326 research outputs found
Magnetic diffusivity tensor and dynamo effects in rotating and shearing turbulence
The turbulent magnetic diffusivity tensor is determined in the presence of
rotation or shear. The question is addressed whether dynamo action from the
shear-current effect can explain large-scale magnetic field generation found in
simulations with shear. For this purpose a set of evolution equations for the
response to imposed test fields is solved with turbulent and mean motions
calculated from the momentum and continuity equations. The corresponding
results for the electromotive force are used to calculate turbulent transport
coefficients. The diagonal components of the turbulent magnetic diffusivity
tensor are found to be very close together, but their values increase slightly
with increasing shear and decrease with increasing rotation rate. In the
presence of shear, the sign of the two off-diagonal components of the turbulent
magnetic diffusion tensor is the same and opposite to the sign of the shear.
This implies that dynamo action from the shear--current effect is impossible,
except perhaps for high magnetic Reynolds numbers. However, even though there
is no alpha effect on the average, the components of the alpha tensor display
Gaussian fluctuations around zero. These fluctuations are strong enough to
drive an incoherent alpha--shear dynamo. The incoherent shear--current effect,
on the other hand, is found to be subdominant.Comment: 12 pages, 13 figures, improved version, accepted by Ap
Multiscale characterization of chemical-mechanical interactions between polymer fibers and cementitious matrix
Together with a series of mechanical tests, the interactions and potential bonding between polymeric fibers and cementitious materials were studied using scanning transmission X-ray microscopy (STXM) and microtomography (mu CT). Experimental results.showed that these techniques have great potential to characterize the polymer fiber-hydrated cement-paste matrix interface, as well as differentiating the chemistry of the two components of a bi-polymer (hybrid) fiber-the polypropylene core and the ethylene acrylic acid copolymer sheath. Similarly, chemical interactions between the hybrid fiber and the cement hydration products were observed, indicating the chemical bonding between the sheath and the hardened cement paste matrix. Microtomography allowed visualization of the performance of the samples, and the distribution and orientation of the two types of fiber in mortar. Beam flexure tests confirmed improved tensile strength of mixes containing hybrid fibers, and expansion bar tests showed similar reductions in expansion for the polypropylene and hybrid fiber mortar bars
Blockade of T-cell activation by dithiocarbamates involves novel mechanisms of inhibition of nuclear factor of activated T cells.
Dithiocarbamates (DTCs) have recently been reported as powerful inhibitors of NF-kappaB activation in a number of cell types. Given the role of this transcription factor in the regulation of gene expression in the inflammatory response, NF-kappaB inhibitors have been suggested as potential therapeutic drugs for inflammatory diseases. We show here that DTCs inhibited both interleukin 2 (IL-2) synthesis and membrane expression of antigens which are induced during T-cell activation. This inhibition, which occurred with a parallel activation of c-Jun transactivating functions and expression, was reflected by transfection experiments at the IL-2 promoter level, and involved not only the inhibition of NF-kappaB-driven reporter activation but also that of nuclear factor of activated T cells (NFAT). Accordingly, electrophoretic mobility shift assays (EMSAs) indicated that pyrrolidine DTC (PDTC) prevented NF-kappaB, and NFAT DNA-binding activity in T cells stimulated with either phorbol myristate acetate plus ionophore or antibodies against the CD3-T-cell receptor complex and simultaneously activated the binding of AP-1. Furthermore, PDTC differentially targeted both NFATp and NFATc family members, inhibiting the transactivation functions of NFATp and mRNA induction of NFATc. Strikingly, Western blotting and immunocytochemical experiments indicated that PDTC promoted a transient and rapid shuttling of NFATp and NFATc, leading to their accelerated export from the nucleus of activated T cells. We propose that the activation of an NFAT kinase by PDTC could be responsible for the rapid shuttling of the NFAT, therefore transiently converting the sustained transactivation of this transcription factor that occurs during lymphocyte activation, and show that c-Jun NH2-terminal kinase (JNK) can act by directly phosphorylating NFATp. In addition, the combined inhibitory effects on NFAT and NF-KB support a potential use of DTCs as immunosuppressants
Lipid-Based Passivation in Nanofluidics
Stretching DNA in nanochannels is a useful tool for direct, visual studies of genomic DNA at the single molecule level. To facilitate the study of the interaction of linear DNA with proteins in nanochannels, we have implemented a highly effective passivation scheme based on lipid bilayers. We demonstrate virtually complete long-term passivation of nanochannel surfaces to a range of relevant reagents, including streptavidin-coated quantum dots, RecA proteins, and RecA-DNA complexes. We show that the performance of the lipid bilayer is significantly better than that of standard bovine serum albumin-based passivation. Finally, we show how the passivated devices allow us to monitor single DNA cleavage events during enzymatic degradation by DNase I. We expect that our approach will open up for detailed, systematic studies of a wide range of protein-DNA interactions with high spatial and temporal resolution
On the rotational evolution of solar- and late-type stars, its magnetic origins, and the possibility of stellar gyrochronology
We propose a simple interpretation of the rotation period data for solar- and
late-type stars. The open cluster and Mt. Wilson star observations suggest that
rotating stars lie primarily on two sequences, initially called I and C. Some
stars lie in the intervening gap. These sequences, and the fractional numbers
of stars on each sequence evolve systematically with cluster age, enabling us
to construct crude rotational isochrones allowing `stellar gyrochronology', a
procedure, upon improvement, likely to yield ages for individual field stars.
