7,942 research outputs found
Hydrodynamic lift on bound vesicles
Bound vesicles subject to lateral forces such as arising from shear flow are
investigated theoretically by combining a lubrication analysis of the bound
part with a scaling approach to the global motion. A minor inclination of the
bound part leads to significant lift due to the additive effects of lateral and
tank-treading motions. With increasing shear rate, the vesicle unbinds from the
substrate at a critical value. Estimates are in agreement with recent
experimental data.Comment: 9 pages, one figur
Generation of specific antibodies against the rap1A, rap1B and rap2 small GTP-binding proteins. Analysis of rap and ras proteins in membranes from mammalian cells
Specific antibodies against rap1A and rap1B small GTP-binding proteins were generated by immunization of rabbits with peptides derived from the C-terminus of the processed proteins. Immunoblot analysis of membranes from several mammalian cell lines and human thrombocytes with affinity-purified antibodies against rap1A or rap1B demonstrated the presence of multiple immunoreactive proteins in the 22-23 kDa range, although at strongly varying levels. Whereas both proteins were present in substantial amounts in membranes from myelocytic HL-60, K-562 and HEL cells, they were hardly detectable in membranes from lymphoma U-937 and S49.1 cyc- cells. Membranes from human thrombocytes and 3T3-Swiss Albino fibroblasts showed strong rap1B immunoreactivity, whereas rap1A protein was present in much lower amounts. In the cytosol of HL-60 cells, only small amounts of rap1A and rap1B proteins were detected, unless the cells were treated with lovastatin, an inhibitor of hydroxymethylglutaryl-coenzyme A reductase, suggesting that both proteins are isoprenylated. By comparison with recombinant proteins, the ratio of rap1A/ras proteins in membranes from HL-60 cells was estimated to be about 4:1. An antiserum directed against the C-terminus of rap2 reacted strongly with recombinant rap2, but not with membranes from tested mammalian cells. In conclusion, rap1A and rap1B proteins are distributed differentially among membranes from various mammalian cell types and are isoprenylated in HL-60 cells
SPH Simulations of Counterrotating Disk Formation in Spiral Galaxies
We present the results of Smoothed Particle Hydrodynamics (SPH) simulations
of the formation of a massive counterrotating disk in a spiral galaxy. The
current study revisits and extends (with SPH) previous work carried out with
sticky particle gas dynamics, in which adiabatic gas infall and a retrograde
gas-rich dwarf merger were tested as the two most likely processes for
producing such a counterrotating disk. We report on experiments with a cold
primary similar to our Galaxy, as well as a hot, compact primary modeled after
NGC 4138. We have also conducted numerical experiments with varying amounts of
prograde gas in the primary disk, and an alternative infall model (a spherical
shell with retrograde angular momentum). The structure of the resulting
counterrotating disks is dramatically different with SPH. The disks we produce
are considerably thinner than the primary disks and those produced with sticky
particles. The time-scales for counterrotating disk formation are shorter with
SPH because the gas loses kinetic energy and angular momentum more rapidly.
Spiral structure is evident in most of the disks, but an exponential radial
profile is not a natural byproduct of these processes. The infalling gas shells
that we tested produce counterrotating bulges and rings rather than disks. The
presence of a considerable amount of preexisting prograde gas in the primary
causes, at least in the absence of star formation, a rapid inflow of gas to the
center and a subsequent hole in the counterrotating disk. In general, our SPH
experiments yield stronger evidence to suggest that the accretion of massive
counterrotating disks drives the evolution of the host galaxies towards earlier
(S0/Sa) Hubble types.Comment: To appear in ApJ. 20 pages LaTex 2-column with 3 tables, 23 figures
(GIF) available at this site. Complete gzipped postscript preprint with
embedded figures available from http://tarkus.pha.jhu.edu/~thakar/cr3.html (3
Mb
Fluctuation-Dissipation Theorem in Nonequilibrium Steady States
In equilibrium, the fluctuation-dissipation theorem (FDT) expresses the
response of an observable to a small perturbation by a correlation function of
this variable with another one that is conjugate to the perturbation with
respect to \emph{energy}. For a nonequilibrium steady state (NESS), the
corresponding FDT is shown to involve in the correlation function a variable
that is conjugate with respect to \emph{entropy}. By splitting up entropy
production into one of the system and one of the medium, it is shown that for
systems with a genuine equilibrium state the FDT of the NESS differs from its
equilibrium form by an additive term involving \emph{total} entropy production.
