18,055 research outputs found
Lattice Boltzmann models for non-ideal fluids with arrested phase-separation
The effects of mid-range repulsion in Lattice Boltzmann models on the
coalescence/breakup behaviour of single-component, non-ideal fluids are
investigated. It is found that mid-range repulsive interactions allow the
formation of spray-like, multi-droplet configurations, with droplet size
directly related to the strength of the repulsive interaction. The simulations
show that just a tiny ten-percent of mid-range repulsive pseudo-energy can
boost the surface/volume ratio of the phase- separated fluid by nearly two
orders of magnitude. Drawing upon a formal analogy with magnetic Ising systems,
a pseudo-potential energy is defined, which is found to behave like a
quasi-conserved quantity for most of the time-evolution. This offers a useful
quantitative indicator of the stability of the various configurations, thus
helping the task of their interpretation and classification. The present
approach appears to be a promising tool for the computational modelling of
complex flow phenomena, such as atomization, spray formation and
micro-emulsions, break-up phenomena and possibly glassy-like systems as well.Comment: 12 pages, 9 figure
Tunnel Spectroscopy of a Proximity Josephson Junction
We present tunnel spectroscopy experiments on the proximity effect in lateral
superconductor-normal metal-superconductor (SNS) Josephson junctions. Our weak
link is embedded into a superconducting (S) ring allowing phase biasing of the
Josephson junction by an external magnetic field. We explore the temperature
and phase dependence of both the induced mini-gap and the modification of the
density of states in the normal (N) metal. Our results agree with a model based
on the quasiclassical theory in the diffusive limit. The device presents an
advanced version of the superconducting quantum interference proximity
transistor (SQUIPT), now reaching flux sensitivities of 3 nA where
is the flux quantum.Comment: 5 pages, 4 figure
Navigation in Curved Space-Time
A covariant and invariant theory of navigation in curved space-time with
respect to electromagnetic beacons is written in terms of J. L. Synge's
two-point invariant world function. Explicit equations are given for navigation
in space-time in the vicinity of the Earth in Schwarzschild coordinates and in
rotating coordinates. The restricted problem of determining an observer's
coordinate time when their spatial position is known is also considered
Heat Kernel Asymptotics on Homogeneous Bundles
We consider Laplacians acting on sections of homogeneous vector bundles over
symmetric spaces. By using an integral representation of the heat semi-group we
find a formal solution for the heat kernel diagonal that gives a generating
function for the whole sequence of heat invariants. We argue that the obtained
formal solution correctly reproduces the exact heat kernel diagonal after a
suitable regularization and analytical continuation.Comment: 29 pages, Proceedings of the 2007 Midwest Geometry Conference in
Honor of Thomas P. Branso
Extinction Curves, Distances, and Clumpiness of Diffuse Interstellar Dust Clouds
We present CCD photometry in UBVRI of several thousand Galactic field stars
in four large (>1 degree^2) regions centered on diffuse interstellar dust
clouds, commonly referred to as ``cirrus'' clouds (with optical depth A_V less
than unity). Our goal in studying these stars is to investigate the properties
of the cirrus clouds. A comparison of the observed stellar surface density
between on-cloud and off-cloud regions as a function of apparent magnitude in
each of the five bands effectively yields a measure of the extinction through
each cloud. For two of the cirrus clouds, this method is used to derive UBVRI
star counts-based extinction curves, and U-band counts are used to place
constraints on the cloud distance. The color distribution of stars and their
location in (U-B, B-V) and (B-V, V-I) color-color space are analyzed in order
to determine the amount of selective extinction (reddening) caused by the
cirrus. The color excesses, A_lambda-A_V, derived from stellar color histogram
offsets for the four clouds, are better fit by a reddening law that rises
steeply towards short wavelengths [R_V==A_V/E(B-V)<=2] than by the standard law
(R_V=3.1). This may be indicative of a higher-than-average abundance of small
dust grains relative to larger grains in diffuse cirrus clouds. The shape of
the counts-based effective extinction curve and a comparison of different
estimates of the dust optical depth (extinction optical depth derived from
background star counts/colors; emission optical depth derived from far infrared
measurements), are used to measure the degree of clumpiness in clouds. The set
of techniques explored in this paper can be readily adapted to the Sloan
Digital Sky Survey data set in order to carry out a systematic, large-scale
study of cirrus clouds.Comment: 22 pages, 13 figures (postscript, gif, jpg). Accepted for publication
in the Astronomical Journal, scheduled for the May 1999 issue. Full
resolution postscript versions of all figures are available at
http://www.ucolick.org/~arpad
Aggregation and Degradation of White Phosphorus Mediated by N-Heterocyclic Carbene Nickel(0) Complexes
Aggregation and Degradation of White Phosphorus Mediated by N-Heterocyclic Carbene Nickel(0) Complexes
Heat Capacity and Magnetic Phase Diagram of the Low-Dimensional Antiferromagnet YBaCuO
A study by specific heat of a polycrystalline sample of the low-dimensional
magnetic system YBaCuO is presented. Magnetic fields up to 14 T are
applied and permit to extract the (,) phase diagram. Below
T, the N\'eel temperature, associated with a
three-dimensional antiferromagnetic long-range ordering, is constant and equals
K. Above , increases linearly with and a
field-induced increase of the entropy at is related to the presence of an
isosbestic point at K, where all the specific heat curves cross.
A comparison is made between YBaCuO and the quasi-two-dimensional
magnetic systems BaNiVO, SrCuOCl, and
PrCuO, for which very similar phase diagrams have been reported. An
effective field-induced magnetic anisotropy is proposed to explain these phase
diagrams.Comment: 14 pages, 7 figure
The Fractal Properties of the Source and BEC
Using simple space-time implementation of the random cascade model we
investigate numerically influence of the possible fractal structure of the
emitting source on Bose-Einstein correlations between identical particles. The
results are then discussed in terms of the non-extensive Tsallis statistics.Comment: LaTeX file and 2 PS files with figures, 8 pages altogether. Talk
presented at the 12th Indian Summer School "Relativistic Heavy Ion Physics,
Prague, Czech Republic, 30 August-3 Sept. 1999; to be published in Czech J.
Phys. (1999). Some typos correcte
Microscopic Model for Granular Stratification and Segregation
We study segregation and stratification of mixtures of grains differing in
size, shape and material properties poured in two-dimensional silos using a
microscopic lattice model for surface flows of grains. The model incorporates
the dissipation of energy in collisions between rolling and static grains and
an energy barrier describing the geometrical asperities of the grains. We study
the phase diagram of the different morphologies predicted by the model as a
function of the two parameters. We find regions of segregation and
stratification, in agreement with experimental finding, as well as a region of
total mixing.Comment: 4 pages, 7 figures, http://polymer.bu.edu/~hmakse/Home.htm
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