931 research outputs found
Eutectic Colony Formation: A Stability Analysis
Experiments have widely shown that a steady-state lamellar eutectic
solidification front is destabilized on a scale much larger than the lamellar
spacing by the rejection of a dilute ternary impurity and forms two-phase cells
commonly referred to as `eutectic colonies'. We extend the stability analysis
of Datye and Langer for a binary eutectic to include the effect of a ternary
impurity. We find that the expressions for the critical onset velocity and
morphological instability wavelength are analogous to those for the classic
Mullins-Sekerka instability of a monophase planar interface, albeit with an
effective surface tension that depends on the geometry of the lamellar
interface and, non-trivially, on interlamellar diffusion. A qualitatively new
aspect of this instability is the occurence of oscillatory modes due to the
interplay between the destabilizing effect of the ternary impurity and the
dynamical feedback of the local change in lamellar spacing on the front motion.
In a transient regime, these modes lead to the formation of large scale
oscillatory microstructures for which there is recent experimental evidence in
a transparent organic system. Moreover, it is shown that the eutectic front
dynamics on a scale larger than the lamellar spacing can be formulated as an
effective monophase interface free boundary problem with a modified
Gibbs-Thomson condition that is coupled to a slow evolution equation for the
lamellar spacing. This formulation provides additional physical insights into
the nature of the instability and a simple means to calculate an approximate
stability spectrum. Finally, we investigate the influence of the ternary
impurity on a short wavelength oscillatory instability that is already present
at off-eutectic compositions in binary eutectics.Comment: 26 pages RevTex, 14 figures (28 EPS files); some minor changes;
references adde
Probing the dynamics of quasicrystal growth using synchrotron live imaging
The dynamics of quasicrystal growth remains an unsolved problem in condensed
matter. By means of synchrotron live imaging, facetted growth proceeding by the
tangential motion of ledges at the solid-melt interface is clearly evidenced
all along the solidification of icosahedral AlPdMn quasicrystals. The effect of
interface kinetics is significant so that nucleation and free growth of new
facetted grains occur in the melt when the solidification rate is increased.
The evolution of these grains is explained in details, which reveals the
crucial role of aluminum rejection, both in the poisoning of grain growth and
driving fluid flow
Analytical solution of generalized Burton--Cabrera--Frank equations for growth and post--growth equilibration on vicinal surfaces
We investigate growth on vicinal surfaces by molecular beam epitaxy making
use of a generalized Burton--Cabrera--Frank model. Our primary aim is to
propose and implement a novel analytical program based on a perturbative
solution of the non--linear equations describing the coupled adatom and dimer
kinetics. These equations are considered as originating from a fully
microscopic description that allows the step boundary conditions to be directly
formulated in terms of the sticking coefficients at each step. As an example,
we study the importance of diffusion barriers for adatoms hopping down
descending steps (Schwoebel effect) during growth and post-growth equilibration
of the surface.Comment: 16 pages, REVTeX 3.0, IC-DDV-94-00
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Identifying a Heart Rate Recovery Criterion After a 6-Minute Walk Test in COPD
Background: Slow heart rate recovery (HRR) after exercise is associated with autonomic dysfunction and increased mortality. What HRR criterion at 1-minute after a 6-minute walk test (6MWT) best defines pulmonary impairment?.
Study Design and Methods: A total of 5008 phase 2 COPDGene (NCT00608764) participants with smoking history were included. A total of 2127 had COPD and, of these, 385 were followed-up 5-years later. Lung surgery, transplant, bronchiectasis, atrial fibrillation, heart failure and pacemakers were exclusionary. HR was measured from pulse oximetry at end-walk and after 1-min seated recovery. A receiver operator characteristic (ROC) identified optimal HRR cut-off. Generalized linear regression determined HRR association with spirometry, chest CT, symptoms and exacerbations.
Results: HRR after 6MWT (bt/min) was categorized in quintiles: ≤ 5 (23.0% of participants), 6– 10 (20.7%), 11– 15 (18.9%), 16– 22 (18.5%) and ≥ 23 (18.9%). Compared to HRR≤ 5, HRR≥ 11 was associated with (p\u3c 0.001): lower pre-walk HR and 1-min post HR; greater end-walk HR; greater 6MWD; greater FEV1%pred; lower airway wall area and wall thickness. HRR was positively associated with FEV1%pred and negatively associated with airway wall thickness. An optimal HRR ≤ 10 bt/min yielded an area under the ROC curve of 0.62 (95% CI 0.58– 0.66) for identifying FEV1\u3c 30%pred. HRR≥ 11 bt/min was the lowest HRR associated with consistently less impairment in 6MWT, spirometry and CT variables. In COPD, HRR≤ 10 bt/min was associated with (p\u3c 0.001): ≥ 2 exacerbations in the previous year (OR=1.76[1.33– 2.34]); CAT≥ 10 (OR=1.42[1.18– 1.71]); mMRC≥ 2 (OR=1.42[1.19– 1.69]); GOLD 4 (OR=1.98[1.44– 2.73]) and GOLD D (OR=1.51[1.18– 1.95]). HRR≤ 10 bt/min was predicted COPD exacerbations at 5-year follow-up (RR=1.83[1.07– 3.12], P=0.027).
