7,172 research outputs found
On the macroion virial contribution to the osmotic pressure in charge-stabilized colloidal suspensions
Our interest goes to the different virial contributions to the equation of
state of charged colloidal suspensions. Neglect of surface effects in the
computation of the colloidal virial term leads to spurious and paradoxical
results. This pitfall is one of the several facets of the danger of a naive
implementation of the so called One Component Model, where the micro-ionic
degrees of freedom are integrated out to only keep in the description the
mesoscopic (colloidal) degrees of freedom. On the other hand, due incorporation
of wall induced forces dissolves the paradox brought forth in the naive
approach, provides a consistent description, and confirms that for salt-free
systems, the colloidal contribution to the pressure is dominated by the
micro-ionic one. Much emphasis is put on the no salt case but the situation
with added electrolyte is also discussed
Colloidal hard-rod fluids near geometrically structured substrates
Density functional theory is used to study colloidal hard-rod fluids near an
individual right-angled wedge or edge as well as near a hard wall which is
periodically patterned with rectangular barriers. The Zwanzig model, in which
the orientations of the rods are restricted to three orthogonal orientations
but their positions can vary continuously, is analyzed by numerical
minimization of the grand potential. Density and orientational order profiles,
excess adsorptions, as well as surface and line tensions are determined. The
calculations exhibit an enrichment [depletion] of rods lying parallel and close
to the corner of the wedge [edge]. For the fluid near the geometrically
patterned wall, complete wetting of the wall -- isotropic liquid interface by a
nematic film occurs as a two-stage process in which first the nematic phase
fills the space between the barriers until an almost planar isotropic --
nematic liquid interface has formed separating the higher-density nematic fluid
in the space between the barriers from the lower-density isotropic bulk fluid.
In the second stage a nematic film of diverging film thickness develops upon
approaching bulk isotropic -- nematic coexistence.Comment: 9 pages, 9 figure
How Leaders Invest Staffing Resources for Learning Improvement
Analyzes staffing challenges that guide school leaders' resource decisions in the context of a learning improvement agenda, staff resource investment strategies that improve learning outcomes equitably, and ways to win support for differential investment
TRADE-OFFS BETWEEN ECONOMIC EFFICIENCY AND CONTAMINATION BY COFFEE PROCESSING A BIOECONOMIC MODEL AT THE WATERSHED LEVEL IN HONDURAS
In Honduras, traditional coffee processing is the cause of two major problems: poor coffee quality and contaminated water. In this paper we present a method that determines the trade-off between economic efficiency and contamination in a Honduran sub-watershed. The method is a bioeconomic model based on mathematical programming that stimulates the functioning of the interlinked economic and ecological processes in the sub-watershed. We compare various scenarii where the model is given the possibility of replacing traditional coffee processing plants with a network of improved ecological plants. For different levels of contamination the model determines the optimal location and size of new coffee processing plants along river streams by minimizing transport, variable and fixed costs. The restrictions of the system are the volume of wet coffee to be processed, the available stream water, and in the alternative scenarii, investment capital and contaminant concentration in the river. We apply the method to a typical sub-watershed in the hillsides of western Honduras and show that coffee quality can be improved and contamination can be reduced substantially at a relatively low cost.coffee, environment, water quality, mathematical programming, transport cost, spatial analysis, watershed, Honduras., Environmental Economics and Policy,
Recent progress in the development of a solar neutron tracking device (SONTRAC)
We report the results of recent calibration data analysis of a prototype scintillating fiber tracking detector system designed to perform imaging, spectroscopy and particle identification on 20 to 250 MeV neutrons and protons. We present the neutron imaging concept and briefly review the detection principle and the prototype description. The prototype detector system records ionization track data on an event-by-event basis allowing event selection criteria to be used in the off-line analysis. Images of acrylic phantoms from the analysis of recent proton beam calibrations are presented as demonstrations of the particle identification, imaging and energy measurement capabilities. The measured position resolution is \u3c 500 micrometers . The measured energy resolution is 14.2 percent at 35 MeV. The detection techniques employed can be applied to measurements in a variety of disciplines including solar and atmospheric physics, radiation therapy and nuclear materials monitoring. These applications are discussed briefly as are alternative detector configurations and future development plans
Colloids in light fields: particle dynamics in random and periodic energy landscapes
