621 research outputs found
Magnetization of multicomponent ferrofluids
The solution of the mean spherical approximation (MSA) integral equation for
isotropic multicomponent dipolar hard sphere fluids without external fields is
used to construct a density functional theory (DFT), which includes external
fields, in order to obtain an analytical expression for the external field
dependence of the magnetization of ferrofluidic mixtures. This DFT is based on
a second-order Taylor series expansion of the free energy density functional of
the anisotropic system around the corresponding isotropic MSA reference system.
The ensuing results for the magnetic properties are in quantitative agreement
with our canonical ensemble Monte Carlo simulation data presented here.Comment: 12 pages including 4 figure
The origin of interparticle potential of electrorheological fluids
The particles of electrorheological fluids can be modelled as dielectric
spheres (DS) immersed in a continuum dielectric. When an external field is
applied, polarization charges are induced on the surfaces of the spheres and
can be represented as point dipoles placed in the centres of the spheres. When
the DSs are close to each other, the induced charge distributions are distorted
by the electric field of the neighbouring DSs. This is the origin of the
interaction potential between the DSs. The calculation of this energy is very
time consuming, therefore, the DS model cannot be used in molecular
simulations. In this paper, we show that the interaction between the point
dipoles appropriately approximates the interaction of DSs. The polarizable
point dipole model provides better results, but this model is not pair-wise
additive, so it is not that practical in particle simulations.Comment: 10 pages, 5 figure
Comment on 'Algebraic perturbation theory for polar fluids: A model for the dielectric constant'
Kalikmanov [Phys. Rev. E 59, 4085 (1999)] proposed a perturbation theory method to calculate the dielectric constant of dipolar hard sphere fluids using an infinitely long cylindrical container to avoid the depolarization. We demonstrate that while the method is very helpful, his theory appears to be incomplete because of the incorrect calculation of the corresponding three-body integrals. It is shown that with the correct consideration of these terms the theory is consistent with the results of earlier work in low-density limit, and at high densities the method yields the equation of Tani et al. [Mol. Phys. 48, 863 (1983)] for the dipolar hard sphere fluid dielectric constant.published_or_final_versio
Magnetization and susceptibility of ferrofluids
A second-order Taylor series expansion of the free energy functional provides
analytical expressions for the magnetic field dependence of the free energy and
of the magnetization of ferrofluids, here modelled by dipolar Yukawa
interaction potentials. The corresponding hard core dipolar Yukawa reference
fluid is studied within the framework of the mean spherical approximation. Our
findings for the magnetic and phase equilibrium properties are in quantitative
agreement with previously published and new Monte Carlo simulation data.Comment: 8 pages including 4 figure
The mean spherical approximation for a dipolar Yukawa fluid
The dipolar hard sphere fluid (DHSF) is a useful model of a polar fluid. However, the DHSF lacks a vapor-liquid transition due to the formation of chain-like structures. Such chains are not characteristic of real polar fluids. A more realistic model of a polar fluid is obtained by adding a Lennard-Jones potential to the intermolecular potential. Very similar results are obtained by adding a Yukawa potential, instead of the Lennard-Jones potential. We call this fluid the dipolar Yukawa fluid (DYF). We show that an analytical solution of the mean spherical approximation (MSA) can be obtained for the DYF. Thus, the DYF has many of the attractive features of the DHSF. We find that, within the MSA, the Yukawa potential modifies only the spherically averaged distribution function. Thus, although the thermodynamic properties of the DYF differ from those of the DHSF, the MSA dielectric constant of the DYF is the same as that of the DHSF. This result, and some other predictions, are tested by simulations and are found to be good approximations. © 1999 American Institute of Physics.published_or_final_versio
Discovery of the spectroscopic binary nature of three bright southern Cepheids
We present an analysis of spectroscopic radial velocity and photometric data
of three bright Galactic Cepheids: LR Trianguli Australis (LR TrA), RZ Velorum
(RZ Vel), and BG Velorum (BG Vel). Based on new radial velocity data, these
Cepheids have been found to be members of spectroscopic binary systems.
The ratio of the peak-to-peak radial velocity amplitude to photometric
amplitude indicates the presence of a companion for LR TrA and BG Vel. IUE
spectra indicate that the companions of RZ Vel and BG Vel cannot be hot stars.
The analysis of all available photometric data revealed that the pulsation
period of RZ Vel and BG Vel varies monotonically, due to stellar evolution.
Moreover, the longest period Cepheid in this sample, RZ Vel, shows period
fluctuations superimposed on the monotonic period increase. The light-time
effect interpretation of the observed pattern needs long-term photometric
monitoring of this Cepheid. The pulsation period of LR TrA has remained
constant since the discovery of its brightness variation.
Using statistical data, it is also shown that a large number of spectroscopic
binaries still remain to be discovered among bright classical Cepheids.Comment: 9 pages, 14 figure
Thermodynamics and structural properties of the dipolar Yukawa fluid
We report computer simulations and a theoretical study of the thermodynamics and structure of a dipolar Yukawa system. A comparison between the analytical mean spherical approximation (MSA) solution, perturbation theory and Monte Carlo simulation data of pressure, internal energy and dielectric constant is given. In the perturbation theory, the MSA solution of hard core Yukawa fluid is used as a reference system. It was found that the MSA solution is reasonable only at lower dipole moments, while the perturbation theory gives good results at low and high values of dipole moment. Liquid-vapor coexistence data of dipolar Yukawa fluid are also obtained by Monte Carlo simulation and by both MSA and perturbation theory. It was found that at high dipole moments the liquid-vapor equilibrium disappears while chain-like structures appear in the low density fluid phase. The appearance of chain-like structures of dipolar Yukawa fluid is discussed in comparison with the Stockmayer fluid. © 1999 American Institute of Physics.published_or_final_versio
Disease course, frequency of relapses and survival of 73 patients with juvenile or adult dermatomyositis
Objective
Our aim is to present the disease course, frequency of relapses and survival of juvenile and adult dermatomyositis (JDM/DM) patients.
Methods
Analysis was performed using data on 73 patients. The median follow-up for 38 JDM patients was 32 months and 78 months for 35 adult DM patients.
Results
23/38 JDM patients (60%) had monophasic, 12/38 (31.6%) had polycyclic and 3138 (7.9%) had chronic disease. Among children treated only with glucocorticoids, 12/20 (60%) had monophasic and 8/20 (40%) had polycyclic disease. 10/17 (58.8%) children, who required second-line immunosuppressive agents, had monophasic and 4/17 (23.5%) had polycyclic disease. 18/35 DM (51.4%) patients had monophasic, 13/35 (37.1%) had polycyclic, 1/35 (2.9%) had chronic disease and 3135 (8.6%) had fulminant myositis. Among DM patients requiring only glucocorticoids, 12/20 (60%) were monophasic and 8/20 (40%) were polycyclic. In patients requiring second-line immunosuppressive agents, 6/15 patients (40%) had monophasic and 5/15 (33.3%) had polycyclic disease. Among patients with polycyclic disease, the risk of relapse was higher during first year than later in the disease course. None of the JDM patients have died, while 4 disease-specific deaths occurred in adult patients. There was no significant difference between the survival of JDM and DM patients.
Discussion
There was no correlation between relapse-free survival and the initial therapeutic regimen. Many of our patients had polycyclic or chronic disease. As relapses can occur after a prolonged disease-free interval, patients should be followed for at least 2 years. Although we found a favourable survival rate, further investigations are needed to assess functional outcome
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