99 research outputs found
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
Anomalous enhancements of low-energy fusion rates in plasmas: the role of ion momentum distributions and inhomogeneous screening
Non-resonant fusion cross-sections significantly higher than corresponding
theoretical predictions are observed in low-energy experiments with deuterated
matrix target. Models based on thermal effects, electron screening, or
quantum-effect dispersion relations have been proposed to explain these
anomalous results: none of them appears to satisfactory reproduce the
experiments. Velocity distributions are fundamental for the reaction rates and
deviations from the Maxwellian limit could play a central role in explaining
the enhancement. We examine two effects: an increase of the tail of the target
Deuteron momentum distribution due to the Galitskii-Yakimets quantum
uncertainty effect, which broadens the energy-momentum relation; and spatial
fluctuations of the Debye-H\"{u}ckel radius leading to an effective increase of
electron screening. Either effect leads to larger reaction rates especially
large at energies below a few keV, reducing the discrepancy between
observations and theoretical expectations.Comment: 6 pages, 3 figure
Magnetic properties of colloidal suspensions of interacting magnetic particles
We review equilibrium thermodynamic properties of systems of magnetic
particles like ferrofluids in which dipolar interactions play an important
role. The review is focussed on two subjects: ({\em i}) the magnetization with
the initial magnetic susceptibility as a special case and ({\em ii}) the phase
transition behavior. Here the condensation ("gas/liquid") transition in the
subsystem of the suspended particles is treated as well as the
isotropic/ferromagnetic transition to a state with spontaneously generated
long--range magnetic order.Comment: Review. 62 pages, 4 figure
Enhancement of fusion rates due to quantum effects in the particles momentum distribution in nonideal media
This study concerns a situation when measurements of the nonresonant
cross-section of nuclear reactions appear highly dependent on the environment
in which the particles interact. An appealing example discussed in the paper is
the interaction of a deuteron beam with a target of deuterated metal Ta. In
these experiments, the reaction cross section for d(d,p)t was shown to be
orders of magnitude greater than what the conventional model predicts for the
low-energy particles. In this paper we take into account the influence of
quantum effects due to the Heisenberg uncertainty principle for particles in a
non-ideal medium elastically interacting with the medium particles. In order to
calculate the nuclear reaction rate in the non-ideal environment we apply both
the Monte Carlo technique and approximate analytical calculation of the Feynman
diagram using nonrelativistic kinetic Green's functions in the medium which
correspond to the generalized energy and momentum distribution functions of
interacting particles. We show a possibility to reduce the 12-fold integral
corresponding to this diagram to a fivefold integral. This can significantly
speed up the computation and control accuracy. Our calculations show that
quantum effects significantly influence reaction rates such as p +7Be, 3He
+4He, p +7Li, and 12C +12C. The new reaction rates may be much higher than the
classical ones for the interior of the Sun and supernova stars. The possibility
to observe the theoretical predictions under laboratory conditions is
discussed
Magnetization of ferrofluids with dipolar interactions - a Born--Mayer expansion
For ferrofluids that are described by a system of hard spheres interacting
via dipolar forces we evaluate the magnetization as a function of the internal
magnetic field with a Born--Mayer technique and an expansion in the dipolar
coupling strength. Two different approximations are presented for the
magnetization considering different contributions to a series expansion in
terms of the volume fraction of the particles and the dipolar coupling
strength.Comment: 19 pages, 11 figures submitted to PR
Hydrodynamic interactions in colloidal ferrofluids: A lattice Boltzmann study
We use lattice Boltzmann simulations, in conjunction with Ewald summation
methods, to investigate the role of hydrodynamic interactions in colloidal
suspensions of dipolar particles, such as ferrofluids. Our work addresses
volume fractions of up to 0.20 and dimensionless dipolar interaction
parameters of up to 8. We compare quantitatively with Brownian
dynamics simulations, in which many-body hydrodynamic interactions are absent.
Monte Carlo data are also used to check the accuracy of static properties
measured with the lattice Boltzmann technique. At equilibrium, hydrodynamic
interactions slow down both the long-time and the short-time decays of the
intermediate scattering function , for wavevectors close to the peak of
the static structure factor , by a factor of roughly two. The long-time
slowing is diminished at high interaction strengths whereas the short-time
slowing (quantified via the hydrodynamic factor ) is less affected by the
dipolar interactions, despite their strong effect on the pair distribution
function arising from cluster formation. Cluster formation is also studied in
transient data following a quench from ; hydrodynamic interactions
slow the formation rate, again by a factor of roughly two
Voyaging in the Pacific
Teaching Oceania is a publication series created with the collaboration of scholars from around the Pacific region to address the need for appropriate literature for undergraduate Pacific Islands Studies students throughout Oceania. The series is designed to take advantage of digital technology to enhance texts with embedded multimedia content, thought-provoking images, and interactive graphs.Teaching Oceania is a publication series created with the collaboration of scholars from around the Pacific region to address the need for appropriate literature for undergraduate Pacific Islands Studies students throughout Oceania. The series is designed to take advantage of digital technology to enhance texts with embedded multimedia content, thought-provoking images, and interactive graphs
ANDES, the high resolution spectrograph for the ELT: project management and system engineering approaches for mastering its preliminary design phase
Myocardial energy depletion and dynamic systolic dysfunction in hypertrophic cardiomyopathy
Evidence indicates that anatomical and physiological phenotypes of hypertrophic cardiomyopathy (HCM) stem from genetically mediated, inefficient cardiomyocyte energy utilization, and subsequent cellular energy depletion. However, HCM often presents clinically with normal left ventricular (LV) systolic function or hyperkinesia. If energy inefficiency is a feature of HCM, why is it not manifest as resting LV systolic dysfunction? In this Perspectives article, we focus on an idiosyncratic form of reversible systolic dysfunction provoked by LV obstruction that we have previously termed the 'lobster claw abnormality' — a mid-systolic drop in LV Doppler ejection velocities. In obstructive HCM, this drop explains the mid-systolic closure of the aortic valve, the bifid aortic pressure trace, and why patients cannot increase stroke volume with exercise. This phenomenon is characteristic of a broader phenomenon in HCM that we have termed dynamic systolic dysfunction. It underlies the development of apical aneurysms, and rare occurrence of cardiogenic shock after obstruction. We posit that dynamic systolic dysfunction is a manifestation of inefficient cardiomyocyte energy utilization. Systolic dysfunction is clinically inapparent at rest; however, it becomes overt through the mechanism of afterload mismatch when LV outflow obstruction is imposed. Energetic insufficiency is also present in nonobstructive HCM. This paradigm might suggest novel therapies. Other pathways that might be central to HCM, such as myofilament Ca2+ hypersensitivity, and enhanced late Na+ current, are discussed
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