9,946 research outputs found
Clustering and gelation of hard spheres induced by the Pickering effect
A mixture of hard-sphere particles and model emulsion droplets is studied
with a Brownian dynamics simulation. We find that the addition of nonwetting
emulsion droplets to a suspension of pure hard spheres can lead to both
gas-liquid and fluid-solid phase separations. Furthermore, we find a stable
fluid of hard-sphere clusters. The stability is due to the saturation of the
attraction that occurs when the surface of the droplets is completely covered
with colloidal particles. At larger emulsion droplet densities a percolation
transition is observed. The resulting networks of colloidal particles show
dynamical and mechanical properties typical of a colloidal gel. The results of
the model are in good qualitative agreement with recent experimental findings
[E. Koos and N. Willenbacher, Science 331, 897 (2011)] in a mixture of
colloidal particles and two immiscible fluids.Comment: 5 figures, 5 page
Kinetic models of ion transport through a nanopore
Kinetic equations for the stationary state distribution function of ions
moving through narrow pores are solved for a number of one-dimensional models
of single ion transport. Ions move through pores of length , under the
action of a constant external field and of a concentration gradient. The
interaction of single ions with the confining pore surface and with water
molecules inside the pore are modelled by a Fokker-Planck term in the kinetic
equation, or by uncorrelated collisions with thermalizing centres distributed
along the pore. The temporary binding of ions to polar residues lining the pore
is modelled by stopping traps or energy barriers. Analytic expressions for the
stationary ion current through the pore are derived for several versions of the
model, as functions of key physical parameters. In all cases, saturation of the
current at high fields is predicted. Such simple models, for which results are
analytic, may prove useful in the study of the current/voltage relations of ion
channels through membranes
Photoacoustic tomography setup using LED illumination
Photoacoustic tomography (PAT) is a hybrid imaging modality that combines optical contrast with ultrasound resolution. Most of the PAT configurations are based on high-energy solid-state lasers such as Nd:YAG laser. In this work, a PAT system that uses light-emitting diode (LED) as a light source is introduced. The system is designed so that the imaged target can be stationary. The target is illuminated by a LED light source from one side and the pressure wave is measured using an acoustic transducer that is rotated around the target. Image reconstruction is based on Bayesian approach to illposed inverse problems. The system was tested with light absorbing targets also in limited-view and sparse angle measurement situations. The results show that LED-based instrumentation and advanced reconstruction methods can form a potential PAT system that can also be applied in limited-view and sparse angle photoacoustic tomography
Measuring the dark matter velocity anisotropy in galaxy clusters
The Universe contains approximately 6 times more dark matter than normal
baryonic matter, and a directly observed fundamental difference between dark
matter and baryons would both be significant for our understanding of dark
matter structures and provide us with information about the basic
characteristics of the dark matter particle. We discuss one distinctive feature
of dark matter structures in equilibrium, namely the property that a local dark
matter temperature may depend on direction. This is in stark contrast to
baryonic gases. We used X-ray observations of two nearby, relaxed galaxy
clusters, under the assumptions of hydrostatic equilibrium and identical dark
matter and gas temperatures in the outer cluster region, to measure this dark
matter temperature anisotropy beta_dm, with non-parametric Monte Carlo methods.
We find that beta_dm is greater than the value predicted for baryonic gases,
beta_gas=0, at more than 3 sigma confidence. The observed value of the
temperature anisotropy is in fair agreement with the results of cosmological
N-body simulations and shows that the equilibration of the dark matter
particles is not governed by local point-like interactions in contrast to
baryonic gases.Comment: 5 pages, 3 figures, extended discussions, matches accepted versio
Three-body interactions in complex fluids: virial coefficients from simulation finite-size effects
A simulation technique is described for quantifying the contribution of
three-body interactions to the thermodynamical properties of coarse-grained
representations of complex fluids. The method is based on comparing the third
virial coefficient for a complex fluid with that of an approximate
coarse-grained model described by a pair potential. To obtain we
introduce a new technique which expresses its value in terms of the measured
volume-dependent asymptote of a certain structural function. The strategy is
applicable to both Molecular Dynamics and Monte Carlo simulation. Its utility
is illustrated via measurements of three-body effects in models of star polymer
and highly size-asymmetrical colloid-polymer mixtures.Comment: 13 pages, 8 figure
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Electric Scooter Injuries and Hospital Admissions in the United States, 2014-2018.
This study investigates trends of injury and hospital admission associated with electric scooter use
Mean properties and Free Energy of a few hard spheres confined in a spherical cavity
We use analytical calculations and event-driven molecular dynamics
simulations to study a small number of hard sphere particles in a spherical
cavity. The cavity is taken also as the thermal bath so that the system
thermalizes by collisions with the wall. In that way, these systems of two,
three and four particles, are considered in the canonical ensemble. We
characterize various mean and thermal properties for a wide range of number
densities. We study the density profiles, the components of the local pressure
tensor, the interface tension, and the adsorption at the wall. This spans from
the ideal gas limit at low densities to the high-packing limit in which there
are significant regions of the cavity for which the particles have no access,
due the conjunction of excluded volume and confinement. The contact density and
the pressure on the wall are obtained by simulations and compared to exact
analytical results. We also obtain the excess free energy for N=4, by using a
simulated-assisted approach in which we combine simulation results with the
knowledge of the exact partition function for two and three particles in a
spherical cavity.Comment: 11 pages, 9 figures and two table
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