1,873 research outputs found
Like-charge attraction through hydrodynamic interaction
We demonstrate that the attractive interaction measured between like-charged
colloidal spheres near a wall can be accounted for by a nonequilibrium
hydrodynamic effect. We present both analytical results and Brownian dynamics
simulations which quantitatively capture the one-wall experiments of Larsen and
Grier (Nature 385, p. 230, 1997).Comment: 10 pages, 4 figure
Strong Attraction between Charged Spheres due to Metastable Ionized States
We report a mechanism which can lead to long range attractions between
like-charged spherical macroions, stemming from the existence of metastable
ionized states. We show that the ground state of a single highly charged
colloid plus a few excess counterions is overcharged. For the case of two
highly charged macroions in their neutralizing divalent counterion solution we
demonstrate that, in the regime of strong Coulomb coupling, the counterion
clouds are very likely to be unevenly distributed, leading to one overcharged
and one undercharged macroion. This long-living metastable configuration in
turn leads to a long range Coulomb attraction.Comment: REVTEX-published versio
Fenestrated capillaries as 'tunnel capillaries': information about blood-tissue-lymph relationships.
Comparing [CII], HI, and CO dynamics of nearby galaxies
The HI and CO components of the interstellar medium (ISM) are usually used to
derive the dynamical mass M_dyn of nearby galaxies. Both components become too
faint to be used as a tracer in observations of high-redshift galaxies. In
those cases, the 158 m line of atomic carbon [CII] may be the only way to
derive M_dyn. As the distribution and kinematics of the ISM tracer affects the
determination of M_dyn, it is important to quantify the relative distributions
of HI, CO and [CII]. HI and CO are well-characterised observationally, however,
for [CII] only very few measurements exist. Here we compare observations of CO,
HI, and [CII] emission of a sample of nearby galaxies, drawn from the HERACLES,
THINGS and KINGFISH surveys. We find that within R_25, the average [CII]
exponential radial profile is slightly shallower than that of the CO, but much
steeper than the HI distribution. This is also reflected in the integrated
spectrum ("global profile"), where the [CII] spectrum looks more like that of
the CO than that of the HI. For one galaxy, a spectrally resolved comparison of
integrated spectra was possible; other comparisons were limited by the
intrinsic line-widths of the galaxies and the coarse velocity resolution of the
[CII] data. Using high-spectral-resolution SOFIA [CII] data of a number of star
forming regions in two nearby galaxies, we find that their [CII] linewidths
agree better with those of the CO than the HI. As the radial extent of a given
ISM tracer is a key input in deriving M_dyn from spatially unresolved data, we
conclude that the relevant length-scale to use in determining M_dyn based on
[CII] data, is that of the well-characterised CO distribution. This length
scale is similar to that of the optical disk.Comment: Accepted for publication in the Astronomical Journa
Impact Evaluation of Training Natural Leaders during a Community-Led Total Sanitation Intervention: A Cluster-Randomized Field Trial in Ghana
We used a cluster-randomized field trial to evaluate training natural leaders (NLs) as an addition to a community-led total sanitation (CLTS) intervention in Ghana. NLs are motivated community members who influence their peers’ behaviors during CLTS. The outcomes were latrine use and quality, which were assessed from surveys and direct observation. From October 2012, Plan International Ghana (Plan) implemented CLTS in 60 villages in three regions in Ghana. After 5 months, Plan trained eight NLs from a randomly selected half of the villages, then continued implementing CLTS in all villages for 12 more months. The NL training led to increased time spent on CLTS by community members, increased latrine construction, and a 19.9 percentage point reduction in open defecation (p < 0.001). The training had the largest impact in small, remote villages with low exposure to prior water and sanitation projects, and may be most effective in socially cohesive villages. For both interventions, latrines built during CLTS were less likely to be constructed of durable materials than pre-existing latrines, but were equally clean, and more often had handwashing materials. CLTS with NL training contributes to three parts of Goal 6 of the Sustainable Development Goals: eliminating open defecation, expanding capacity-building, and strengthening community participation
Faraday Tomography of the North Polar Spur: Constraints on the distance to the Spur and on the Magnetic Field of the Galaxy
We present radio continuum and polarization images of the North Polar Spur
(NPS) from the Global Magneto-Ionic Medium Survey (GMIMS) conducted with the
Dominion Radio Astrophysical Observatory 26-m Telescope. We fit polarization
angle versus wavelength squared over 2048 frequency channels from 1280 to 1750
MHz to obtain a Faraday Rotation Measure (RM) map of the NPS. Combining this RM
map with a published Faraday depth map of the entire Galaxy in this direction,
we derive the Faraday depth introduced by the NPS and the Galactic interstellar
medium (ISM) in front of and behind the NPS. The Faraday depth contributed by
the NPS is close to zero, indicating that the NPS is an emitting only feature.
