2,411 research outputs found
Active elastohydrodynamics of vesicles in narrow, blind constrictions
Fluid-resistance limited transport of vesicles through narrow constrictions
is a recurring theme in many biological and engineering applications. Inspired
by the motor-driven movement of soft membrane-bound vesicles into closed
neuronal dendritic spines, here we study this problem using a combination of
passive three-dimensional simulations and a simplified semi-analytical theory
for active transport of vesicles that are forced through such constrictions by
molecular motors. We show that the motion of these objects is characterized by
two dimensionless quantities related to the geometry and the strength of
forcing relative to the vesicle elasticity. We use numerical simulations to
characterize the transit time for a vesicle forced by fluid pressure through a
constriction in a channel, and find that relative to an open channel, transport
into a blind end leads to the formation of an effective lubrication layer that
strongly impedes motion. When the fluid pressure forcing is complemented by
forces due to molecular motors that are responsible for vesicle trafficking
into dendritic spines, we find that the competition between motor forcing and
fluid drag results in multistable dynamics reminiscent of the real system. Our
study highlights the role of non-local hydrodynamic effects in determining the
kinetics of vesicular transport in constricted geometries
OVI, NV and CIV in the Galactic Halo: II. Velocity-Resolved Observations with Hubble and FUSE
We present a survey of NV and OVI (and where available CIV) in the Galactic
halo, using data from the Far Ultraviolet Spectroscopic Explorer (FUSE) and the
Hubble Space Telescope (HST) along 34 sightlines. These ions are usually
produced in nonequilibrium processes such as shocks, evaporative interfaces, or
rapidly cooling gas, and thus trace the dynamics of the interstellar medium.
Searching for global trends in integrated and velocity-resolved column density
ratios, we find large variations in most measures, with some evidence for a
systematic trend of higher ionization (lower NV/OVI column density ratio) at
larger positive line-of-sight velocities. The slopes of log[N(NV)/N(OVI)] per
unit velocity range from -0.015 to +0.005, with a mean of
-0.0032+/-0.0022(r)+/-0.0014(sys) dex/(km/s). We compare this dataset with
models of velocity-resolved high-ion signatures of several common physical
structures. The dispersion of the ratios, OVI/NV/CIV, supports the growing
belief that no single model can account for hot halo gas, and in fact some
models predict much stronger trends than are observed. It is important to
understand the signatures of different physical structures to interpret
specific lines of sight and future global surveys.Comment: ApJ in press 43 pages, 22 fig
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