127,736 research outputs found
Electric field assisted nanocolloidal gold deposition
The deposition of nanocolloidal gold particles under the influence of an externally applied electric field is studied in situ by means of spectroscopic ellipsometry. The variation of the relative coverage with time, as a function of applied potential, is determined using a principal component analysis. Calibration of the absolute coverage is done by means of ex situ electron microscopy. The results reveal that the deposition rate is directly related to the electrochemical current. A threshold potential exists for current and therewith also deposition to occur. The spatial distribution of nanoparticles deposited in an applied field exhibits a higher degree of order as compared to the random, irreversibly deposited nanocolloids at chemically functionalized surfaces. The experimental findings are discussed in terms of a simple electrochemical model
R fluids
A theory of collisionless fluids is developed in a unified picture, where
nonrotating figures with anisotropic random velocity component distributions
and rotating figures with isotropic random velocity component distributions,
make adjoints configurations to the same system. R fluids are defined and mean
and rms angular velocities and mean and rms tangential velocity components are
expressed, by weighting on the moment of inertia and the mass, respectively.
The definition of figure rotation is extended to R fluids. The generalized
tensor virial equations are formulated for R fluids and further attention is
devoted to axisymmetric configurations where, for selected coordinate axes, a
variation in figure rotation has to be counterbalanced by a variation in
anisotropy excess and vice versa. A microscopical analysis of systematic and
random motions is performed under a few general hypotheses, by reversing the
sign of tangential or axial velocity components of an assigned fraction of
particles, leaving the distribution function and other parameters unchanged
(Meza 2002). The application of the reversion process to tangential velocity
components, implies the conversion of random motion rotation kinetic energy
into systematic motion rotation kinetic energy. The application of the
reversion process to axial velocity components, implies the conversion of
random motion translation kinetic energy into systematic motion translation
kinetic energy, and the loss related to a change of reference frame is
expressed in terms of systematic (imaginary) motion rotation kinetic energy. A
procedure is sketched for deriving the spin parameter distribution (including
imaginary rotation) from a sample of observed or simulated large-scale
collisionless fluids i.e. galaxies and galaxy clusters.Comment: 29 pages, 2 figure
Unsupervised machine learning for detection of phase transitions in off-lattice systems II. Applications
We outline how principal component analysis (PCA) can be applied to particle
configuration data to detect a variety of phase transitions in off-lattice
systems, both in and out of equilibrium. Specifically, we discuss its
application to study 1) the nonequilibrium random organization (RandOrg) model
that exhibits a phase transition from quiescent to steady-state behavior as a
function of density, 2) orientationally and positionally driven equilibrium
phase transitions for hard ellipses, and 3) compositionally driven demixing
transitions in the non-additive binary Widom-Rowlinson mixture
Halo stochasticity in global clustering analysis
In the present work we study the statistics of haloes, which in the halo
model determines the distribution of galaxies. Haloes are known to be biased
tracer of dark matter, and at large scales it is usually assumed there is no
intrinsic stochasticity between the two fields. Following the work of Seljak &
Warren (2004), we explore how correct this assumption is and, moving a step
further, we try to qualify the nature of stochasticity. We use Principal
Component Analysis applied to the outputs of a cosmological N-body simulation
to: (1) explore the behaviour of stochasticity in the correlation between
haloes of different masses; (2) explore the behaviour of stochasticity in the
correlation between haloes and dark matter. We show results obtained using a
catalogue with 2.1 million haloes, from a PMFAST simulation with box size of
1000h^{-1}Mpc. In the relation between different populations of haloes we find
that stochasticity is not-negligible even at large scales. In agreement with
the conclusions of Tegmark & Bromley (1999) who studied the correlations of
different galaxy populations, we found that the shot-noise subtracted
stochasticity is qualitatively different from `enhanced' shot noise and,
specifically, it is dominated by a single stochastic eigenvalue. We call this
the `minimally stochastic' scenario, as opposed to shot noise which is
`maximally stochastic'. In the correlation between haloes and dark matter, we
find that stochasticity is minimized, as expected, near the dark matter peak (k
~ 0.02 h Mpc^{-1} for a LambdaCDM cosmology) and, even at large scales, it is
of the order of 15 per cent above the shot noise. Moreover, we find that the
reconstruction of the dark matter distribution is improved when we use
eigenvectors as tracers of the bias. [Abridged]Comment: 9 pages, 12 figures. Submitted to MNRA
Correlations within Eigenvectors and Transition Amplitudes in the Two-Body Random Interaction Model
It is shown that the two-body character of the interaction in a many-body
system gives rise to specific correlations between the components of compound
states, even if this interaction is completely random. Surprisingly, these
correlations increase with the increase of the number of active (valence)
particles. Statistical theory of transition amplitudes between compound states,
which takes into account these correlation is developed and tested within the
framework of the Two-Body Random Interaction Model. It is demonstrated that a
feature, which can be called ``correlation resonance'', appears in the
distribution of the transition matrix amplitudes, since the correlations
strongly reduce the transition amplitudes at the tails and increase them near
the maximum of the distribution.Comment: 20 pages in Latex and 15 Postscript figures, to appear in Phys. Rev.
E (1996
Mapping aerial metal deposition in metropolitan areas from tree bark : a case study in Sheffield, England
We investigated the use of metals accumulated on tree bark for mapping their deposition across metropolitan Sheffield by sampling 642 trees of three common species. Mean concentrations of metals were generally an order of
magnitude greater than in samples from a remote uncontaminated site. We found trivially small differences among tree species with respect to metal concentrations on
bark, and in subsequent statistical analyses did not discriminate between them. We mapped the concentrations of As, Cd and Ni by lognormal universal kriging using parameters estimated by residual maximum likelihood ({\sc reml}). The concentrations of Ni and Cd were greatest close to a large steel works, their probable source, and declined markedly within 500~metres of it and from there more gradually over several kilometres. Arsenic was much more
evenly distributed, probably as a result of locally mined coal burned in domestic fires for many years. Tree bark seems to integrate airborne pollution over time, and our findings show that sampling and analysing it are cost-effective means of mapping and identifying sources
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