762 research outputs found
Exposition domiciliaire aux moisissures : quel impact sur le statut aspergillaire du patient atteint de mucoviscidose ?
National audienc
Giant slip lengths of a simple fluid at vibrating solid interfaces
It has been shown recently [PRL 102, 254503 (2009)] that in the plane-plane
configuration a mechanical resonator vibrating close to a rigid wall in a
simple fluid can be overdamped to a frozen regime. Here, by solving
analytically the Navier Stokes equations with partial slip boundary conditions
at the solid fluid interface, we develop a theoretical approach justifying and
extending these earlier findings. We show in particular that in the perfect
slip regime the above mentioned results are, in the plane-plane configuration,
very general and robust with respect to lever geometry considerations. We
compare the results with those obtained previously for the sphere moving
perpendicularly and close to a plane in a simple fluid and discuss in more
details the differences concerning the dependence of the friction forces with
the gap distance separating the moving object (i.e., plane or sphere) from the
fixed plane. Finally, we show that the submicron fluidic effect reported in the
reference above, and discussed further in the present work, can have dramatic
implications in the design of nano-electromechanical systems (NEMS).Comment: submitted to PRE (see also PRL 102, 254503 (2009)
Unveiling the intruder deformed 0 state in Si
The 0 state in Si has been populated at the {\sc Ganil/Lise3}
facility through the -decay of a newly discovered 1 isomer in
Al of 26(1) ms half-life. The simultaneous detection of pairs
allowed the determination of the excitation energy E(0)=2719(3) keV and
the half-life T=19.4(7) ns, from which an electric monopole strength of
(E0)=13.0(0.9) was deduced. The 2 state is
observed to decay both to the 0 ground state and to the newly observed
0 state (via a 607(2) keV transition) with a ratio
R(2)=1380(717). Gathering all
information, a weak mixing with the 0 and a large deformation parameter
of =0.29(4) are found for the 0 state, in good agreement with
shell model calculations using a new {\sc sdpf-u-mix} interaction allowing
\textit{np-nh} excitations across the N=20 shell gap.Comment: 5 pages, 3 figures, accepted for publication in Physical Review
Letter
Casimir-Polder force between an atom and a dielectric plate: thermodynamics and experiment
The low-temperature behavior of the Casimir-Polder free energy and entropy
for an atom near a dielectric plate are found on the basis of the Lifshitz
theory. The obtained results are shown to be thermodynamically consistent if
the dc conductivity of the plate material is disregarded. With inclusion of dc
conductivity, both the standard Lifshitz theory (for all dielectrics) and its
generalization taking into account screening effects (for a wide range of
dielectrics) violate the Nernst heat theorem. The inclusion of the screening
effects is also shown to be inconsistent with experimental data of Casimir
force measurements. The physical reasons for this inconsistency are elucidated.Comment: 10 pages, 1 figure; improved discussion; to appear in J. Phys. A:
Math. Theor. (Fast Track Communications
Dynamique des ongulés sauvages dans la région méditerranéenne française
La politique de restauration des populations d’ongulés, entreprise à partir des années 70, a montré son efficacité aussi bien au niveau national qu’en région méditerranéenne, où tous les ongulés sont en expansion : le sanglier, le chevreuil, le cerf, le mouflon et même le chamois qui descend des montagnes pour coloniser les forêts de chêne vert. Toutefois, les méthodes de comptage appliquées depuis plusieurs décennies ne permettent plus une gestion efficace des ongulés sauvages car elles sous-estiment systématiquement les effectifs et ne font qu’aggraver le phénomène d’explosion des populations. Le résultat en est une dégradation de la qualité des animaux et l’apparition de dégâts sur la végétation. Ainsi, depuis quelques années, une nouvelle approche de gestion durable commence à être appliquée par certains gestionnaires. Cette gestion, basée sur les indicateurs de changement écologique (ICE), cherche à évaluer l’interaction entre les ongulés sauvages et le milieu pour maintenir un équilibre entre les niveaux de populations et les ressources alimentaires
On electrostatic and Casimir force measurements between conducting surfaces in a sphere-plane configuration
We report on measurements of forces acting between two conducting surfaces in
a spherical-plane configuration in the 35 nm-1 micrometer separation range. The
measurements are obtained by performing electrostatic calibrations followed by
a residual analysis after subtracting the electrostatic-dependent component. We
find in all runs optimal fitting of the calibrations for exponents smaller than
the one predicted by electrostatics for an ideal sphere-plane geometry. We also
find that the external bias potential necessary to minimize the electrostatic
contribution depends on the sphere-plane distance. In spite of these anomalies,
