19,928 research outputs found
Particles with selective wetting affect spinodal decomposition microstructures
We have used mesoscale simulations to study the effect of immobile particles
on microstructure formation during spinodal decomposition in ternary mixtures
such as polymer blends. Specifically, we have explored a regime of
interparticle spacings (which are a few times the characteristic spinodal
length scale) in which we might expect interesting new effects arising from
interactions among wetting, spinodal decomposition and coarsening. In this
paper, we report three new effects for systems in which the particle phase has
a strong preference for being wetted by one of the components (say, A). In the
presence of particles, microstructures are not bicontinuous in a symmetric
mixture. An asymmetric mixture, on the other hand, first forms a
non-bicontinuous microstructure which then evolves into a bicontinuous one at
intermediate times. Moreover, while wetting of the particle phase by the
preferred component (A) creates alternating A-rich and B-rich layers around the
particles, curvature-driven coarsening leads to shrinking and disappearance of
the first A-rich layer, leaving a layer of the non-preferred component in
contact with the particle. At late simulation times, domains of the matrix
components coarsen following the Lifshitz-Slyozov-Wagner law, .Comment: Accepted for publication in PCCP on 24th May 201
A mass-loss rate determination for zeta Puppis from the quantitative analysis of X-ray emission line profiles
We fit every emission line in the high-resolution Chandra grating spectrum of
zeta Pup with an empirical line profile model that accounts for the effects of
Doppler broadening and attenuation by the bulk wind. For each of sixteen lines
or line complexes that can be reliably measured, we determine a best-fitting
fiducial optical depth, tau_* = kappa*Mdot/4{pi}R_{\ast}v_{\infty}, and place
confidence limits on this parameter. These sixteen lines include seven that
have not previously been reported on in the literature. The extended wavelength
range of these lines allows us to infer, for the first time, a clear increase
in tau_* with line wavelength, as expected from the wavelength increase of
bound-free absorption opacity. The small overall values of tau_*, reflected in
the rather modest asymmetry in the line profiles, can moreover all be fit
simultaneously by simply assuming a moderate mass-loss rate of 3.5 \pm 0.3
\times 10^{-6} Msun/yr, without any need to invoke porosity effects in the
wind. The quoted uncertainty is statistical, but the largest source of
uncertainty in the derived mass-loss rate is due to the uncertainty in the
elemental abundances of zeta Pup, which affects the continuum opacity of the
wind, and which we estimate to be a factor of two. Even so, the mass-loss rate
we find is significantly below the most recent smooth-wind H-alpha mass-loss
rate determinations for zeta Pup, but is in line with newer determinations that
account for small-scale wind clumping. If zeta Pup is representative of other
massive stars, these results will have important implications for stellar and
galactic evolution.Comment: Accepted for publication in the Monthly Notices of the Royal
Astronomical Society. 17 pages, including 14 figures (7 color
The Spectroscopic Footprint of the Fast Solar Wind
We analyze a large, complex equatorial coronal hole (ECH) and its immediate
surroundings with a focus on the roots of the fast solar wind. We start by
demonstrating that our ECH is indeed a source of the fast solar wind at 1AU by
examining in situ plasma measurements in conjunction with recently developed
measures of magnetic conditions of the photosphere, inner heliosphere and the
mapping of the solar wind source region. We focus the bulk of our analysis on
interpreting the thermal and spatial dependence of the non-thermal line widths
in the ECH as measured by SOHO/SUMER by placing the measurements in context
with recent studies of ubiquitous Alfven waves in the solar atmosphere and line
profile asymmetries (indicative of episodic heating and mass loading of the
coronal plasma) that originate in the strong, unipolar magnetic flux
concentrations that comprise the supergranular network. The results presented
in this paper are consistent with a picture where a significant portion of the
energy responsible for the transport of heated mass into the fast solar wind is
provided by episodically occurring small-scale events (likely driven by
magnetic reconnection) in the upper chromosphere and transition region of the
strong magnetic flux regions that comprise the supergranular network.Comment: 25 pages, accepted to appear in the Astrophysical Journal. Supporting
movies can be found in http://download.hao.ucar.edu/pub/mscott/papers/ECH
Modeling the near-infrared lines of O-type stars
We use a grid of 30 line-blanketed unified stellar photosphere and wind
models for O-type stars; computed with the code CMFGEN in order to evaluate its
potential in the near-infrared spectral domain. The grid includes dwarfs,
giants and supergiants. We analyse the equivalent width behaviour of the 20
strongest lines of hydrogen and helium in spectral windows that can be observed
using ground-based instrumentation and compare the results with observations.
