37,768 research outputs found
The relationship between induced fluid structure and boundary slip in nanoscale polymer films
The molecular mechanism of slip at the interface between polymer melts and
weakly attractive smooth surfaces is investigated using molecular dynamics
simulations. In agreement with our previous studies on slip flow of
shear-thinning fluids, it is shown that the slip length passes through a local
minimum at low shear rates and then increases rapidly at higher shear rates. We
found that at sufficiently high shear rates, the slip flow over atomically flat
crystalline surfaces is anisotropic. It is demonstrated numerically that the
friction coefficient at the liquid-solid interface (the ratio of viscosity and
slip length) undergoes a transition from a constant value to the power-law
decay as a function of the slip velocity. The characteristic velocity of the
transition correlates well with the diffusion velocity of fluid monomers in the
first fluid layer near the solid wall at equilibrium. We also show that in the
linear regime, the friction coefficient is well described by a function of a
single variable, which is a product of the magnitude of surface-induced peak in
the structure factor and the contact density of the adjacent fluid layer. The
universal relationship between the friction coefficient and induced fluid
structure holds for a number of material parameters of the interface: fluid
density, chain length, wall-fluid interaction energy, wall density, lattice
type and orientation, thermal or solid walls.Comment: 33 pages, 14 figure
High magnetic field pulsars and magnetars: a unified picture
We propose a unified picture of high magnetic field radio pulsars and
magnetars by arguing that they are all rotating high-field neutron stars, but
have different orientations of their magnetic axes with respective to their
rotation axes. In strong magnetic fields where photon splitting suppresses pair
creation near the surface, the high-field pulsars can have active inner
accelerators while the anomalous X-ray pulsars cannot. This can account for the
very different observed emission characteristics of the anomalous X-ray pulsar
1E 2259+586 and the high field radio pulsar PSR J1814-1744. A predicted
consequence of this picture is that radio pulsars having surface magnetic field
greater than about G should not exist.Comment: 5 pages, emulateapj style, accepted for publication in the ApJ
Letter
Slip boundary conditions for shear flow of polymer melts past atomically flat surfaces
Molecular dynamics simulations are carried out to investigate the dynamic
behavior of the slip length in thin polymer films confined between atomically
smooth thermal surfaces. For weak wall-fluid interactions, the shear rate
dependence of the slip length acquires a distinct local minimum followed by a
rapid growth at higher shear rates. With increasing fluid density, the position
of the local minimum is shifted to lower shear rates. We found that the ratio
of the shear viscosity to the slip length, which defines the friction
coefficient at the liquid/solid interface, undergoes a transition from a nearly
constant value to the power law decay as a function of the slip velocity. In a
wide range of shear rates and fluid densities, the friction coefficient is
determined by the product of the value of surface induced peak in the structure
factor and the contact density of the first fluid layer near the solid wall.Comment: 27 pages, 11 figure
Interferometric Observations of V838 Monocerotis
We have used long-baseline near-IR interferometry to resolve the peculiar
eruptive variable V838 Mon and to provide the first direct measurement of its
angular size. Assuming a uniform disk model for the emission we derive an
apparent angular diameter at the time of observations (November-December 2004)
of milli-arcseconds. For a nominal distance of kpc,
this implies a linear radius of . However, the data are
somewhat better fit by elliptical disk or binary component models, and we
suggest that the emission may be strongly affected by ejecta from the outburst.Comment: 12 pages, 1 two-part encapsulated postscript figure. Accepted by
ApJL. Added a table of observation
Magnetars and pulsars: a missing link
There is growing evidence that soft gamma-ray repeaters (SGRs) and anomalous
X-ray pulsars (AXPs) are isolated neutron stars with superstrong magnetic
fields, i.e., magnetars, marking them a distinguished species from the
conventional species of spindown-powered isolated neutron stars, i.e., radio
pulsars. The current arguments in favor of the magnetar interpretation of
SGR/AXP phenomenology will be outlined, and the two energy sources in
magnetars, i.e. a magnetic dissipation energy and a spindown energy, will be
reviewed. I will then discuss a missing link between magnetars and pulsars,
i.e., lack of the observational evidence of the spindown-powered behaviors in
known magnetars. Some recent theoretical efforts in studying such behaviors
