11,267 research outputs found
Improved version of the eikonal model for absorbing spherical particles
We present a new expression of the scattering amplitude, valid for spherical
absorbing objects, which leads to an improved version of the eikonal method
outside the diffraction region. Limitations of this method are discussed and
numerical results are presented and compared successfully with the Mie theory.Comment: 7 pages, postscript figures available on cpt.univ-mrs.fr, to appear
in J. Mod. Optic
The unreasonable effectiveness of equilibrium-like theory for interpreting non-equilibrium experiments
There has been great interest in applying the results of statistical
mechanics to single molecule experiements. Recent work has highlighted
so-called non-equilibrium work-energy relations and Fluctuation Theorems which
take on an equilibrium-like (time independent) form. Here I give a very simple
heuristic example where an equilibrium result (the barometric law for colloidal
particles) arises from theory describing the {\em thermodynamically}
non-equilibrium phenomenon of a single colloidal particle falling through
solution due to gravity. This simple result arises from the fact that the
particle, even while falling, is in {\em mechanical} equilibrium (gravitational
force equal the viscous drag force) at every instant. The results are
generalized by appeal to the central limit theorem. The resulting time
independent equations that hold for thermodynamically non-equilibrium (and even
non-stationary) processes offer great possibilities for rapid determination of
thermodynamic parameters from single molecule experiments.Comment: 6 page
Droplets move over viscoelastic substrates by surfing a ridge
Liquid drops on soft solids generate strong deformations below the contact
line, resulting from a balance of capillary and elastic forces. The movement of
these drops may cause strong, potentially singular dissipation in the soft
solid. Here we show that a drop on a soft substrate moves by surfing a ridge:
the initially flat solid surface is deformed into a sharp ridge whose
orientation angle depends on the contact line velocity. We measure this angle
for water on a silicone gel and develop a theory based on the substrate
rheology. We quantitatively recover the dynamic contact angle and provide a
mechanism for stick-slip motion when a drop is forced strongly: the contact
line depins and slides down the wetting ridge, forming a new one after a
transient. We anticipate that our theory will have implications in problems
such as self-organization of cell tissues or the design of capillarity-based
microrheometers.Comment: 9 pages, 5 figure
Knowledge-based Virtual Reconstruction of Museum Artifacts
Within the framework of heritage preservation, 3D scanning and modeling for heritage documentation has increased significantly in recent years, mainly due to the evolution of laser and image-based techniques, modeling software, powerful computers and virtual reality. 3D laser acquisition constitutes a real development opportunity for 3D modeling based previously on theoretical data. The representation of the object information rely on the knowledge of its historic and theoretical frame to reconstitute a posteriori its previous states.
This project proposes an approach dealing with data extraction based on architectural knowledge and Laser statement informing measurements, the whole leading to 3D reconstruction. The experimented Khmer objects are exposed at Guimet museum in Paris. The purpose of this digital modeling meets the need of exploitable models for simulation projects, prototyping, exhibitions, promoting cultural tourism and particularly for archiving against any likely disaster and as an aided tool for the formulation of virtual museum concept
Collective Atomic Recoil Laser as a synchronization transition
We consider here a model previously introduced to describe the collective
behavior of an ensemble of cold atoms interacting with a coherent
electromagnetic field. The atomic motion along the self-generated
spatially-periodic force field can be interpreted as the rotation of a phase
oscillator. This suggests a relationship with synchronization transitions
occurring in globally coupled rotators. In fact, we show that whenever the
field dynamics can be adiabatically eliminated, the model reduces to a
self-consistent equation for the probability distribution of the atomic
"phases". In this limit, there exists a formal equivalence with the Kuramoto
model, though with important differences in the self-consistency conditions.
Depending on the field-cavity detuning, we show that the onset of synchronized
behavior may occur through either a first- or second-order phase transition.
