15,549 research outputs found
The crossover from single file to Fickian diffusion
The crossover from single-file diffusion, where the mean-square displacement
scales as ~t^(1/2), to normal Fickian diffusion, where ~tL$ such that (L- 2 \sigma)/\sigma = \delta_c << 1 the particles can be
described as hopping past one-another in an average time t_{hop}. For shorter
times t << t_{hop} the particles still exhibit sub-diffusive behaviour, but at
longer times t > t_{hop}, normal Fickian diffusion sets in with an effective
diffusion constant D_{hop} ~ t_{hop}^(1/2). For the Brownian particles, t_{hop}
~ 1/\delta_c^(2) when \delta << 1, but when hydrodynamic interactions are
included, we find a stronger dependence than \delta_c^{-2}. We attribute this
difference to short-range lubrication forces that make it more difficult for
particles to hop past each other in very narrow channels
The Effects of Inter-particle Attractions on Colloidal Sedimentation
We use a mesoscopic simulation technique to study the effect of short-ranged
inter-particle attraction on the steady-state sedimentation of colloidal
suspensions. Attractions increase the average sedimentation velocity
compared to the pure hard-sphere case, and for strong enough attractions, a
non-monotonic dependence on the packing fraction with a maximum velocity
at intermediate is observed. Attractions also strongly enhance
hydrodynamic velocity fluctuations, which show a pronounced maximum size as a
function of . These results are linked to a complex interplay between
hydrodynamics and the formation and break-up of transient many-particle
clusters.Comment: 4 pages 4 figure
Pathogen-host reorganization during Chlamydia invasion revealed by cryo-electron tomography
Invasion of host cells is a key early event during bacterial infection, but the underlying pathogen-host interactions are yet to be fully visualised in three-dimensional detail. We have captured snapshots of the early stages of bacterial-mediated endocytosis in situ by exploiting the small size of chlamydial elementary bodies (EBs) for whole cell cryo-electron tomography. Chlamydiae are obligate intracellular bacteria that infect eukaryotic cells and cause sexually transmitted infections and trachoma, the leading cause of preventable blindness. We demonstrate that Chlamydia trachomatis LGV2 EBs are intrinsically polarised. One pole is characterised by a tubular inner membrane invagination, while the other exhibits asymmetric periplasmic expansion to accommodate an array of type III secretion systems (T3SSs). Strikingly, EBs orient with their T3SS-containing pole facing target cells, enabling the T3SSs to directly contact the cellular plasma membrane. This contact induces enveloping macropinosomes, actin-rich filopodia and phagocytic cups to zipper tightly around the internalising bacteria. Once encapsulated into tight early vacuoles, EB polarity and the T3SSs are lost. Our findings reveal previously undescribed structural transitions in both pathogen and host during the initial steps of chlamydial invasion
Minimal Universal Extra Dimensions
Highly degenerate spectra associated with universal extra dimensions (UED)
provide an interesting phenomenology not only from the point of view of
cosmology and astrophysics, but also for colliders. We study these exotic
signals for the simplest case, called minimal UED, where it is natural to find
slow charged particles, displaced vertices, tracks with non-vanishing impact
parameters, track kinks, and even vanishing charged tracks.Comment: 6 pages, 3 figures. Contributed to XXIII International Symposium on
Lepton and Photon Interactions at High Energy, Aug 13-18, 2007, Daegu, Kore
Hydrodynamic and Brownian Fluctuations in Sedimenting Suspensions
We use a mesoscopic computer simulation method to study the interplay between
hydrodynamic and Brownian fluctuations during steady-state sedimentation of
hard sphere particles for Peclet numbers (Pe) ranging from 0.1 to 15. Even when
the hydrodynamic interactions are an order of magnitude weaker than Brownian
forces, they still induce backflow effects that dominate the reduction of the
average sedimentation velocity with increasing particle packing fraction.
Velocity fluctuations, on the other hand, begin to show nonequilibrium
hydrodynamic character for Pe > 1Comment: 4 pages 4 figures, RevTex, to appear in Phys. Rev. Lett. New version
with some minor correction
Victims and Survivors: Displaced Persons and Other War Victims in Vietnam, 1954-1975
https://digitalcommons.cedarville.edu/cr_launch_party_2016/1002/thumbnail.jp
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