4,236 research outputs found
Real-Time Maps of Fluid Flow Fields in Porous Biomaterials
Mechanical forces such as fluid shear have been shown to enhance cell growth
and differentiation, but knowledge of their mechanistic effect on cells is
limited because the local flow patterns and associated metrics are not
precisely known. Here we present real-time, noninvasive measures of local
hydrodynamics in 3D biomaterials based on nuclear magnetic resonance. Microflow
maps were further used to derive pressure, shear and fluid permeability fields.
Finally, remodeling of collagen gels in response to precise fluid flow
parameters was correlated with structural changes. It is anticipated that
accurate flow maps within 3D matrices will be a critical step towards
understanding cell behavior in response to controlled flow dynamics.Comment: 23 pages, 4 figure
Accuracy threshold for concatenated error detection in one dimension
Estimates of the quantum accuracy threshold often tacitly assume that it is
possible to interact arbitrary pairs of qubits in a quantum computer with a
failure rate that is independent of the distance between them. None of the many
physical systems that are candidates for quantum computing possess this
property. Here we study the performance of a concatenated error-detection code
in a system that permits only nearest-neighbor interactions in one dimension.
We make use of a new message-passing scheme that maximizes the number of errors
that can be reliably corrected by the code. Our numerical results indicate that
arbitrarily accurate universal quantum computation is possible if the
probability of failure of each elementary physical operation is below
approximately 10^{-5}. This threshold is three orders of magnitude lower than
the highest known.Comment: 7 pages, 4 figures, now with error bar
Molecular Hydrogen Line Emission from the Reflection Nebula Parsamyan 18
The newly-commissioned University of New South Wales Infrared Fabry-Perot
(UNSWIRF) has been used to image molecular hydrogen emission at 2.12 and 2.25
microns in the reflection nebula Parsamyan 18. P 18 is known to exhibit low
values of the (1-0)/(2-1) S(1) ratio suggestive of UV-pumped fluorescence
rather than thermal excitation by shocks. Our line ratio mapping reveals the
full extent of this fluorescent emission from extended arc-like features, as
well as a more concentrated thermal component in regions closer to the central
exciting star. We show that the emission morphology, line fluxes, and gas
density are consistent with the predictions of photodissociation region (PDR)
theory. Those regions with the highest intrinsic 1-0 S(1) intensities also tend
to show the highest (1-0)/(2-1) S(1) line ratios. Furthermore, variations in
the line ratio can be attributed to intrinsic fluctuations in the incident
radiation field and/or the gas density, through the self-shielding action of
H_2. An isolated knot of emission discovered just outside P 18, and having both
an unusually high (1-0)/(2-1) S(1) ratio and relative velocity provides
additional evidence for an outflow source associated with P 18.Comment: 8 pages, 6 figures, LaTeX, uses mn-1.4.sty. Accepted for publication
in MNRA
Sigma Terms of Light-Quark Hadrons
A calculation of the current-quark mass dependence of hadron masses can help
in using observational data to place constraints on the variation of nature's
fundamental parameters. A hadron's sigma-term is a measure of this dependence.
The connection between a hadron's sigma-term and the Feynman-Hellmann theorem
is illustrated with an explicit calculation for the pion using a rainbow-ladder
truncation of the Dyson-Schwinger equations: in the vicinity of the chiral
limit sigma_pi = m_pi/2. This truncation also provides a decent estimate of
sigma_rho because the two dominant self-energy corrections to the rho-meson's
mass largely cancel in their contribution to sigma_rho. The truncation is less
accurate for the omega, however, because there is little to compete with an
omega->rho+pi self-energy contribution that magnifies the value of sigma_omega
by ~25%. A Poincare' covariant Faddeev equation, which describes baryons as
composites of confined-quarks and -nonpointlike-diquarks, is solved to obtain
the current-quark mass dependence of the masses of the nucleon and Delta, and
thereby sigma_N and sigma_Delta. This "quark-core" piece is augmented by the
"pion cloud" contribution, which is positive. The analysis yields sigma_N~60MeV
and sigma_Delta~50MeV.Comment: 22 pages, reference list expande
The reductive activation of CO2 across a TiâTi double bond: synthetic, structural, and mechanistic studies