Our main findings are that: i) HeI/HeII line ratios in the J, H and K bands
correlate well with the optical ratio employed in spectral classification, and
can therefore be used to determine the spectral type; ii) in supergiant stars
the transition from the stellar photosphere to the wind follows a shallower
density gradient than the standard approach followed in our models, which can
be mimicked by adopting a lower gravity in our prescription of the density
stratification. iii) the Brackett gamma line poses a number of peculiar
problems which partly might be related to wind clumping, and iv) the Brackett
alpha line is an excellent mass-loss indicator. For the first and last item we
provide quantitative calibrations.Comment: 14 pages, 7 figures, accepted by A&
A Mass-Loss Rate Determination For Zeta Puppis From The Quantitative Analysis Of X-Ray Emission-Line Profiles
We fit every emission line in the high-resolution Chandra grating spectrum of. Pup with an empirical line profile model that accounts for the effects of Doppler broadening and attenuation by the bulk wind. For each of 16 lines or line complexes that can be reliably measured, we determine a best-fitting fiducial optical depth, tau(*) equivalent to kappa(M) over dot/4 pi R(*)upsilon(infinity), and place confidence limits on this parameter. These 16 lines include seven that have not previously been reported on in the literature. The extended wavelength range of these lines allows us to infer, for the first time, a clear increase in tau(*) with line wavelength, as expected from the wavelength increase of bound-free absorption opacity. The small overall values of tau(*), reflected in the rather modest asymmetry in the line profiles, can moreover all be fitted simultaneously by simply assuming a moderate mass-loss rate of 3.5 +/- 0.3 x 10(-6) M(circle dot) yr(-1), without any need to invoke porosity effects in the wind. The quoted uncertainty is statistical, but the largest source of uncertainty in the derived mass-loss rate is due to the uncertainty in the elemental abundances of zeta Pup, which affects the continuum opacity of the wind, and which we estimate to be a factor of 2. Even so, the mass-loss rate we find is significantly below the most recent smooth-wind H alpha mass-loss rate determinations for zeta Pup, but is in line with newer determinations that account for small-scale wind clumping. If zeta Pup is representative of other massive stars, these results will have important implications for stellar and Galactic evolution
Spin coating of an evaporating polymer solution
We consider a mathematical model of spin coating of a single polymer blended in a solvent. The model describes the one-dimensional development of the thin layer of the mixture as the layer thins due to flow created by a balance of viscous forces and centrifugal forces and due to evaporation of the solvent. In the model both the diffusivity of the solvent in the polymer and the viscosity of the mixture are very rapidly varying functions of the solvent volume fraction. Guided by numerical solutions an asymptotic analysis reveals a number of different possible behaviours of the thinning layer dependent on the nondimensional parameters describing the system.\ud
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The main practical interest is in controlling the appearance and development of a ``skin'' on the polymer where the solvent concentration reduces rapidly on the outer surface leaving the bulk of the layer still with high concentrations of solvent. The critical parameters controlling this behaviour are found to be the ratio of the diffusion to advection time scales, the ratio of the evaporation to advection time scales and , the ratio of the diffusivity of the initial mixture and the pure polymer. In particular, our analysis shows that for very small evaporation with skin formation can be prevented
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