1,573 research outputs found
A liquid state theory that remains successful in the critical region
A thermodynamically self-consistent Ornstein-Zernike approximation (SCOZA) is
applied to a fluid of spherical particles with a pair potential given by a
hard-core repulsion and a Yukawa attractive tail . This
potential allows one to take advantage of the known analytical properties of
the solution to the Ornstein-Zernike equation for the case in which the direct
correlation function outside the repulsive core is given by a linear
combination of two Yukawa tails and the radial distribution function
satisfies the exact core condition for . The predictions for the
thermodynamics, the critical point, and the coexistence curve are compared here
to other theories and to simulation results. In order to unambiguously assess
the ability of the SCOZA to locate the critical point and the phase boundary of
the system, a new set of simulations has also been performed. The method
adopted combines Monte Carlo and finite-size scaling techniques and is
especially adapted to deal with critical fluctuations and phase separation. It
is found that the version of the SCOZA considered here provides very good
overall thermodynamics and a remarkably accurate critical point and coexistence
curve. For the interaction range considered here, given by , the
critical density and temperature predicted by the theory agree with the
simulation results to about 0.6%.Comment: Prepared for the John Barker festschrift issue of Molecular Physics.
22 pages Latex, 6 ps figure
Liquid-gas phase behaviour of an argon-like fluid modelled by the hard-core two-Yukawa potential
We study a model for an argon-like fluid parameterised in terms of a
hard-core repulsion and a two-Yukawa potential. The liquid-gas phase behaviour
of the model is obtained from the thermodynamically self-consistent
Ornstein-Zernike approximation (SCOZA) of Hoye and Stell, the solution of which
lends itself particularly well to a pair potential of this form. The
predictions for the critical point and the coexistence curve are compared to
new high resolution simulation data and to other liquid-state theories,
including the hierarchical reference theory (HRT) of Parola and Reatto. Both
SCOZA and HRT deliver results that are considerably more accurate than standard
integral-equation approaches. Among the versions of SCOZA considered, the one
yielding the best agreement with simulation successfully predicts the critical
point parameters to within 1%.Comment: 10 pages 6 figure
Liquid-vapor interface of a polydisperse fluid
We report a Grand Canonical Monte Carlo simulation study of the liquid-vapor
interface of a model fluid exhibiting polydispersity in terms of the particle
size . The bulk density distribution, , of the system
is controlled by the imposed chemical potential distribution . We
choose the latter such that assumes a Schulz form with
associated degree of polydispersity . By introducing a smooth
attractive wall, a planar liquid-vapor interface is formed for bulk state
points within the region of liquid-vapor coexistence. Owing to fractionation,
the pure liquid phase is enriched in large particles, with respect to the
coexisting vapor. We investigate how the spatial non-uniformity of the density
near the liquid-vapor interface affects the evolution of the local distribution
of particle sizes between the limiting pure phase forms. We find (as previously
predicted by density functional theory, Bellier-Castella {\em et al}, Phys.
Rev. {\bf E65}, 021503 (2002)) a segregation of smaller particles to the
interface. The magnitude of this effect is quantified for various via
measurements of the relative adsorption. Additionally, we consider the utility
of various estimators for the interfacial width and highlight the difficulties
of isolating the intrinsic contribution of polydispersity to this width.Comment: 9 pages, 10 Fig
Wetting transitions in polydisperse fluids
The properties of the coexisting bulk gas and liquid phases of a polydisperse
fluid depend not only on the prevailing temperature, but also on the overall
parent density. As a result, a polydisperse fluid near a wall will exhibit
density-driven wetting transitions inside the coexistence region. We propose a
likely topology for the wetting phase diagram, which we test using Monte Carlo
simulations of a model polydisperse fluid at an attractive wall, tracing the
wetting line inside the cloud curve and identifying the relationship to
prewetting.Comment: 4 Pages, 4 figures. Accepted for publication in Physical Review
Letter
Accurate simulation estimates of cloud points of polydisperse fluids
We describe two distinct approaches to obtaining cloud point densities and
coexistence properties of polydisperse fluid mixtures by Monte Carlo simulation
within the grand canonical ensemble. The first method determines the chemical
potential distribution (with the polydisperse attribute)
under the constraint that the ensemble average of the particle density
distribution matches a prescribed parent form. Within the region
of phase coexistence (delineated by the cloud curve) this leads to a
distribution of the fluctuating overall particle density n, p(n), that
necessarily has unequal peak weights in order to satisfy a generalized lever
rule. A theoretical analysis shows that as a consequence, finite-size
corrections to estimates of coexistence properties are power laws in the system
size. The second method assigns such that an equal peak weight
criterion is satisfied for p(n)\mu(\sigma)$. We show how to ascertain the
requisite weight factor operationally. A theoretical analysis of the second
method suggests that it leads to finite-size corrections to estimates of
coexistence properties which are {\em exponentially small} in the system size.