The age and color dependences of the sequences allow the identification of the
underlying mechanism, which appears to be primarily magnetic. The majority of
solar- and late-type stars possess a dominant Sun-like, or Interface magnetic
field, which connects the convective envelope both to the radiative interior of
the star and to the exterior where winds can drain off angular momentum. These
stars spin down Skumanich-style. An age-decreasing fraction of young G, K, and
M stars, which are rapid rotators, possess only a Convective field which is not
only inefficient in depleting angular momentum, but also incapable of coupling
the surface convection zone to the inner radiative zone, so that only the outer
zone is spun down, and on an exponential timescale. These stars do not yet
possess large-scale dynamos. The large-scale magnetic field associated with the
dynamo, apparently created by the shear between the decoupled radiative and
convective zones, (re)couples the convective and radiative zones and drives a
star from the Convective to the Interface sequence through the gap on a
timescale that increases as stellar mass decreases. (Abstract is truncated
here.)Comment: 32 pages, 6 figures (4 in color), accepted by Ap
Interference of flavonoids with enzymatic assays for the determination of free fatty acid and triglyceride levels
Flavonoids are bioactive food compounds with potential lipid-lowering effects. Commercially available enzymatic assays are widely used to determine free fatty acid (FFA) and triglyceride (TG) levels both in vivo in plasma or serum and in vitro in cell culture medium or cell lysate. However, we have observed that various flavonoids interfere with peroxidases used in these enzymatic assays, resulting in incorrect lower FFA and TG levels than actually present. Furthermore, addition of isorhamnetin or the major metabolite of the flavonoid quercetin in human and rat plasma, quercetin-3-O-glucuronide, to murine serum also resulted in a significant reduction of the detected TG levels, while a trend was seen for FFA levels. It is concluded that when applying these assays, vigilance is needed and alternative analytical methods, directly assessing FFA or TG levels, should be used for studying the biological effects of flavonoids on FFA and TG levels
Femoral fracture following knee ligament reconstruction surgery due to an unpredictable complication of bioabsorbable screw fixation: a case report and review of literature
We report an unusual case of femoral fracture from minimal trauma, due to the rapid disappearance of a bioabsorbable interference screw used for reconstruction of the posterolateral corner of the knee. The literature on bone tunnel fractures following knee ligament reconstruction surgery is also reviewed
The case for a distributed solar dynamo shaped by near-surface shear
Arguments for and against the widely accepted picture of a solar dynamo being
seated in the tachocline are reviewed and alternative ideas concerning dynamos
operating in the bulk of the convection zone, or perhaps even in the
near-surface shear layer, are discussed. Based on the angular velocities of
magnetic tracers it is argued that the observations are compatible with a
distributed dynamo that may be strongly shaped by the near-surface shear layer.
Direct simulations of dynamo action in a slab with turbulence and shear are
presented to discuss filling factor and tilt angles of bipolar regions in such
a model.Comment: 10 pages, 6 figures, Astrophys. J. 625 (scheduled for the 1 June 2005
issue
Overcoming the Back Reaction on Turbulent Motions in the Presence of Magnetic Fields
Standard magnetohydrodynamic theories, such as the mean field dynamo theory,
have been criticized when the back reaction of the magnetic field on turbulent
motions is neglected. For the dynamo, this back reaction has been argued to
suppress the turbulent motions required for optimal mean field production. Here
it is suggested that if the magnetic field is spatially intermittent, for
example residing in flux tubes, the back reaction on turbulent flows may be
significantly reduced.Comment: Accepted to Physical Review Letters, 15 pages, plain Te
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