will be reviewed along with some predictions testable in the near future.Comment: Invited talk at the Sixth Pacific Rim Conference on Stellar
Astrophysics, a tribute to Helmut A. Abt, July 11-17, 2002, Xi'an. To appear
in the proceedings (eds. K. S. Cheng, K. C. Leung & T. P. Li
Magnetic Photon Splitting: Computations of Proper-time Rates and Spectra
The splitting of photons in the presence of an intense magnetic field has
recently found astrophysical applications in polar cap models of gamma-ray
pulsars and in magnetar scenarios for soft gamma repeaters. Numerical
computation of the polarization-dependent rates of this third order QED process
for arbitrary field strengths and energies below pair creation threshold is
difficult: thus early analyses focused on analytic developments and simpler
asymptotic forms. The recent astrophysical interest spurred the use of the
S-matrix approach by Mentzel, Berg and Wunner to determine splitting rates. In
this paper, we present numerical computations of a full proper-time expression
for the rate of splitting that was obtained by Stoneham, and is exact up to the
pair creation threshold. While the numerical results derived here are in accord
with the earlier asymptotic forms due to Adler, our computed rates still differ
by as much as factors of 3 from the S-matrix re-evaluation of Wilke and Wunner,
reflecting the extreme difficulty of generating accurate S-matrix numerics for
fields below about \teq{4.4\times 10^{13}}Gauss. We find that our proper-time
rates appear very accurate, and exceed Adler's asymptotic specializations
significantly only for photon energies just below pair threshold and for
supercritical fields, but always by less than a factor of around 2.6. We also
provide a useful analytic series expansion for the scattering amplitude valid
at low energies.Comment: 13 pages, AASTeX format, including 3 eps figures, ApJ in pres
Positronium collapse and the maximum magnetic field in pure QED
A maximum value for the magnetic field is determined, which provides the full
compensation of the positronium rest mass by the binding energy in the maximum
symmetry state and disappearance of the energy gap separating the
electron-positron system from the vacuum. The compensation becomes possible
owing to the falling to the center phenomenon. The maximum magnetic field may
be related to the vacuum and describe its structure.Comment: 4 pages, accepted for publication in Phys. Rev. Letter
Halo Excitation of He in Inelastic and Charge-Exchange Reactions
Four-body distorted wave theory appropriate for nucleon-nucleus reactions
leading to 3-body continuum excitations of two-neutron Borromean halo nuclei is
developed. The peculiarities of the halo bound state and 3-body continuum are
fully taken into account by using the method of hyperspherical harmonics. The
procedure is applied for A=6 test-bench nuclei; thus we report detailed studies
of inclusive cross sections for inelastic He(p,p')He and
charge-exchange Li(n,p)He reactions at nucleon energy 50 MeV. The
theoretical low-energy spectra exhibit two resonance-like structures. The first
(narrow) is the excitation of the well-known three-body resonance. The
second (broad) bump is a composition of overlapping soft modes of
multipolarities whose relative weights depend on
transferred momentum and reaction type. Inelastic scattering is the most
selective tool for studying the soft dipole excitation mode.Comment: Submitted to Phys. Rev. C., 11 figures using eps
Electric field of a pointlike charge in a strong magnetic field and ground state of a hydrogenlike atom
In an external constant magnetic field, so strong that the electron Larmour
length is much shorter than its Compton length, we consider the modification of
the Coulomb potential of a point charge owing to the vacuum polarization. We
establish a short-range component of the static interaction in the Larmour
scale, expressed as a Yukawa-like law, and reveal the corresponding "photon
mass" parameter. The electrostatic force regains its long-range character in
the Compton scale: the tail of the potential follows an anisotropic Coulomb
law, decreasing away from the charge slower along the magnetic field and faster
across. In the infinite-magnetic-field limit the potential is confined to an
infinitely thin string passing though the charge parallel to the external
field. This is the first evidence for dimensional reduction in the photon
sector of quantum electrodynamics. The one-dimensional form of the potential on
the string is derived that includes a delta-function centered in the charge.
The nonrelativistic ground-state energy of a hydrogenlike atom is found with
its use and shown not to be infinite in the infinite-field limit, contrary to
what was commonly accepted before, when the vacuum polarization had been
ignored. These results may be useful for studying properties of matter at the
surface of extremely magnetized neutron stars.Comment: 45 pages, 6 figures, accepted to Phys. Rev.
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