1,215 research outputs found
First-order layering and critical wetting transitions in non-additive hard sphere mixtures
Using fundamental-measure density functional theory we investigate entropic
wetting in an asymmetric binary mixture of hard spheres with positive
non-additivity. We consider a general planar hard wall, where preferential
adsorption is induced by a difference in closest approach of the different
species and the wall. Close to bulk fluid-fluid coexistence the phase rich in
the minority component adsorbs either through a series of first-order layering
transitions, where an increasing number of liquid layers adsorbs sequentially,
or via a critical wetting transition, where a thick film grows continuously.Comment: 4 pages, 4 figure
Restoration of Lake Hakanoa: Results of model simulations
This report was requested by Waikato District Council. It covers the lake water quality of, and possible restoration scenarios for, Lake Hakanoa a riverine lake situated in Huntly. The lake is used as a recreational resource by the community. In the past it has been reported to have had very poor water quality and is known to be eutrophic. It is currently in an algal-dominated, devegetated state and has low water clarity. The shallowness of this lake makes it potentially susceptible to resuspension of sediments through wind action. A community group, Friends of Hakanoa, has been responsible for the formation of a path around the perimeter of the lake, retiring about 3.6% of the catchment from pastoral farming and creating a riparian margin. Results from more recent reports and this report indicate a trend of improving water quality which may be related to recent restoration actions such as re-establishment of a riparian margin
The van Hove distribution function for Brownian hard spheres: dynamical test particle theory and computer simulations for bulk dynamics
We describe a test particle approach based on dynamical density functional
theory (DDFT) for studying the correlated time evolution of the particles that
constitute a fluid. Our theory provides a means of calculating the van Hove
distribution function by treating its self and distinct parts as the two
components of a binary fluid mixture, with the `self' component having only one
particle, the `distinct' component consisting of all the other particles, and
using DDFT to calculate the time evolution of the density profiles for the two
components. We apply this approach to a bulk fluid of Brownian hard spheres and
compare to results for the van Hove function and the intermediate scattering
function from Brownian dynamics computer simulations. We find good agreement at
low and intermediate densities using the very simple Ramakrishnan-Yussouff
[Phys. Rev. B 19, 2775 (1979)] approximation for the excess free energy
functional. Since the DDFT is based on the equilibrium Helmholtz free energy
functional, we can probe a free energy landscape that underlies the dynamics.
Within the mean-field approximation we find that as the particle density
increases, this landscape develops a minimum, while an exact treatment of a
model confined situation shows that for an ergodic fluid this landscape should
be monotonic. We discuss possible implications for slow, glassy and arrested
dynamics at high densities.Comment: Submitted to Journal of Chemical Physic
Dynamics in inhomogeneous liquids and glasses via the test particle limit
We show that one may view the self and the distinct part of the van Hove
dynamic correlation function of a simple fluid as the one-body density
distributions of a binary mixture that evolve in time according to dynamical
density functional theory. For a test case of soft core Brownian particles the
theory yields results for the van Hove function that agree quantitatively with
those of our Brownian dynamics computer simulations. At sufficiently high
densities the free energy landscape underlying the dynamics exhibits a barrier
as a function of the mean particle displacement, shedding new light on the
nature of glass formation. For hard spheres confined between parallel planar
walls the barrier height oscillates in-phase with the local density, implying
that the mobility is maximal between layers, which should be experimentally
observable in confined colloidal dispersions.Comment: 4 pages, 3 figure
Solvent mediated interactions between model colloids and interfaces: A microscopic approach
We determine the solvent mediated contribution to the effective potentials
for model colloidal or nano- particles dispersed in a binary solvent that
exhibits fluid-fluid phase separation. Using a simple density functional theory
we calculate the density profiles of both solvent species in the presence of
the `colloids', which are treated as external potentials, and determine the
solvent mediated (SM) potentials. Specifically, we calculate SM potentials
between (i) two colloids, (ii) a colloid and a planar fluid-fluid interface,
and (iii) a colloid and a planar wall with an adsorbed wetting film. We
consider three different types of colloidal particles: colloid A which prefers
the bulk solvent phase rich in species 2, colloid C which prefers the solvent
phase rich in species 1, and `neutral' colloid B which has no strong preference
for either phase, i.e. the free energies to insert the colloid into either of
the coexisting bulk phases are almost equal. When a colloid which has a
preference for one of the two solvent phases is inserted into the disfavored
phase at statepoints close to coexistence a thick adsorbed `wetting' film of
the preferred phase may form around the colloids. The presence of the adsorbed
film has a profound influence on the form of the SM potentials.Comment: 17 Pages, 13 Figures. Accepted for publication in Journal of Chemical
Physic
The Origin and Universality of the Stellar Initial Mass Function
We review current theories for the origin of the Stellar Initial Mass
Function (IMF) with particular focus on the extent to which the IMF can be
considered universal across various environments. To place the issue in an
observational context, we summarize the techniques used to determine the IMF
for different stellar populations, the uncertainties affecting the results, and
the evidence for systematic departures from universality under extreme
circumstances. We next consider theories for the formation of prestellar cores
by turbulent fragmentation and the possible impact of various thermal,
hydrodynamic and magneto-hydrodynamic instabilities. We address the conversion
of prestellar cores into stars and evaluate the roles played by different
processes: competitive accretion, dynamical fragmentation, ejection and
starvation, filament fragmentation and filamentary accretion flows, disk
formation and fragmentation, critical scales imposed by thermodynamics, and
magnetic braking. We present explanations for the characteristic shapes of the
Present-Day Prestellar Core Mass Function and the IMF and consider what
significance can be attached to their apparent similarity. Substantial
computational advances have occurred in recent years, and we review the
numerical simulations that have been performed to predict the IMF directly and
discuss the influence of dynamics, time-dependent phenomena, and initial
conditions.Comment: 24 pages, 6 figures. Accepted for publication as a chapter in
Protostars and Planets VI, University of Arizona Press (2014), eds. H.