Conclusion: HRR≤ 10 bt/min after 6MWT in COPD is associated with more severe expiratory flow limitation, airway wall thickening, worse dyspnoea and quality of life, and future exacerbations, suggesting that an abnormal HRR≤ 10 bt/min after a 6MWT may be used in a comprehensive assessment in COPD for risk of severity, symptoms and future exacerbations
Measurements of differential cross sections of Z/gamma*+jets+X events in proton anti-proton collisions at sqrt{s}=1.96 TeV
We present cross section measurements for Z/gamma*+jets+X production,
differential in the transverse momenta of the three leading jets. The data
sample was collected with the D0 detector at the Fermilab Tevatron proton
anti-proton collider at a center-of-mass energy of 1.96 TeV and corresponds to
an integrated luminosity of 1 fb-1. Leading and next-to-leading order
perturbative QCD predictions are compared with the measurements, and agreement
is found within the theoretical and experimental uncertainties. We also make
comparisons with the predictions of four event generators. Two
parton-shower-based generators show significant shape and normalization
differences with respect to the data. In contrast, two generators combining
tree-level matrix elements with a parton shower give a reasonable description
of the the shapes observed in data, but the predicted normalizations show
significant differences with respect to the data, reflecting large scale
uncertainties. For specific choices of scales, the normalizations for either
generator can be made to agree with the measurements.Comment: Published in PLB. 11 pages, 3 figure
Measurement of the t-channel single top quark production cross section
The D0 collaboration reports direct evidence for electroweak production of
single top quarks through the t-channel exchange of a virtual W boson. This is
the first analysis to isolate an individual single top quark production
channel. We select events containing an isolated electron or muon, missing
transverse energy, and two, three or four jets from 2.3 fb^-1 of ppbar
collisions at the Fermilab Tevatron Collider. One or two of the jets are
identified as containing a b hadron. We combine three multivariate techniques
optimized for the t-channel process to measure the t- and s-channel cross
sections simultaneously. We measure cross sections of 3.14 +0.94 -0.80 pb for
the t-channel and 1.05 +-0.81 pb for the s-channel. The measured t-channel
result is found to have a significance of 4.8 standard deviations and is
consistent with the standard model prediction.Comment: 7 pages, 6 figure
A General Approach for Predicting the Filtration of Soft and Permeable Colloids: The Milk Example
Membrane filtration operations (ultra-, microfiltration) are now extensively used for concentrating or separating an ever-growing variety of colloidal dispersions. However, the phenomena that determine the efficiency of these operations are not yet fully understood. This is especially the case when dealing with colloids that are soft, deformable, and permeable. In this paper, we propose a methodology for building a model that is able to predict the performance (flux, concentration profiles) of the filtration of such objects in relation with the operating conditions. This is done by focusing on the case of milk filtration, all experiments being performed with dispersions of milk casein micelles, which are sort of ″natural″ colloidal microgels. Using this example, we develop the general idea that a filtration model can always be built for a given colloidal dispersion as long as this dispersion has been characterized in terms of osmotic pressure Π and hydraulic permeability k. For soft and permeable colloids, the major issue is that the permeability k cannot be assessed in a trivial way like in the case for hard-sphere colloids. To get around this difficulty, we follow two distinct approaches to actually measure k: a direct approach, involving osmotic stress experiments, and a reverse-calculation approach, that consists of estimating k through well-controlled filtration experiments. The resulting filtration model is then validated against experimental measurements obtained from combined milk filtration/SAXS experiments. We also give precise examples of how the model can be used, as well as a brief discussion on the possible universality of the approach presented here
Measurement of trilinear gauge boson couplings from WW + WZ to lnu jj events in pp-bar collisions at sqrt{s}=1.96 TeV
We present a direct measurement of trilinear gauge boson couplings at gammaWW
and ZWW vertices in WW and WZ events produced in pp-bar collisions at
sqrt{s}=1.96 TeV. We consider events with one electron or muon, missing
transverse energy, and at least two jets. The data were collected using the D0
detector and correspond to 1.1/fb of integrated luminosity. Considering two
different relations between the couplings at the gammaWW and ZWW vertices, we
measure these couplings at 68% C.L. to be kappa_{gamma}=1.07^{+0.26}_{-0.29},
lambda =0.00^{+0.06}_{-0.06} and g_{1}^{Z}=1.04^{+0.09}_{-0.09} in a scenario
respecting SU(2)_L x U(1)_Y gauge symmetry and kappa =1.04^{+0.11}_{-0.11} and
lambda=0.00^{+0.06}_{-0.06} in an "equal couplings" scenario.Comment: 14 pages, 7 figures, published in Phys. Rev. D, updated to published
versio
Observation of ZZ production in ppbar collisions at sqrt(s) = 1.96 TeV
We present an observation for ZZ -> l+l-l'+l'- (l, l' = e or mu) production
in ppbar collisions at a center-of-mass energy of sqrt(s) = 1.96 TeV. Using 1.7
fb-1 of data collected by the D0 experiment at the Fermilab Tevatron Collider,
we observe three candidate events with an expected background of 0.14 +0.03
-0.02 events. The significance of this observation is 5.3 standard deviations.
The combination of D0 results in this channel, as well as in ZZ -> l+l-nunubar,
yields a significance of 5.7 standard deviations and a combined cross section
of sigma(ZZ) = 1.60 +/- 0.63 (stat.) +0.16 -0.17 (syst.) pb.Comment: 7 pages, 1 figure, 2 tables Modified slightly following review
proces
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