258 research outputs found
Structural properties of a calcium aluminosilicate glass from molecular-dynamics simulations: A finite size effects study
We study a calcium aluminosilicate glass of composition
(SiO)-(AlO)-(CaO) by means of
molecular-dynamics (MD) simulations, using a potential made of two-body and
three-body interactions. In order to prepare small samples that can
subsequently be studied by first-principles, the finite size effects on the
liquid dynamics and on the glass structural properties are investigated. We
find that finite size effects affect the Si-O-Si and Si-O-Al angular
distributions, the first peaks of the Si-O, Al-O and Ca-O pair correlation
functions, the Ca coordination and the oxygen atoms environment in the smallest
system (100 atoms). We give evidence that these finite size effects can be
directly attributed to the use of three-body interactions.Comment: 36 pages, 14 figures. Journal of Chem. Phys., in pres
Detection of the tagged or untagged photons in acousto-optic imaging of thick highly scattering media by photorefractive adaptive holography
We propose an original adaptive wavefront holographic setup based on the
photorefractive effect (PR), to make real-time measurements of acousto-optic
signals in thick scattering media, with a high flux collection at high rates
for breast tumor detection. We describe here our present state of art and
understanding on the problem of breast imaging with PR detection of the
acousto-optic signal
Back-translation for discovering distant protein homologies
Frameshift mutations in protein-coding DNA sequences produce a drastic change
in the resulting protein sequence, which prevents classic protein alignment
methods from revealing the proteins' common origin. Moreover, when a large
number of substitutions are additionally involved in the divergence, the
homology detection becomes difficult even at the DNA level. To cope with this
situation, we propose a novel method to infer distant homology relations of two
proteins, that accounts for frameshift and point mutations that may have
affected the coding sequences. We design a dynamic programming alignment
algorithm over memory-efficient graph representations of the complete set of
putative DNA sequences of each protein, with the goal of determining the two
putative DNA sequences which have the best scoring alignment under a powerful
scoring system designed to reflect the most probable evolutionary process. This
allows us to uncover evolutionary information that is not captured by
traditional alignment methods, which is confirmed by biologically significant
examples.Comment: The 9th International Workshop in Algorithms in Bioinformatics
(WABI), Philadelphia : \'Etats-Unis d'Am\'erique (2009
Molecular dynamics simulation of radiation damage in glasses
Molecular dynamics simulations of the ballistic effects arising from displacement cascades in glasses have been investigated in silica and in a SiO 2 -B 2 O 3 -Na 2 O glass. In both glasses the T-O-T′ angle (where T and T′ are network formers) diminishes, despite radiation causes opposite effects: while the ternary glass swells and silica becomes denser. We show that radiation-induced modifications of macroscopic glass properties result from structural change at medium/range, reflecting an increasing disorder and internal energy of the system. A local thermal quenching model is proposed to account for the effects of ballistic collisions. The core of a displacement cascade is heated by the passage of the projectile, then rapidly quenched, leading to a process that mimics a local thermal quenching. The observed changes in both the mechanical and structural properties of glasses eventually reach saturation at 2 10 18 α/g as the accumulated energy increases. The passage of a single projectile is sufficient to reach the maximum degree of damage, confirming the hypothesis postulated in the swelling model proposed by J.A.C. Marples
The importance of major mergers in the build up of stellar mass in brightest cluster galaxies at z=1
Recent independent results from numerical simulations and observations have
shown that brightest cluster galaxies (BCGs) have increased their stellar mass
by a factor of almost two between z~0.9 and z~0.2. The numerical simulations
further suggest that more than half this mass is accreted through major
mergers. Using a sample of 18 distant galaxy clusters with over 600
spectroscopically confirmed cluster members between them, we search for
observational evidence that major mergers do play a significant role. We find a
major merger rate of 0.38 +/- 0.14 mergers per Gyr at z~1. While the
uncertainties, which stem from the small size of our sample, are relatively
large, our rate is consistent with the results that are derived from numerical
simulations. If we assume that this rate continues to the present day and that
half of the mass of the companion is accreted onto the BCG during these
mergers, then we find that this rate can explain the growth in the stellar mass
of the BCGs that is observed and predicted by simulations. Major mergers
therefore appear to be playing an important role, perhaps even the dominant
one, in the build up of stellar mass in these extraordinary galaxies.Comment: 15 pages, 6 figures, accepted for publication in MNRAS. Reduced data
will be made available through the ESO archiv
Shear instabilities of freely standing thermotropic smectic-A films
In this Letter we discuss theoretically the instabilities of thermotropic
freely standing smectic-A films under shear flow\cite{re:wu}. We show that, in
Couette geometry, the centrifugal force pushes the liquid crystal toward the
outer boundary and induces smectic layer dilation close to the outer boundary.