Beuther, R. S. Klessen, C. P. Dullemond, Th. Hennin
A Physical Model for the Origin of Quasar Lifetimes
We propose a model of quasar lifetimes in which observational quasar
lifetimes and an intrinsic lifetime of rapid accretion are strongly
distinguished by the physics of obscuration by surrounding gas and dust.
Quasars are powered by gas funneled to galaxy centers, but for a large part of
the accretion lifetime are heavily obscured by the large gas densities powering
accretion. In this phase, starbursts and black hole growth are fueled but the
quasar is buried. Eventually, feedback from accretion energy disperses
surrounding gas, creating a window in which the black hole is observable
optically as a quasar, until accretion rates drop below those required to
maintain a quasar luminosity. We model this process and measure the unobscured
and intrinsic quasar lifetimes in a hydrodynamical simulation of a major galaxy
merger. The source luminosity is determined from the black hole accretion rate,
calculated from local gas properties. We calculate the column density of
hydrogen to the source along multiple lines of sight and use these column
densities and gas metallicities to determine B-band attenuation of the source.
Defining the observable quasar lifetime as the total time with an observed
B-band luminosity above some limit L_B,min, we find lifetimes ~10-20 Myr for
L_B,min=10^11 L_sun (M_B=-23), in good agreement with observationally
determined quasar lifetimes. This is significantly smaller than the intrinsic
lifetime ~100 Myr obtained if attenuation is neglected. The ratio of observed
to intrinsic lifetime is also strong function of both the limiting luminosity
and the observed frequency.Comment: 5 pages, 4 figures, submitted to ApJ Letter
Design of ceramic-polymer optical composites for building energy efficiency: Infrared property control and transparent bulk thermal insulators
Of ~34 billion of energy waste annually. Materials design of windows, roofs and insulation is an opportunity for energy efficiency improvements, by optimizing solar absorption, transmission, infrared emission and thermal insulation. This presentation will discuss both static and dynamic/active approaches to improved energy efficiency in windows through materials design and performance improvements.
Please click Additional Files below to see the full abstract
Cluster Alignments and Ellipticities in LCDM Cosmology
The ellipticities and alignments of clusters of galaxies, and their evolution
with redshift, are examined in the context of a Lambda-dominated cold dark
matter cosmology. We use a large-scale, high-resolution N-body simulation to
model the matter distribution in a light cone containing ~10^6 clusters out to
redshifts of z=3. Cluster ellipticities are determined as a function of mass,
radius, and redshift, both in 3D and in projection. We find strong cluster
ellipticities: the mean ellipticity increases with redshift from 0.3 at z=0 to
0.5 at z=3, for both 3D and 2D ellipticities; the evolution is well-fit by
e=0.33+0.05z. The ellipticities increase with cluster mass and with cluster
radius; the main cluster body is more elliptical than the cluster cores, but
the increase of ellipticities with redshift is preserved. Using the fitted
cluster ellipsoids, we determine the alignment of clusters as a function of
their separation. We find strong alignment of clusters for separations <100
Mpc/h; the alignment increases with decreasing separation and with increasing
redshift. The evolution of clusters from highly aligned and elongated systems
at early times to lower alignment and elongation at present reflects the
hierarchical and filamentary nature of structure formation. These measures of
cluster ellipticity and alignment will provide a new test of the current
cosmological model when compared with upcoming cluster surveys.Comment: 29 pages including 13 figures, to appear in ApJ Jan. 2005 (corrected
typos, added reference
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