The scaling predictions for both methods are tested via Monte Carlo simulations
of a novel polydisperse lattice gas model near its cloud curve, the results
showing excellent quantitative agreement with the theory.Comment: 8 pages, 6 figure
A conceptual framework to assess the impact of training on equipment cost and availability in the military context
Designing military support is challenging and current practices need to be reviewed and improved. This paper gives an overview of the Industry current practices in designing military support under Ministry of Defence/Industry agreements (in particular for Contracting for Availability (CfA)), and identifies challenges and opportunities for improvement. E.g. training delivery was identified as an important opportunity for improving the CfA in-service phase. Thus, an innovative conceptual framework is presented to assess the impact of training on the equipment availability and cost. Additionally, guidelines for improving the current training delivery strategies are presented, which can also be applied to other Industry contexts
Free energies of crystalline solids: a lattice-switch Monte Carlo method
We present a method for the direct evaluation of the difference between the
free energies of two crystalline structures, of different symmetry. The method
rests on a Monte Carlo procedure which allows one to sample along a path,
through atomic-displacement-space, leading from one structure to the other by
way of an intervening transformation that switches one set of lattice vectors
for another. The configurations of both structures can thus be sampled within a
single Monte Carlo process, and the difference between their free energies
evaluated directly from the ratio of the measured probabilities of each. The
method is used to determine the difference between the free energies of the fcc
and hcp crystalline phases of a system of hard spheres.Comment: 5 pages Revtex, 3 figure
Phase behaviour and particle-size cutoff effects in polydisperse fluids
We report a joint simulation and theoretical study of the liquid-vapor phase
behaviour of a fluid in which polydispersity in the particle size couples to
the strength of the interparticle interactions. Attention is focussed on the
case in which the particles diameters are distributed according to a fixed
Schulz form with degree of polydispersity . The coexistence
properties of this model are studied using grand canonical ensemble Monte Carlo
simulations and moment free energy calculations. We obtain the cloud and shadow
curves as well as the daughter phase density distributions and fractional
volumes along selected isothermal dilution lines. In contrast to the case of
size-{\em independent} interaction strengths (N.B. Wilding, M. Fasolo and P.
Sollich, J. Chem. Phys. {\bf 121}, 6887 (2004)), the cloud and shadow curves
are found to be well separated, with the critical point lying significantly
below the cloud curve maximum. For densities below the critical value, we
observe that the phase behaviour is highly sensitive to the choice of upper
cutoff on the particle size distribution. We elucidate the origins of this
effect in terms of extremely pronounced fractionation effects and discuss the
likely appearance of new phases in the limit of very large values of the
cutoff.Comment: 12 pages, 15 figure
Cosmic ray propagation and star formation history of NGC 1961
We present new radio continuum data at 4 frequencies on the supermassive,
peculiar galaxy NGC 1961. These observations allow us to separate the thermal
and the nonthermal radio emission and to determine the nonthermal spectral
index distribution. This spectral index distribution in the galactic disk is
unusual: at the maxima of the radio emission the synchrotron spectrum is very
steep, indicating aged cosmic ray electrons. Away from the maxima the spectrum
is much flatter. The steep spectrum of the synchrotron emission at the maxima
indicates that a strong decline of the star formation rate has taken place at
these sites. The extended radio emission is a sign of recent cosmic ray
acceleration, probably by recent star formation. We suggest that a violent
event in the past, most likely a merger or a collision with an intergalactic
gas cloud, has caused the various unusual features of the galaxy.Comment: 9 pages, latex with MN-macros, 20 figures, accepted for publication
in MNRA
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