Under strong shear, this effect induces a layer buckling instability. The
critical shear rate is proportional to , where is the thickness
of the film.Comment: 12 pages, 2 figure
Self-healing capacity of nuclear glass observed by NMR spectroscopy
Safe management of high level nuclear waste is a worldwide significant issue for which vitrification has been selected by many countries. There exists a crucial need for improving our understanding of the ageing of the glass under irradiation. While external irradiation by ions provides a rapid simulation of damage induced by alpha decays, short lived actinide doping is more representative of the reality. Here, we report radiological NMR experiments to compare the damage in International Simplified Glass (ISG) when irradiated by these two methods. In the 0.1 mole percent 244Cm doped glass, accumulation of high alpha decay only shows small modifications of the local structure, in sharp contrast to heavy ion irradiation. These results reveal the ability of the alpha particle to partially repair the damage generated by the heavy recoil nuclei highlighting the radiation resistance of nuclear glass and the difficulty to accurately simulate its behaviour by single ion beam irradiations
Shear induced instabilities in layered liquids
Motivated by the experimentally observed shear-induced destabilization and
reorientation of smectic A like systems, we consider an extended formulation of
smectic A hydrodynamics. We include both, the smectic layering (via the layer
displacement u and the layer normal p) and the director n of the underlying
nematic order in our macroscopic hydrodynamic description and allow both
directions to differ in non equilibrium situations. In an homeotropically
aligned sample the nematic director does couple to an applied simple shear,
whereas the smectic layering stays unchanged. This difference leads to a finite
(but usually small) angle between n and p, which we find to be equivalent to an
effective dilatation of the layers. This effective dilatation leads, above a
certain threshold, to an undulation instability of the layers. We generalize
our earlier approach [Rheol. Acta, vol.39(3), 15] and include the cross
couplings with the velocity field and the order parameters for orientational
and positional order and show how the order parameters interact with the
undulation instability. We explore the influence of various material parameters
on the instability. Comparing our results to recent experiments and molecular
dynamic simulations, we find a good qualitative agreement.Comment: 15 pages, 12 figures, accepted for publication in PR
The Thermal Structural Transition of α-Crystallin Inhibits the Heat Induced Self-Aggregation
-crystallin, the major constituent of human lens, is a member of the heat-shock proteins family and it is known to have a quaternary structural transition at . The presence of calcium ions and/or temperature changes induce supramolecular self-aggregation, a process of relevance in the cataractogenesis. Here we investigate the potential effect of the bovine -crystallin's structural transition on the self-aggregation process. Along all the temperatures investigated, aggregation proceeds by forming intermediate molecular assemblies that successively aggregate in clusters. The final morphology of the aggregates, above and below , is similar, but the aggregation kinetics are completely different. The size of the intermediate molecular assemblies, and their repulsive energy barrier show a marked increase while crossing . Our results highlight the key role of heat modified form of -crystallin in protecting from aggregation and preserving the transparency of the lens under hyperthermic conditions
The Importance of Major Mergers in the Build Up of Stellar Mass in Brightest Cluster Galaxies at \u3cem\u3ez\u3c/em\u3e = 1
Recent independent results from numerical simulations and observations have shown that brightest cluster galaxies (BCGs) have increased their stellar mass by a factor of almost 2 between z ∼ 0.9 and z ∼ 0.2. The numerical simulations further suggest that more than half this mass is accreted through major mergers. Using a sample of 18 distant galaxy clusters with over 600 spectroscopically confirmed cluster members between them, we search for observational evidence that major mergers do play a significant role. We find a major merger rate of 0.38 ± 0.14 mergers per Gyr at z ∼ 1. While the uncertainties, which stem from the small size of our sample, are relatively large, our rate is consistent with the results that are derived from numerical simulations. If we assume that this rate continues to the present day and that half of the mass of the companion is accreted on to the BCG during these mergers, then we find that this rate can explain the growth in the stellar mass of the BCGs that is observed and predicted by simulations. Major mergers therefore appear to be playing an important role, perhaps even the dominant one, in the build up of stellar mass in these extraordinary galaxies
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