2,154 research outputs found
Constraining coherent low frequency radio flares from compact binary mergers
The presence and detectability of coherent radio emission from compact binary
mergers (containing at least one neutron star) remains poorly constrained due
to large uncertainties in the models. These compact binary mergers may
initially be detected as Short Gamma-ray Bursts (SGRBs) or via their
gravitational wave emission. Several radio facilities have developed rapid
response modes enabling them to trigger on these events and search for this
emission. For this paper, we constrain this coherent radio emission using the
deepest available constraints for GRB 150424A, which were obtained via a
triggered observation with the Murchison Widefield Array. We then expand this
analysis to determine the properties of magnetar merger remnants that may be
formed via a general population of binary neutron star mergers. Our results
demonstrate that many of the potential coherent emission mechanisms that have
been proposed for such events can be detected or very tightly constrained by
the complementary strategies used by the current generation of low-frequency
radio telescopes.Comment: 19 pages, submitted to MNRA
A paediatric telecardiology service for district hospitals in south-east England: an observational study.
The attached article is a Publisher version of the final published version which may be accessed at the link below. Copyright © 2010 BMJ Publishing Group Ltd & Royal College of Paediatrics and Child Health. All rights reservedOBJECTIVES: To compare caseloads of new patients assessed by paediatric cardiologists face-to-face or during teleconferences, and assess NHS costs for the alternative referral arrangements. DESIGN: Prospective cohort study over 15 months. SETTING: Four district hospitals in south-east England and a London paediatric cardiology centre. PATIENTS: Babies and children. INTERVENTION: A telecardiology service introduced alongside outreach clinics. MEASUREMENTS: Clinical outcomes and mean NHS costs per patient. RESULTS: 266 new patients were studied: 75 had teleconsultations (19 of 42 newborns and 56 of 224 infants and children). Teleconsultation patients generally were younger (49% being under 1 year compared with 32% seen personally (p = 0.025)) and their symptoms were not as severe. A cardiac intervention was undertaken immediately or planned for five telemedicine patients (7%) and 30 conventional patients (16%). However, similar proportions of patients were discharged after being assessed (32% telemedicine and 39% conventional). During scheduled teleconferences the mean duration of time per patient in sessions involving real-time echocardiography was 14.4 min, and 8.5 min in sessions where pre-recorded videos were transmitted. Mean cost comparisons for telemedicine and face-to-face patients over 14-day and 6-month follow-up showed the telecardiology service to be cost-neutral for the three hospitals with infrequently-held outreach clinics (1519 UK pounds vs 1724 UK pounds respectively after 14 days). CONCLUSION: Paediatric cardiology centres with small cadres of specialists are under pressure to cope with ever-expanding caseloads of new patients with suspected anomalies. Innovative use of telecardiology alongside conventional outreach services should suitably, and economically, enhance access to these specialists.The Department of Health and the Charitable Funds Committee of the Royal Brompton and Harefield NHS Trust funded the project
Are stress-free membranes really 'tensionless'?
In recent years it has been argued that the tension parameter driving the
fluctuations of fluid membranes, differs from the imposed lateral stress, the
'frame tension'. In particular, stress-free membranes were predicted to have a
residual fluctuation tension. In the present paper, this argument is
reconsidered and shown to be inherently inconsistent -- in the sense that a
linearized theory, the Monge model, is used to predict a nonlinear effect.
Furthermore, numerical simulations of one-dimensional stiff membranes are
presented which clearly demonstrate, first, that the internal 'intrinsic'
stress in membranes indeed differs from the frame tension as conjectured, but
second, that the fluctuations are nevertheless driven by the frame tension.
With this assumption, the predictions of the Monge model agree excellently with
the simulation data for stiffness and tension values spanning several orders of
magnitude
Direct calculation of interfacial tensions from computer simulation: Results for freely jointed tangent hard sphere chains
We develop a methodology for the calculation of surface free energies based
on the probability distribution of a wandering interface. Using a simple
extension of the NpT sampling, we allow the interface area to randomly probe
the available space and evaluate the surface free energy from histogram
analysis and the corresponding average. The method is suitable for studying
systems with either continuous or discontinuous potentials, as it does not
require explicit evaluation of the virial. The proposed algorithm is compared
with known results for the surface tension of Lennard--Jones and Square Well
fluid, as well as for the interface tension of a bead--spring polymer model and
good agreement is found. We also calculate interfacial tensions of freely
jointed tangent hard sphere chains on athermal walls for a wide range of chain
lengths and densities. The results are compared with three different
theoretical approaches, Scaled Particle Theory, the Yu and Wu density
functional theory and an analytical approximation based on the latter approach.
