2,975 research outputs found
Capillarity-Driven Flows at the Continuum Limit
We experimentally investigate the dynamics of capillary-driven flows at the
nanoscale, using an original platform that combines nanoscale pores and
microfluidic features. Our results show a coherent picture across multiple
experiments including imbibition, poroelastic transient flows, and a
drying-based method that we introduce. In particular, we exploit extreme drying
stresses - up to 100 MPa of tension - to drive nanoflows and provide
quantitative tests of continuum theories of fluid mechanics and thermodynamics
(e.g. Kelvin-Laplace equation) across an unprecedented range. We isolate the
breakdown of continuum as a negative slip length of molecular dimension.Comment: 5 pages; 4 figure
Machine Learning for Neuroimaging with Scikit-Learn
Statistical machine learning methods are increasingly used for neuroimaging
data analysis. Their main virtue is their ability to model high-dimensional
datasets, e.g. multivariate analysis of activation images or resting-state time
series. Supervised learning is typically used in decoding or encoding settings
to relate brain images to behavioral or clinical observations, while
unsupervised learning can uncover hidden structures in sets of images (e.g.
resting state functional MRI) or find sub-populations in large cohorts. By
considering different functional neuroimaging applications, we illustrate how
scikit-learn, a Python machine learning library, can be used to perform some
key analysis steps. Scikit-learn contains a very large set of statistical
learning algorithms, both supervised and unsupervised, and its application to
neuroimaging data provides a versatile tool to study the brain.Comment: Frontiers in neuroscience, Frontiers Research Foundation, 2013, pp.1
A Lunar Backup Record of Humanity
The risk of a catastrophic or existential disaster for our civilization is
increasing this century. A significant motivation for a near-term space
settlement is the opportunity to safeguard civilization in the event of a
planetary-scale disaster. A catastrophic event could destroy the significant
cultural, scientific, and technological progress on Earth. However, early space
settlements can preserve records of human activity by maintaining a backup data
storage system. The backup can also store information about the events leading
up to the disaster. The system would improve the ability of early space
settlers to recover our civilization after collapse. We show that advances in
laser communications and data storage enable the development of a data storage
system on the lunar surface with a sufficient uplink data rate and storage
capacity to preserve valuable information about the achievements of our
civilization and the chronology of the disaster.Comment: 12 pages, 2 figures; accepted for publication in the journal
"Signals" (2022
Effect of Gravitational Lensing on Measurements of the Sunyaev-Zel'dovich Effect
The Sunyaev-Zel'dovich (SZ) effect of a cluster of galaxies is usually
measured after background radio sources are removed from the cluster field.
Gravitational lensing by the cluster potential leads to a systematic deficit in
the residual intensity of unresolved sources behind the cluster core relative
to a control field far from the cluster center. As a result, the measured
decrement in the Rayleigh-Jeans temperature of the cosmic microwave background
is overestimated. We calculate the associated systematic bias which is
inevitably introduced into measurements of the Hubble constant using the SZ
effect. For the cluster A2218, we find that observations at 15 GHz with a beam
radius of 0'.4 and a source removal threshold of 100 microJy underestimate the
Hubble constant by 6-10%. If the profile of the gas pressure declines more
steeply with radius than that of the dark matter density, then the ratio of
lensing to SZ decrements increases towards the outer part of the cluster.Comment: 11 pages, 3 figures, submitted to ApJ
Gravitational self-torque and spin precession in compact binaries
We calculate the effect of self-interaction on the "geodetic" spin precession
of a compact body in a strong-field orbit around a black hole. Specifically, we
consider the spin precession angle per radian of orbital revolution for
a particle carrying mass and spin in a circular orbit
around a Schwarzschild black hole of mass . We compute
through in perturbation theory, i.e, including the correction
(obtained numerically) due to the torque exerted by the
conservative piece of the gravitational self-field. Comparison with a
post-Newtonian (PN) expression for , derived here through 3PN
order, shows good agreement but also reveals strong-field features which are
not captured by the latter approximation. Our results can inform
semi-analytical models of the strong-field dynamics in astrophysical binaries,
important for ongoing and future gravitational-wave searches.Comment: 5 pages, 1 table, 1 figure. Minor changes to match published versio
Apprentissage d'atlas fonctionnel du cerveau modélisant la variabilité inter-individuelle
Recent studies have shown that resting-state spontaneous brain activity unveils intrinsic cerebral functioning and complete information brought by prototype task study. From these signals, we will set up a functional atlas of the brain, along with an across-subject variability model. The novelty of our approach lies in the integration of neuroscientific priors and inter-individual variability in a probabilistic description of the rest activity. These models will be applied to large datasets. This variability, ignored until now, may lead to learning of fuzzy atlases, thus limited in term of resolution. This program yields both numerical and algorithmic challenges because of the data volume but also because of the complexity of modelisation.De récentes études ont montré que l'activité spontanée du cerveau observée au repos permet d'étudier l'organisation fonctionnelle cérébrale en complément de l'information fournie par les protocoles de tâches. A partir de ces signaux, nous allons extraire un atlas fonctionnel du cerveau modélisant la variabilité inter-sujet. La nouveauté de notre approche réside dans l'intégration d'a-prioris neuroscientifiques et de la variabilité inter-sujet directement dans un modèles probabiliste de l'activité de repos. Ces modèles seront appliqués sur de larges jeux de données. Cette variabilité, ignorée jusqu'à présent, cont nous permettre d'extraire des atlas flous, donc limités en terme de résolution. Des challenges à la fois numériques et algorithmiques sont à relever de par la taille des jeux de données étudiés et la complexité de la modélisation considérée
Simulation of large photomultipliers for experiments in astroparticle physics
We have developed an accurate simulation model of the large 9 inch
photomultiplier tubes (PMT) used in water-Cherenkov detectors of cosmic-ray
induced extensive air-showers. This work was carried out as part of the
development of the Offline simulation software for the Pierre Auger Observatory
surface array, but our findings may be relevant also for other astrophysics
experiments that employ similar large PMTs.
The implementation is realistic in terms of geometrical dimensions, optical
processes at various surfaces, thin-film treatment of the photocathode, and
photon reflections on the inner structure of the PMT. With the quantum
efficiency obtained for this advanced model we have calibrated a much simpler
and a more rudimentary model of the PMT which is more practical for massive
simulation productions. We show that the quantum efficiency declared by
manufactures of the PMTs is usually determined under conditions substantially
different from those relevant for the particular experiment and thus requires
careful (re)interpretation when applied to the experimental data or when used
in simulations. In principle, the effective quantum efficiency could vary
depending on the optical characteristics of individual events.Comment: 8 pages, 11 figure
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