46,407 research outputs found
Scaling forces to asteroid surfaces: The role of cohesion
The scaling of physical forces to the extremely low ambient gravitational
acceleration regimes found on the surfaces of small asteroids is performed.
Resulting from this, it is found that van der Waals cohesive forces between
regolith grains on asteroid surfaces should be a dominant force and compete
with particle weights and be greater, in general, than electrostatic and solar
radiation pressure forces. Based on this scaling, we interpret previous
experiments performed on cohesive powders in the terrestrial environment as
being relevant for the understanding of processes on asteroid surfaces. The
implications of these terrestrial experiments for interpreting observations of
asteroid surfaces and macro-porosity are considered, and yield interpretations
that differ from previously assumed processes for these environments. Based on
this understanding, we propose a new model for the end state of small, rapidly
rotating asteroids which allows them to be comprised of relatively fine
regolith grains held together by van der Waals cohesive forces.Comment: 54 pages, 7 figure
Symmetries and exact solutions of the BPS Skyrme model
The BPS Skyrme model is a specific subclass of Skyrme-type field theories
which possesses both a BPS bound and infinitely many soliton solutions
(skyrmions) saturating that bound, a property that makes the model a very
convenient first approximation to the study of some properties of nuclei and
hadrons. A related property, the existence of a large group of symmetry
transformations, allows for solutions of rather general shapes, among which
some of them will be relevant to the description of physical nuclei.
We study here the classical symmetries of the BPS Skyrme model, applying them
to construct soliton solutions with some prescribed shapes, what constitutes a
further important step for the reliable application of the model to strong
interaction physics.Comment: Latex, 17 page
D-Brane Interactions in a Gravitational Shock Wave Background
We study D-branes in the background of a gravitational shock wave. We
consider the case of parallel D-branes located on opposite sides with respect
to the shock wave. Their interaction is studied by evaluating the cylinder
diagram using the boundary states technique. Boundary states are defined at
each D-brane and their scalar product is evaluated after propagation through
the shock wave. Taking the limit where the gravitational shock wave vanishes we
show that the amplitude evaluated is consistent with the flat space-time
result.Comment: To be published in Modern Physics Letters A, revised version with
references added, 12 page
Reliable First-Principles Alloy Thermodynamics via Truncated Cluster Expansions
In alloys cluster expansions (CE) are increasingly used to combine
first-principles electronic-structure and Monte Carlo methods to predict
thermodynamic properties. As a basis-set expansion in terms of lattice
geometrical clusters and effective cluster interactions, the CE is exact if
infinite, but is tractable only if truncated. Yet until now a truncation
procedure was not well-defined and did not guarantee a reliable truncated CE.
We present an optimal truncation procedure for CE basis sets that provides
reliable thermodynamics. We then exemplify its importance in NiV, where the
CE has failed unpredictably, and now show agreement to a range of measured
values, predict new low-energy structures, and explain the cause of previous
failures.Comment: 4 pages, 2 figure
Nod2 Deficiency in mice is Associated with Microbiota Variation Favouring the Expansion of mucosal CD4+ LAP+ Regulatory Cells
Nucleotide-binding Oligomerization Domain-2 (NOD2) mutations are associated with an increased risk to develop Crohn's Disease. In previous studies, we have shown that Nod2-/- mice manifest increased proportion of Lamina Propria (LP) CD4+ LAP+ Foxp3- regulatory cells, when compared with Nod2+/+ mice, while CD4+ Foxp3 + regulatory cells were not affected. Here, we investigated the Nod2 gut microbiota, by 16S rRNA pyrosequencing, at steady state and after TNBS-colitis induction in mice reared separately or in cohousing, correlating the microbial profiles with LP regulatory T cells proportion and tissue cytokines content. We found that enrichment of Rikenella and Alistipes (Rikenellaceae) in Nod2-/- mice at 8 weeks of age reared separately was associated with increased proportion of CD4+ LAP+ Foxp3- cells and less severe TNBS-colitis. In co-housed mice the acquisition of Rickenellaceae by Nod2+/+ mice was associated with increased CD4+ LAP+ Foxp3- proportion and less severe colitis. Severe colitis was associated with enrichment of gram-negative pathobionts (Escherichia and Enterococcus), while less severe colitis with protective bacteria (Barnesiella, Odoribacter and Clostridium IV). Environmental factors acting on genetic background with different outcomes according to their impact on microbiota, predispose in different ways to inflammation. These results open a new scenario for therapeutic attempt to re-establish eubiosis in Inflammatory Bowel Disease patients with NOD2 polymorphisms
Microcanonical treatment of black hole decay at the Large Hadron Collider
This study of corrections to the canonical picture of black hole decay in
large extra dimensions examines the effects of back-reaction corrected and
microcanonical emission at the LHC. We provide statistical interpretations of
the different multiparticle number densities in terms of black hole decay to
standard model particles. Provided new heavy particles of mass near the
fundamental Planck scale are not discovered, differences between these
corrections and thermal decay will be insignificant at the LHC.Comment: small additions and clarifications, format for J. Phys.
New porous medium Poisson-Nernst-Planck equations for strongly oscillating electric potentials
We consider the Poisson-Nernst-Planck system which is well-accepted for
describing dilute electrolytes as well as transport of charged species in
homogeneous environments. Here, we study these equations in porous media whose
electric permittivities show a contrast compared to the electric permittivity
of the electrolyte phase. Our main result is the derivation of convenient
low-dimensional equations, that is, of effective macroscopic porous media
Poisson-Nernst-Planck equations, which reliably describe ionic transport. The
contrast in the electric permittivities between liquid and solid phase and the
heterogeneity of the porous medium induce strongly oscillating electric
potentials (fields). In order to account for this special physical scenario, we
introduce a modified asymptotic multiple-scale expansion which takes advantage
of the nonlinearly coupled structure of the ionic transport equations. This
allows for a systematic upscaling resulting in a new effective porous medium
formulation which shows a new transport term on the macroscale. Solvability of
all arising equations is rigorously verified. This emergence of a new transport
term indicates promising physical insights into the influence of the microscale
material properties on the macroscale. Hence, systematic upscaling strategies
provide a source and a prospective tool to capitalize intrinsic scale effects
for scientific, engineering, and industrial applications
Meissner state in finite superconducting cylinders with uniform applied magnetic field
We study the magnetic response of superconductors in the presence of low
values of a uniform applied magnetic field. We report measurements of DC
magnetization and AC magnetic susceptibility performed on niobium cylinders of
different length-to-radius ratios, which show a dramatic enhance of the initial
magnetization for thin samples, due to the demagnetizing effects. The
experimental results are analyzed by applying a model that calculates the
magnetic response of the superconductor, taking into account the effects of the
demagnetizing fields. We use the results of magnetization and current and field
distributions of perfectly diamagnetic cylinders to discuss the physics of the
demagnetizing effects in the Meissner state of type-II superconductors.Comment: Accepted to be published in Phys. Rev. B; 15 pages, 7 ps figure
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