14,948 research outputs found
Force on a sphere via the generalized reciprocal theorem
An approach based on the generalized reciprocal theorem is presented to derive the well-known result for the drag force exerted on a rigid sphere translating in a viscous fluid in an arbitrary manner. The use of generalized reciprocal theorem allows one to bypass the calculation of stress distribution over the particle surface in order to compute the force
Hydrodynamic stress on fractal aggregates of spheres
We calculate the average hydrodynamic stress on fractal aggregates of spheres using Stokesian dynamics. We find that for fractal aggregates of force-free particles, the stress does not grow as the cube of the radius of gyration, but rather as the number of particles in the aggregate. This behavior is only found for random aggregates of force-free particles held together by hydrodynamic lubrication forces. The stress on aggregates of particles rigidly connected by interparticle forces grows as the radius of gyration cubed. We explain this behavior by examining the transmission of the tension along connecting lines in an aggregate and use the concept of a persistance length in order to characterize this stress transmission within an aggregate
Searching Gravitational Waves from Pulsars, Using Laser Beam Interferometers
We use recent population synthesis results to investigate the distribution of
pulsars in the frequency space, having a gravitational strain high enough to be
detected by the future generations of laser beam interferometers.
We find that until detectors become able to recover the entire population,
the frequency distribution of the 'detectable' population will be very
dependent on the detector noise curve. Assuming a mean equatorial deformation
, the optimal frequency is around 450 Hz for interferometers
of the first generation (LIGO or VIRGO) and shifts toward 85 Hz for advanced
detectors. An interesting result for future detection stategies is the
significant narrowing of the distribution when improving the sensitivity: with
an advanced detector, it is possible to have 90% of detection probability while
exploring less than 20% of the parameter space (7.5% in the case of ). In addition, we show that in most cases the spindown of
'detectable' pulsars represents a period shift of less than a tens of
nanoseconds after one year of observation, making them easier to follow in the
frequency space.Comment: 5 pages, 3 figures accepted for publication in Astronomy &
Astrophysic
Stability of degenerate Cauchy horizons in black hole spacetimes
In the multihorizon black hole spacetimes, it is possible that there are
degenerate Cauchy horizons with vanishing surface gravities. We investigate the
stability of the degenerate Cauchy horizon in black hole spacetimes. Despite
the asymptotic behavior of spacetimes (flat, anti-de Sitter, or de Sitter), we
find that the Cauchy horizon is stable against the classical perturbations, but
unstable quantum mechanically.Comment: Revtex, 4 pages, no figures, references adde
Gem-induced cytoskeleton remodeling increases cellular migration of HTLV-1-infected cells, formation of infected-to-target T-cell conjugates and viral transmission
Efficient HTLV-1 viral transmission occurs through cell-to-cell contacts. The Tax viral transcriptional activator protein facilitates this process. Using a comparative transcriptomic analysis, we recently identified a series of genes up-regulated in HTLV-1 Tax expressing T-lymphocytes. We focused our attention towards genes that are important for cytoskeleton dynamic and thus may possibly modulate cell-to-cell contacts. We first demonstrate that Gem, a member of the small GTP-binding proteins within the Ras superfamily, is expressed both at the RNA and protein levels in Tax-expressing cells and in HTLV-1-infected cell lines. Using a series of ChIP assays, we show that Tax recruits CREB and CREB Binding Protein (CBP) onto a c-AMP Responsive Element (CRE) present in the gem promoter. This CRE sequence is required to drive Tax-activated gem transcription. Since Gem is involved in cytoskeleton remodeling, we investigated its role in infected cells motility. We show that Gem co-localizes with F-actin and is involved both in T-cell spontaneous cell migration as well as chemotaxis in the presence of SDF-1/CXCL12. Importantly, gem knock-down in HTLV-1-infected cells decreases cell migration and conjugate formation. Finally, we demonstrate that Gem plays an important role in cell-to-cell viral transmission
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