by implementing a parametrixation-dependent subtraction of the electrostatic
contribution we have found evidence for short-distance attractive forces of
magnitude comparable to the expected Casimir-Lifshitz force. We finally discuss
the relevance of our findings in the more general context of Casimir-Lifshitz
force measurements, with particular regard to the critical issues of the
electrical and geometrical characterization of the involved surfaces.Comment: 22 pages, 15 figure
X-ray pushing of a mechanical microswing
Nanoelectromechanical Systems (NEMS) are among the best candidates to measure
interactions at nanoscale [1-6], especially when resonating oscillators are
used with high quality factor [7, 8]. Despite many efforts [9, 10], efficient
and easy actuation in NEMS remains an issue [11]. The mechanism that we
propose, thermally mediated Center Of Mass (COM) displacements, represents a
new actuation scheme for NEMS and MEMS. To demonstrate this scheme efficiency
we show how mechanical nanodis- placements of a MEMS is triggered using
modulated X-ray microbeams. The MEMS is a microswing constituted by a Ge
microcrystal attached to a Si microcantilever. The interaction is mediated by
the Ge absorption of the intensity modulated X-ray microbeam impinging on the
microcrystal. The small but finite thermal expansion of the Ge microcrystal is
large enough to force a nanodisplacement of the Ge microcrystal COM glued on a
Si microlever. The inverse mechanism can be envisaged: MEMS can be used to
shape X-ray beams. A Si microlever can be a high frequency X-ray beam chopper
for time studies in biology and chemistry.Comment: 5 pages, 4 figur
Sub-surface Oxygen and Surface Oxide Formation at Ag(111): A Density-functional Theory Investigation
To help provide insight into the remarkable catalytic behavior of the
oxygen/silver system for heterogeneous oxidation reactions, purely sub-surface
oxygen, and structures involving both on-surface and sub-surface oxygen, as
well as oxide-like structures at the Ag(111) surface have been studied for a
wide range of coverages and adsorption sites using density-functional theory.
Adsorption on the surface in fcc sites is energetically favorable for low
coverages, while for higher coverage a thin surface-oxide structure is
energetically favorable. This structure has been proposed to correspond to the
experimentally observed (4x4) phase. With increasing O concentrations, thicker
oxide-like structures resembling compressed Ag2O(111) surfaces are
energetically favored. Due to the relatively low thermal stability of these
structures, and the very low sticking probability of O2 at Ag(111), their
formation and observation may require the use of atomic oxygen (or ozone, O3)
and low temperatures. We also investigate diffusion of O into the sub-surface
region at low coverage (0.11 ML), and the effect of surface Ag vacancies in the
adsorption of atomic oxygen and ozone-like species. The present studies,
together with our earlier investigations of on-surface and
surface-substitutional adsorption, provide a comprehensive picture of the
behavior and chemical nature of the interaction of oxygen and Ag(111), as well
as of the initial stages of oxide formation.Comment: 17 pages including 14 figures, Related publications can be found at
http://www.fhi-berlin.mpg.de/th/paper.htm
Unraveling biogeochemical phosphorus dynamics in hyperarid Mars‐analogue soils using stable oxygen isotopes in phosphate
With annual precipitation less than 20 mm and extreme UV intensity, the Atacama Desert in northern Chile has long been utilized as an analogue for recent Mars. In these hyperarid environments, water and biomass are extremely limited, and thus, it becomes difficult to generate a full picture of biogeochemical phosphate‐water dynamics. To address this problem, we sampled soils from five Atacama study sites and conducted three main analyses—stable oxygen isotopes in phosphate, enzyme pathway predictions, and cell culture experiments. We found that high sedimentation rates decrease the relative size of the organic phosphorus pool, which appears to hinder extremophiles. Phosphoenzyme and pathway prediction analyses imply that inorganic pyrophosphatase is the most likely catalytic agent to cycle P in these environments, and this process will rapidly overtake other P utilization strategies. In these soils, the biogenic δ18O signatures of the soil phosphate (δ18OPO4) can slowly overprint lithogenic δ18OPO4 values over a timescale of tens to hundreds of millions of years when annual precipitation is more than 10 mm. The δ18OPO4 of calcium‐bound phosphate minerals seems to preserve the δ18O signature of the water used for biogeochemical P cycling, pointing toward sporadic rainfall and gypsum hydration water as key moisture sources. Where precipitation is less than 2 mm, biological cycling is restricted and bedrock δ18OPO4 values are preserved. This study demonstrates the utility of δ18OPO4 values as indicative of biogeochemical cycling and hydrodynamics in an extremely dry Mars‐analogue environment
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