The dynamics of colloidal particles in potential energy landscapes have
mainly been investigated theoretically. In contrast, here we discuss the
experimental realization of potential energy landscapes with the help of light
fields and the observation of the particle dynamics by video microscopy. The
experimentally observed dynamics in periodic and random potentials are compared
to simulation and theoretical results in terms of, e.g. the mean-squared
displacement, the time-dependent diffusion coefficient or the non-Gaussian
parameter. The dynamics are initially diffusive followed by intermediate
subdiffusive behaviour which again becomes diffusive at long times. How
pronounced and extended the different regimes are, depends on the specific
conditions, in particular the shape of the potential as well as its roughness
or amplitude but also the particle concentration. Here we focus on dilute
systems, but the dynamics of interacting systems in external potentials, and
thus the interplay between particle-particle and particle-potential
interactions, is also mentioned briefly. Furthermore, the observed dynamics of
dilute systems resemble the dynamics of concentrated systems close to their
glass transition, with which it is compared. The effect of certain potential
energy landscapes on the dynamics of individual particles appears similar to
the effect of interparticle interactions in the absence of an external
potential
Teaching & Learning The Spanish Aspect Using Blogs and Wikis: An Exploratory Study
This study investigated the influence of asynchronous computer text based technologies on the students’ performance when learning the preterite and the imperfect aspects in Spanish. Two research questions guided the study: Research Question 1) Is there a difference in students’ achievement levels in Spanish preterite and imperfect between those using wiki technologies and those using blog technologies after controlling for pre-intervention achievement levels? and Research Question 2) Are there differences in satisfaction levels for students learning Spanish preterite and imperfect via blog technologies as compared to those learning via wiki technologies? Results indicate that there were not significant differences between students who use blog or wiki technologies on performance levels when controlling for pre-existing knowledge. Results also indicated that there were not significant differences in satisfaction levels between those students using a wiki and those using a blog. These results suggest that wikis and blogs are good potential tools that may facilitate the teaching and learning of problematic grammar structures in a narrative context
A scintillating plastic fiber tracking detector for neutron and proton imaging and spectroscopy
We report the results of recent calibration data analysis of a prototype scintillating fiber tracking detector system designed to perform imaging, spectroscopy and particle identification on 20 to 250 MeV neutrons and protons. We present the neutron imaging concept and briefly review the detection principle and the prototype description. The prototype detector system records ionization track data on an event-by-event basis allowing event selection criteria to be used in the off-line analysis. Images of acrylic phantoms from the analysis of recent proton beam calibrations (14 to 65 MeV range) are presented as demonstrations of the particle identification, imaging and energy measurement capabilities. The measured position resolution is c 500 pm. The measured energy resolution (AE/E, FWHM) is 14.2% at 35 MeV. An effective technique for track identification and data compression is presented. The detection techniques employed can be applied to measurements in a variety of disciplines including solar and atmospheric physics, radiation therapy and nuclear materials monitoring. These applications are discussed briefly as are alternative detector configurations and future development plans
Feedback in the Antennae Galaxies (NGC 4038/9): I. High-Resolution Infrared Spectroscopy of Winds from Super Star Clusters
We present high-resolution (R ~ 24,600) near-IR spectroscopy of the youngest
super star clusters (SSCs) in the prototypical starburst merger, the Antennae
Galaxies. These SSCs are young (3-7 Myr old) and massive (10^5 - 10^7 M_sun for
a Kroupa IMF) and their spectra are characterized by broad, extended Br-gamma
emission, so we refer to them as emission-line clusters (ELCs) to distinguish
them from older SSCs. The Brgamma lines of most ELCs have supersonic widths
(60-110 km/s FWHM) and non-Gaussian wings whose velocities exceed the clusters'
escape velocities. This high-velocity unbound gas is flowing out in winds that
are powered by the clusters' massive O and W-R stars over the course of at
least several crossing times. The large sizes of some ELCs relative to those of
older SSCs may be due to expansion caused by these outflows; many of the ELCs
may not survive as bound stellar systems, but rather dissipate rapidly into the
field population. The observed tendency of older ELCs to be more compact than
young ones is consistent with the preferential survival of the most
concentrated clusters at a given age.Comment: Accepted to Ap
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