The Faraday depth caused by the ISM in front of the NPS is consistent with zero
at b>50 degree, implying that this part of the NPS is local at a distance of
approximately several hundred parsecs. The Faraday depth contributed by the ISM
behind the NPS gradually increases with Galactic latitude up to b=44 degree,
and decreases at higher Galactic latitudes. This implies that either the part
of the NPS at b<44 degree is distant or the NPS is local but there is a sign
change of the large-scale magnetic field. If the NPS is local, there is then no
evidence for a large-scale anti-symmetry pattern in the Faraday depth of the
Milky Way. The Faraday depth introduced by the ISM behind the NPS at latitudes
b>50 degree can be explained by including a coherent vertical magnetic field.Comment: 9 pages, 8 figures, accepted for publication in ApJ. Some figures
have been degraded to reduce sizes, for a high resolution version, see
http://physics.usyd.edu.au/~xhsun/ms_nps.pd
A Detailed Study of the Radio--FIR Correlation in NGC6946 with Herschel-PACS/SPIRE from KINGFISH
We derive the distribution of the synchrotron spectral index across NGC6946
and investigate the correlation between the radio continuum (synchrotron) and
far-infrared (FIR) emission using the KINGFISH Herschel PACS and SPIRE data.
The radio--FIR correlation is studied as a function of star formation rate,
magnetic field strength, radiation field strength, and the total gas surface
brightness. The synchrotron emission follows both star-forming regions and the
so-called magnetic arms present in the inter-arm regions. The synchrotron
spectral index is steepest along the magnetic arms (), while
it is flat in places of giant H{\sc ii} regions and in the center of the galaxy
(). The map of provides an observational
evidence for aging and energy loss of cosmic ray electrons propagating in the
disk of the galaxy. Variations in the synchrotron--FIR correlation across the
galaxy are shown to be a function of both star formation and magnetic fields.
We find that the synchrotron emission correlates better with cold rather than
with warm dust emission, when the interstellar radiation field is the main
heating source of dust. The synchrotron--FIR correlation suggests a coupling
between the magnetic field and the gas density. NGC6946 shows a power-law
behavior between the total (turbulent) magnetic field strength B and the star
formation rate surface density with an index of
0.14\,(0.16)0.01. This indicates an efficient production of the turbulent
magnetic field with the increasing gas turbulence expected in actively star
forming regions. The scale-by-scale analysis of the synchrotron--FIR
correlation indicates that the ISM affects the propagation of old/diffused
cosmic ray electrons, resulting in a diffusion coefficient of \,cm\,s for 2.2\,GeV CREs.Comment: 23 pages, 13 figures, accepted for publication in Astronomy &
Astrophysics Journa
Casimir Forces between Spherical Particles in a Critical Fluid and Conformal Invariance
Mesoscopic particles immersed in a critical fluid experience long-range
Casimir forces due to critical fluctuations. Using field theoretical methods,
we investigate the Casimir interaction between two spherical particles and
between a single particle and a planar boundary of the fluid. We exploit the
conformal symmetry at the critical point to map both cases onto a highly
symmetric geometry where the fluid is bounded by two concentric spheres with
radii R_- and R_+. In this geometry the singular part of the free energy F only
depends upon the ratio R_-/R_+, and the stress tensor, which we use to
calculate F, has a particularly simple form. Different boundary conditions
(surface universality classes) are considered, which either break or preserve
the order-parameter symmetry. We also consider profiles of thermodynamic
densities in the presence of two spheres. Explicit results are presented for an
ordinary critical point to leading order in epsilon=4-d and, in the case of
preserved symmetry, for the Gaussian model in arbitrary spatial dimension d.
Fundamental short-distance properties, such as profile behavior near a surface
or the behavior if a sphere has a `small' radius, are discussed and verified.
The relevance for colloidal solutions is pointed out.Comment: 37 pages, 2 postscript figures, REVTEX 3.0, published in Phys. Rev. B
51, 13717 (1995
Phase behaviour of a model of colloidal particles with a fluctuating internal state
Colloidal particles are not simple rigid particles, in general an isolated
particle is a system with many degrees of freedom in its own right, e.g., the
counterions around a charged colloidal particle.The behaviour of model
colloidal particles, with a simple phenomenological model to account for these
degrees of freedom, is studied. It is found that the interaction between the
particles is not pairwise additive. It is even possible that the interaction
between a triplet of particles is attractive while the pair interaction is
repulsive. When this is so the liquid phase is either stable only in a small
region of the phase diagram or absent altogether.Comment: 12 pages including 4 figure
Hard Spheres in Vesicles: Curvature-Induced Forces and Particle-Induced Curvature
We explore the interplay of membrane curvature and nonspecific binding due to
excluded-volume effects among colloidal particles inside lipid bilayer
vesicles. We trapped submicron spheres of two different sizes inside a
pear-shaped, multilamellar vesicle and found the larger spheres to be pinned to
the vesicle's surface and pushed in the direction of increasing curvature. A
simple model predicts that hard spheres can induce shape changes in flexible
vesicles. The results demonstrate an important relationship between the shape
of a vesicle or pore and the arrangement of particles within it.Comment: LaTeX with epsfig; ps available at
http://dept.physics.upenn.edu/~nelson/index.shtml Phys Rev Lett in press
(1997
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