[Image: see text] The reactivity of the bis(pentalene)dititanium double-sandwich compound Ti(2)Pn(â )(2) (1) (Pn(â ) = 1,4-{Si(i)Pr(3)}(2)C(8)H(4)) with CO(2) is investigated in detail using spectroscopic, X-ray crystallographic, and computational studies. When the CO(2) reaction is performed at â78 °C, the 1:1 adduct 4 is formed, and low-temperature spectroscopic measurements are consistent with a CO(2) molecule bound symmetrically to the two Ti centers in a ÎŒ:η(2),η(2) binding mode, a structure also indicated by theory. Upon warming to room temperature the coordinated CO(2) is quantitatively reduced over a period of minutes to give the bis(oxo)-bridged dimer 2 and the dicarbonyl complex 3. In situ NMR studies indicated that this decomposition proceeds in a stepwise process via monooxo (5) and monocarbonyl (7) double-sandwich complexes, which have been independently synthesized and structurally characterized. 5 is thermally unstable with respect to a ÎŒ-O dimer in which the TiâTi bond has been cleaved and one pentalene ligand binds in an η(8) fashion to each of the formally Ti(III) centers. The molecular structure of 7 shows a âside-onâ bound carbonyl ligand. Bonding of the double-sandwich species Ti(2)Pn(2) (Pn = C(8)H(6)) to other fragments has been investigated by density functional theory calculations and fragment analysis, providing insight into the CO(2) reaction pathway consistent with the experimentally observed intermediates. A key step in the proposed mechanism is disproportionation of a mono(oxo) di-Ti(III) species to yield di-Ti(II) and di-Ti(IV) products. 1 forms a structurally characterized, thermally stable CS(2) adduct 8 that shows symmetrical binding to the Ti(2) unit and supports the formulation of 4. The reaction of 1 with COS forms a thermally unstable complex 9 that undergoes scission to give mono(ÎŒ-S) mono(CO) species 10. Ph(3)PS is an effective sulfur transfer agent for 1, enabling the synthesis of mono(ÎŒ-S) complex 11 with a double-sandwich structure and bis(ÎŒ-S) dimer 12 in which the TiâTi bond has been cleaved
An Electrochemical Study of Frustrated Lewis Pairs: A Metal-free Route to Hydrogen Oxidation
[Image: see text] Frustrated Lewis pairs have found many applications in the heterolytic activation of H(2) and subsequent hydrogenation of small molecules through delivery of the resulting proton and hydride equivalents. Herein, we describe how H(2) can be preactivated using classical frustrated Lewis pair chemistry and combined with in situ nonaqueous electrochemical oxidation of the resulting borohydride. Our approach allows hydrogen to be cleanly converted into two protons and two electrons in situ, and reduces the potential (the required energetic driving force) for nonaqueous H(2) oxidation by 610 mV (117.7 kJ mol(â1)). This significant energy reduction opens routes to the development of nonaqueous hydrogen energy technology
Lifetime determination of excited states in Cd-106
Two separate experiments using the Differential Decay Curve Method have been performed to extract mean lifetimes of excited states in 106 Cd. The inedium-spin states of interest were populated by the Mo-98(C-12, 4n) Cd-106 reaction performed at the Wright Nuclear Structure Lab., Yale University. From this experiment, two isomeric state mean lifetimes have been deduced. The low-lying states were populated by the Mo-96(C-13, 3n)Cd-106 reaction performed at the Institut fur Kernphysik, Universitat zu Koln. The mean lifetime of the I-pi = 2(1)(+) state was deduced, tentatively, as 16.4(9) ps. This value differs from the previously accepted literature value from Coulomb excitation of 10.43(9) ps
Title: will be set by the publisher Editors: will be set by the publisher EAS Publications Series, Vol.?, 2009 THE BEST SITE ON EARTH?
Abstract. We compare the merits of potential observatory sites on the Antarctic Plateau, in regard to the boundary layer, cloud cover, free atmosphere seeing, aurorae, airglow, and precipitable water vapour. We find that (a) all Antarctic sites are likely compromised for optical work by airglow and aurorae; (b) Dome A is the best existing site in almost all respects; (c) there is an even better site (âRidge Aâ) 150kms SW of Dome A; (d) Dome F is a remarkably good site except for aurorae; (e) Dome C probably has the least cloud cover of any of the sites, and might be able to use a predicted âOH hole â in the Spring. The Antarctic plateau probably contains the best astronomical sites on Earth, but none of the existing bases were situated with astronomy in mind. In Saunders et al.(2009), we use published data and models, and unpublished meteorological and other information, to try to compare the merits of the potential sites. Here, we summarise only the new findings and conclusions. We include boundary layer thickness, cloud cover, auroral emission, airglow, precipitable water vapour, an
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