301 research outputs found
Plasmas in Saturn's magnetosphere
The solar wind plasma analyzer on board Pioneer 2 provides first observations of low-energy positive ions in the magnetosphere of Saturn. Measurable intensities of ions within the energy-per-unit charge (E/Q) range 100 eV to 8 keV are present over the planetocentric radial distance range about 4 to 16 R sub S in the dayside magnetosphere. The plasmas are found to be rigidly corotating with the planet out to distances of at least 10 R sub S. At radial distances beyond 10 R sub S, the bulk flows appear to be in the corotation direction but with lesser speeds than those expected from rigid corotation. At radial distances beyond the orbit of Rhea at 8.8 R sub S, the dominant ions are most likely protons and the corresponding typical densities and temperatures are 0.5/cu cm and 1,000,000 K, respectively, with substantial fluctuations. It is concluded that the most likely source of these plasmas in the photodissociation of water frost on the surface of the ring material with subsequent ionization of the products and radially outward diffusion. The presence of this plasma torus is expected to have a large influence on the dynamics of Saturn's magnetosphere since the pressure ratio beta of these plasmas approaches unity at radial distances as close to the planet as 6.5 R sub S. On the basis of these observational evidences it is anticipated that quasi-periodic outward flows of plasma, accompanied with a reconfiguration of the magnetosphere beyond about 6.5 R sub S, will occur in the local night sector in order to relieve the plasma pressure from accretion of plasma from the rings
Cluster-mining: An approach for determining core structures of metallic nanoparticles from atomic pair distribution function data
We present a novel approach for finding and evaluating structural models of
small metallic nanoparticles. Rather than fitting a single model with many
degrees of freedom, the approach algorithmically builds libraries of
nanoparticle clusters from multiple structural motifs, and individually fits
them to experimental PDFs. Each cluster-fit is highly constrained. The
approach, called cluster-mining, returns all candidate structure models that
are consistent with the data as measured by a goodness of fit. It is highly
automated, easy to use, and yields models that are more physically realistic
and result in better agreement to the data than models based on cubic
close-packed crystallographic cores, often reported in the literature for
metallic nanoparticles
Fluid Induced Particle Size Segregation in Sheared Granular Assemblies
We perform a two-dimensional molecular-dynamics study of a model for sheared
bidisperse granular systems under conditions of simple shear and Poiseuille
flow. We propose a mechanism for particle-size segregation based on the
observation that segregation occurs if the viscous length scale introduced by a
liquid in the system is smaller than of the order of the particle size. We show
that the ratio of shear rate to viscosity must be small if one wants to find
size segregation. In this case the particles in the system arrange themselves
in bands of big and small particles oriented along the direction of the flow.
Similarly, in Poiseuille flow we find the formation of particle bands. Here, in
addition, the variety of time scales in the flow leads to an aggregation of
particles in the zones of low shear rate and can suppress size segregation in
these regions. The results have been verified against simulations using a full
Navier-Stokes description for the liquid.Comment: 11 pages, REVTEX format, ps figures compressed uuencoded separately
or by e-mail from [email protected]. A postscript version of the
paper will be available from
http://www.ica1.uni-stuttgart.de/local/WWW/papers/papers.htm
An optical fiber based interferometer to measure velocity profiles in sheared complex fluids
We describe an optical fiber based interferometer to measure velocity
profiles in sheared complex fluids using Dynamic Light Scattering (DLS). After
a review of the theoretical problem of DLS under shear, a detailed description
of the setup is given. We outline the various experimental difficulties induced
by refraction when using a Couette cell. We also show that homodyne DLS is not
well suited to measure quantitative velocity profiles in narrow-gap Couette
geometries. On the other hand, the heterodyne technique allows us to determine
the velocity field inside the gap of a Couette cell. All the technical features
of the setup, namely its spatial resolution (--m) and its
temporal resolution ( s per point, min per profile) are
discussed, as well as the calibration procedure with a Newtonian fluid. As
briefly shown on oil-in-water emulsions, such a setup permits one to record
both velocity profiles and rheological data simultaneouslyComment: 13 pages, 16 figures, Submitted to Eur. Phys. J. A
Shear-banding in a lyotropic lamellar phase, Part 1: Time-averaged velocity profiles
Using velocity profile measurements based on dynamic light scattering and
coupled to structural and rheological measurements in a Couette cell, we
present evidences for a shear-banding scenario in the shear flow of the onion
texture of a lyotropic lamellar phase. Time-averaged measurements clearly show
the presence of structural shear-banding in the vicinity of a shear-induced
transition, associated to the nucleation and growth of a highly sheared band in
the flow. Our experiments also reveal the presence of slip at the walls of the
Couette cell. Using a simple mechanical approach, we demonstrate that our data
confirms the classical assumption of the shear-banding picture, in which the
interface between bands lies at a given stress . We also outline
the presence of large temporal fluctuations of the flow field, which are the
subject of the second part of this paper [Salmon {\it et al.}, submitted to
Phys. Rev. E]
Brownian Dynamics Simulation of Polydisperse Hard Spheres
Standard algorithms for the numerical integration of the Langevin equation
require that interactions are slowly varying during to the integration
timestep. This in not the case for hard-body systems, where there is no
clearcut between the correlation time of the noise and the timescale of the
interactions. Starting from a short time approximation of the Smoluchowsky
equation, we introduce an algorithm for the simulation of the overdamped
Brownian dynamics of polydisperse hard-spheres in absence of hydrodynamics
interactions and briefly discuss the extension to the case of external drifts
Shear-banding in a lyotropic lamellar phase, Part 2: Temporal fluctuations
We analyze the temporal fluctuations of the flow field associated to a
shear-induced transition in a lyotropic lamellar phase: the layering transition
of the onion texture. In the first part of this work [Salmon et al., submitted
to Phys. Rev. E], we have evidenced banded flows at the onset of this
shear-induced transition which are well accounted for by the classical picture
of shear-banding. In the present paper, we focus on the temporal fluctuations
of the flow field recorded in the coexistence domain. These striking dynamics
are very slow (100--1000s) and cannot be due to external mechanical noise.
Using velocimetry coupled to structural measurements, we show that these
fluctuations are due to a motion of the interface separating the two
differently sheared bands. Such a motion seems to be governed by the
fluctuations of , the local stress at the interface between the
two bands. Our results thus provide more evidence for the relevance of the
classical mechanical approach of shear-banding even if the mechanism leading to
the fluctuations of remains unclear
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