A related variant of the FDT not requiring explicit knowledge of the stationary
state is particularly useful for coupled Langevin systems. The \emph{a priori}
surprising freedom apparently involved in different forms of the FDT in a NESS
is clarified.Comment: 6 pages; EPL, in pres
Wrinkling of microcapsules in shear flow
Elastic capsules can exhibit short wavelength wrinkling in external shear
flow. We analyse this instability of the capsule shape and use the length scale
separation between the capsule radius and the wrinkling wavelength to derive
analytical results both for the threshold value of the shear rate and for the
critical wave-length of the wrinkling. These results can be used to deduce
elastic parameters from experiments.Comment: 4 pages, 2 figures, submitted to PR
Tidal controls on trace gas dynamics in a seagrass meadow of the Ria Formosa lagoon (southern Portugal)
Coastal zones are important source regions for a variety of trace gases, including halocarbons and sulfur-bearing species. While salt marshes, macroalgae and phyto-plankton communities have been intensively studied, little is known about trace gas fluxes in seagrass meadows. Here we report results of a newly developed dynamic flux chamber system that can be deployed in intertidal areas over full tidal cycles allowing for highly time-resolved measurements. The fluxes of CO2, methane (CH4) and a range of volatile organic compounds (VOCs) showed a complex dynamic mediated by tide and light. In contrast to most previous studies, our data indicate significantly enhanced fluxes during tidal immersion relative to periods of air exposure. Short emission peaks occurred with onset of the feeder current at the sampling site. We suggest an overall strong effect of advective transport processes to explain the elevated fluxes during tidal immersion. Many emission estimates from tidally influenced coastal areas still rely on measurements carried out during low tide only. Hence, our results may have significant implications for budgeting trace gases in coastal areas. This dynamic flux chamber system provides intensive time series data of community respiration (at night) and net community production (during the day) of shallow coastal systems.German Federal Ministry of Education and Research (BMBF) [03F0611E, 03F0662E]; EU FP7 ASSEMBLE research infrastructure initiative
Reconstructing the global topology of the universe from the cosmic microwave background
If the universe is multiply-connected and sufficiently small, then the last
scattering surface wraps around the universe and intersects itself. Each circle
of intersection appears as two distinct circles on the microwave sky. The
present article shows how to use the matched circles to explicitly reconstruct
the global topology of space.Comment: 6 pages, 2 figures, IOP format. To be published in the proceedings of
the Cleveland Cosmology and Topology Workshop 17-19 Oct 1997. Submitted to
Class. Quant. Gra
Mobility and Diffusion of a Tagged Particle in a Driven Colloidal Suspension
We study numerically the influence of density and strain rate on the
diffusion and mobility of a single tagged particle in a sheared colloidal
suspension. We determine independently the time-dependent velocity
autocorrelation functions and, through a novel method, the response functions
with respect to a small force. While both the diffusion coefficient and the
mobility depend on the strain rate the latter exhibits a rather weak
dependency. Somewhat surprisingly, we find that the initial decay of response
and correlation functions coincide, allowing for an interpretation in terms of
an 'effective temperature'. Such a phenomenological effective temperature
recovers the Einstein relation in nonequilibrium. We show that our data is well
described by two expansions to lowest order in the strain rate.Comment: submitted to EP
Dynamics of non-equilibrium membrane bud formation
The dynamical response of a lipid membrane to a local perturbation of its
molecular symmetry is investigated theoretically. A density asymmetry between
the two membrane leaflets is predominantly released by in-plane lipid diffusion
or membrane curvature, depending upon the spatial extent of the perturbation.
It may result in the formation of non-equilibrium structures (buds), for which
a dynamical size selection is observed. A preferred size in the micrometer
range is predicted, as a signature of the crossover between membrane and
solvent dominated dynamical membrane response.Comment: 7 pages 3 figure
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