Furthermore, we find a secondary threshold, above which a periodic self-pulsing
regime sets in, that is immediately followed by the unlocking of the
forward-field frequency. At yet higher, but still experimentally meaningful,
input intensities, irregular, chaotic oscillations may eventually appear.
Finally, we derive a simpler model, involving only five scalar variables, which
is able to reproduce the entire phenomenology exhibited by the original model
Three-dimensional Gross-Pitaevskii solitary waves in optical lattices: stabilization using the artificial quartic kinetic energy induced by lattice shaking
In this Letter, we show that a three-dimensional Bose-Einstein solitary wave
can become stable if the dispersion law is changed from quadratic to quartic.
We suggest a way to realize the quartic dispersion, using shaken optical
lattices. Estimates show that the resulting solitary waves can occupy as little
as -th of the Brillouin zone in each of the three directions and
contain as many as atoms, thus representing a \textit{fully
mobile} macroscopic three-dimensional object.Comment: 8 pages, 1 figure, accepted in Phys. Lett.
Image effects in transport at metal-molecule interfaces
We present a method for incorporating image-charge effects into the
description of charge transport through molecular devices. A simple model
allows us to calculate the adjustment of the transport levels, due to the
polarization of the electrodes as charge is added to and removed from the
molecule. For this, we use the charge distributions of the molecule between two
metal electrodes in several charge states, rather than in gas phase, as
obtained from a density-functional theory-based transport code. This enables us
to efficiently model level shifts and gap renormalization caused by
image-charge effects, which are essential for understanding molecular transport
experiments. We apply the method to benzene di-amine molecules and compare our
results with the standard approach based on gas phase charges. Finally, we give
a detailed account of the application of our approach to porphyrin-derivative
devices recently studied experimentally by Perrin et al. [Nat. Nanotechnol. 8,
282 (2013)], which demonstrates the importance of accounting for image-charge
effects when modeling transport through molecular junctions
High resolution observations of friction-induced oxide and its interaction with the worn surface
A detailed transmission electron microscopy study of oxide and oxygen-containing phase formation during the sliding wear of metals, composites and coatings is provided. A wide range of different materials types are reported in order to compare and contrast their oxidational wear behaviour: a low carbon stainless steel, a H21 tool steel containing 7%TiC particles, a 17%Cr white iron,an Al–Si/30%SiC composite, an Al–alloy (6092)–15%Ni3Al composite and finally a 3rd generation TiAlN/CrN ‘superhard’ multilayer coating. For the ferrous alloys, nanoscale oxides and oxygen-containing phases were formed that exhibited excellent adhesion to the substrate. In all cases, an increase in oxide coverage of the surface was associated with a decrease in Lancaster wear coefficient. The oxide at the surface of the 316L and H21+7%TiC was found to deform with the substrate, forming a mechanically mixed layer that enhanced surface wear resistance. Evidence of oxidational wear is presented for the wear of the Al–Si–30%SiC composite, but this did not give a beneficial effect in wear, a result of the brittle nature of the oxide that resulted in detachment of fine (150nm) thick fragments. The worn surface of the Al–alloy (6092)–15%Ni3Al and TiAlN/CrN coating was characterized by reaction with the counterface and subsequent oxidation, the product of which enhanced wear resistance. The observations are related to the classical theory of oxidational wear
Distinguishing an ejected blob from alternative flare models at the Galactic centre with GRAVITY
The black hole at the Galactic centre exhibits regularly flares of radiation,
the origin of which is still not understood. In this article, we study the
ability of the near-future GRAVITY infrared instrument to constrain the nature
of these events. We develop realistic simulations of GRAVITY astrometric data
sets for various flare models. We show that the instrument will be able to
distinguish an ejected blob from alternative flare models, provided the blob
inclination is >= 45deg, the flare brightest magnitude is 14 <= mK <= 15 and
the flare duration is >= 1h30.Comment: 11 pages, 9 figures, accepted by MNRA
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