jamming transition from high volume fractions. We find that the
construction history parametrized by the compression rate during the
preparation protocol has a strong effect on the micromechanical properties of
granular materials but not on emulsions. This leads the granular system to jam
at different volume fractions depending on the histories. Isostaticity is found
in the packings close to the jamming transition in emulsions and in granular
materials at slow compression rates and infinite friction. Heterogeneity of
interparticle forces increases as the packings approach the jamming transition
which is demonstrated by the exponential tail in force distributions and the
small values of the participation number measuring spatial localization of the
forces. However, no signatures of the jamming transition are observed in
structural properties, like the radial distribution functions and the
distributions of contacts.Comment: Submitted to PR
Glasslike Arrest in Spinodal Decomposition as a Route to Colloidal Gelation
Colloid-polymer mixtures can undergo spinodal decomposition into colloid-rich
and colloid-poor regions. Gelation results when interconnected colloid-rich
regions solidify. We show that this occurs when these regions undergo a glass
transition, leading to dynamic arrest of the spinodal decomposition. The
characteristic length scale of the gel decreases with increasing quench depth,
and the nonergodicity parameter exhibits a pronounced dependence on scattering
vector. Mode coupling theory gives a good description of the dynamics, provided
we use the full static structure as input.Comment: 14 pages, 4 figures; replaced with published versio
Heterogeneous Dynamics of Coarsening Systems
We show by means of experiments, theory and simulations, that the slow
dynamics of coarsening systems displays dynamic heterogeneity similar to that
observed in glass-forming systems. We measure dynamic heterogeneity via novel
multi-point functions which quantify the emergence of dynamic, as opposed to
static, correlations of fluctuations. Experiments are performed on a coarsening
foam using Time Resolved Correlation, a recently introduced light scattering
method. Theoretically we study the Ising model, and present exact results in
one dimension, and numerical results in two dimensions. For all systems the
same dynamic scaling of fluctuations with domain size is observed.Comment: Minor changes; to be published in Phys. Rev. Let
Critical packing in granular shear bands
In a realistic three-dimensional setup, we simulate the slow deformation of
idealized granular media composed of spheres undergoing an axisymmetric
triaxial shear test. We follow the self-organization of the spontaneous strain
localization process leading to a shear band and demonstrate the existence of a
critical packing density inside this failure zone. The asymptotic criticality
arising from the dynamic equilibrium of dilation and compaction is found to be
restricted to the shear band, while the density outside of it keeps the memory
of the initial packing. The critical density of the shear band depends on
friction (and grain geometry) and in the limit of infinite friction it defines
a specific packing state, namely the \emph{dynamic random loose packing}
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