Whereas SPT only yields qualitative results, the last two approaches are found
to yield very good agreement with simulations.Comment: 20 pages, 6 figures, Phys. Rev. E in press
In-plane structure and ordering at liquid sodium surfaces and interfaces from ab initio molecular dynamics
Atoms at liquid metal surfaces are known to form layers parallel to the
surface. We analyze the two-dimensional arrangement of atoms within such layers
at the surface of liquid sodium, using ab initio molecular dynamics (MD)
simulations based on density functional theory. Nearest neighbor distributions
at the surface indicate mostly 5-fold coordination, though there are noticeable
fractions of 4-fold and 6-fold coordinated atoms. Bond angle distributions
suggest a movement toward the angles corresponding to a six-fold coordinated
hexagonal arrangement of the atoms as the temperature is decreased towards the
solidification point. We rationalize these results with a distorted hexagonal
order at the surface, showing a mixture of regions of five and six-fold
coordination. The liquid surface results are compared with classical MD
simulations of the liquid surface, with similar effects appearing, and with ab
initio MD simulations for a model solid-liquid interface, where a pronounced
shift towards hexagonal ordering is observed as the temperature is lowered
Coarse-Grained Simulations of Membranes under Tension
We investigate the properties of membranes under tension by Monte-Carlo
simulations of a generic coarse-grained model for lipid bilayers. We give a
comprising overview of the behavior of several membrane characteristics, such
as the area per lipid, the monolayer overlap, the nematic order, and pressure
profiles. Both the low-temperature regime, where the membranes are in a gel
phase, and the high-temperature regime, where they are in the fluid phase, are
considered. In the gel state, the membrane is hardly influenced by tension. In
the fluid state, high tensions lead to structural changes in the membrane,
which result in different compressibility regimes. The ripple state, which is
found at tension zero in the transition regime between the fluid and the gel
phase, disappears under tension and gives way to an interdigitated phase. We
also study the membrane fluctuations in the fluid phase. In the low tension
regime the data can be fitted nicely to a suitably extended elastic theory. At
higher tensions the elastic fit consistently underestimates the strength of
long-wavelength fluctuations. Finally, we investigate the influence of tension
on the effective interaction between simple transmembrane inclusions and show
that tension can be used to tune the hydrophobic mismatch interaction between
membrane proteins.Comment: 14 pages, 14 figures, accepted for publication in The Journal of
Chemical Physic
Phase field theory of interfaces and crystal nucleation in a eutectic system of fcc structure: I. Transitions in the one-phase liquid region
The published version of this Article can be accessed from the link below - Copyright @ 2007 American Institute of PhysicsThe phase field theory (PFT) has been applied to predict equilibrium interfacial properties and nucleation barrier in the binary eutectic system Ag-Cu using double well and interpolation functions deduced from a Ginzburg-Landau expansion that considers fcc (face centered cubic) crystal symmetries. The temperature and composition dependent free energies of the liquid and solid phases are taken from CALculation of PHAse Diagrams-type calculations. The model parameters of PFT are fixed so as to recover an interface thickness of approximately 1 nm from molecular dynamics simulations and the interfacial free energies from the experimental dihedral angles available for the pure components. A nontrivial temperature and composition dependence for the equilibrium interfacial free energy is observed. Mapping the possible nucleation pathways, we find that the Ag and Cu rich critical fluctuations compete against each other in the neighborhood of the eutectic composition. The Tolman length is positive and shows a maximum as a function of undercooling. The PFT predictions for the critical undercooling are found to be consistent with experimental results. These results support the view that heterogeneous nucleation took place in the undercooling experiments available at present. We also present calculations using the classical droplet model classical nucleation theory (CNT) and a phenomenological diffuse interface theory (DIT). While the predictions of the CNT with a purely entropic interfacial free energy underestimate the critical undercooling, the DIT results appear to be in a reasonable agreement with the PFT predictions.This work has been supported by the Hungarian Academy of Sciences under Contract No. OTKA-K-62588 and by the ESA PECS Contract Nos. 98005, 98021, and 98043
Solvent-free coarse-grained lipid model for large-scale simulations
A coarse-grained molecular model, which consists of a spherical particle and
an orientation vector, is proposed to simulate lipid membrane on a large length
scale. The solvent is implicitly represented by an effective attractive
interaction between particles. A bilayer structure is formed by
orientation-dependent (tilt and bending) potentials. In this model, the
membrane properties (bending rigidity, line tension of membrane edge, area
compression modulus, lateral diffusion coefficient, and flip-flop rate) can be
varied over broad ranges. The stability of the bilayer membrane is investigated
via droplet-vesicle transition. The rupture of the bilayer and worm-like
micelle formation can be induced by an increase in the spontaneous curvature of
the monolayer membrane.Comment: 13 pages, 19 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
Solvent mediated interactions between model colloids and interfaces: A microscopic approach
We determine the solvent mediated contribution to the effective potentials
for model colloidal or nano- particles dispersed in a binary solvent that
exhibits fluid-fluid phase separation. Using a simple density functional theory
we calculate the density profiles of both solvent species in the presence of
the `colloids', which are treated as external potentials, and determine the
solvent mediated (SM) potentials. Specifically, we calculate SM potentials
between (i) two colloids, (ii) a colloid and a planar fluid-fluid interface,
and (iii) a colloid and a planar wall with an adsorbed wetting film. We
consider three different types of colloidal particles: colloid A which prefers
the bulk solvent phase rich in species 2, colloid C which prefers the solvent
phase rich in species 1, and `neutral' colloid B which has no strong preference
for either phase, i.e. the free energies to insert the colloid into either of
the coexisting bulk phases are almost equal. When a colloid which has a
preference for one of the two solvent phases is inserted into the disfavored
phase at statepoints close to coexistence a thick adsorbed `wetting' film of
the preferred phase may form around the colloids. The presence of the adsorbed
film has a profound influence on the form of the SM potentials.Comment: 17 Pages, 13 Figures. Accepted for publication in Journal of Chemical
Physic
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