870 research outputs found
The construction, utilization, and evaluation of a series of exercises designed to improve the oral language of second grade children
Thesis (Ed.M.)--Boston Universit
Nonlinear Dynamics of Capacitive Charging and Desalination by Porous Electrodes
The rapid and efficient exchange of ions between porous electrodes and
aqueous solutions is important in many applications, such as electrical energy
storage by super-capacitors, water desalination and purification by capacitive
deionization (or desalination), and capacitive extraction of renewable energy
from a salinity difference. Here, we present a unified mean-field theory for
capacitive charging and desalination by ideally polarizable porous electrodes
(without Faradaic reactions or specific adsorption of ions) in the limit of
thin double layers (compared to typical pore dimensions). We illustrate the
theory in the case of a dilute, symmetric, binary electrolyte using the
Gouy-Chapman-Stern (GCS) model of the double layer, for which simple formulae
are available for salt adsorption and capacitive charging of the diffuse part
of the double layer. We solve the full GCS mean-field theory numerically for
realistic parameters in capacitive deionization, and we derive reduced models
for two limiting regimes with different time scales: (i) In the
"super-capacitor regime" of small voltages and/or early times where the porous
electrode acts like a transmission line, governed by a linear diffusion
equation for the electrostatic potential, scaled to the RC time of a single
pore. (ii) In the "desalination regime" of large voltages and long times, the
porous electrode slowly adsorbs neutral salt, governed by coupled, nonlinear
diffusion equations for the pore-averaged potential and salt concentration
Evaluation Research and Institutional Pressures: Challenges in Public-Nonprofit Contracting
This article examines the connection between program evaluation research and decision-making by public managers. Drawing on neo-institutional theory, a framework is presented for diagnosing the pressures and conditions that lead alternatively toward or away the rational use of evaluation research. Three cases of public-nonprofit contracting for the delivery of major programs are presented to clarify the way coercive, mimetic, and normative pressures interfere with a sound connection being made between research and implementation. The article concludes by considering how public managers can respond to the isomorphic pressures in their environment that make it hard to act on data relating to program performance.This publication is Hauser Center Working Paper No. 23. The Hauser Center Working Paper Series was launched during the summer of 2000. The Series enables the Hauser Center to share with a broad audience important works-in-progress written by Hauser Center scholars and researchers
The Social and Political Dimensions of the Ebola Response: Global Inequality, Climate Change, and Infectious Disease
The 2014 Ebola crisis has highlighted public-health vulnerabilities in Liberia, Sierra
Leone, and Guinea – countries ravaged by extreme poverty, deforestation and
mining-related disruption of livelihoods and ecosystems, and bloody civil wars in
the cases of Liberia and Sierra Leone. Ebola’s emergence and impact are grounded
in the legacy of colonialism and its creation of enduring inequalities within African
nations and globally, via neoliberalism and the Washington Consensus. Recent
experiences with new and emerging diseases such as SARS and various strains of
HN influenzas have demonstrated the effectiveness of a coordinated local and
global public health and education-oriented response to contain epidemics. To what
extent is international assistance to fight Ebola strengthening local public health and
medical capacity in a sustainable way, so that other emerging disease threats, which
are accelerating with climate change, may be met successfully? This chapter
considers the wide-ranging socio-political, medical, legal and environmental factors
that have contributed to the rapid spread of Ebola, with particular emphasis on the
politics of the global and public health response and the role of gender, social
inequality, colonialism and racism as they relate to the mobilization and
establishment of the public health infrastructure required to combat Ebola and other
emerging diseases in times of climate change
Exploratory investigation of drivers of attainment in ethnic minority adult learners
There is evidence that ethnic minority learners in further education in England either under-achieve or are under-represented because they face various inhibitors connected to their ethnicity. Motivators may be in place, however, which increase attainment specifically for some ethnic groups. This exploratory study intends to examine what works and what does not among South Asian (Pakistani and Bangladeshi heritage) females and black male adult learners in FE. A mixed-method study was carried out using questionnaires and focus groups with 68 ethnic minority students in three further education colleges in England. The combination of the results showed that being a member of a minority culture and/or religion may increase feelings of isolation in academic settings; teaching staff who are knowledgeable about the student’s culture increase feelings of inclusion; and role models are crucially important. Results are discussed in light of British data of school experiences of ethnic minority learners
Diffuse-Charge Dynamics in Electrochemical Systems
The response of a model micro-electrochemical system to a time-dependent
applied voltage is analyzed. The article begins with a fresh historical review
including electrochemistry, colloidal science, and microfluidics. The model
problem consists of a symmetric binary electrolyte between parallel-plate,
blocking electrodes which suddenly apply a voltage. Compact Stern layers on the
electrodes are also taken into account. The Nernst-Planck-Poisson equations are
first linearized and solved by Laplace transforms for small voltages, and
numerical solutions are obtained for large voltages. The ``weakly nonlinear''
limit of thin double layers is then analyzed by matched asymptotic expansions
in the small parameter , where is the
screening length and the electrode separation. At leading order, the system
initially behaves like an RC circuit with a response time of
(not ), where is the ionic diffusivity, but nonlinearity
violates this common picture and introduce multiple time scales. The charging
process slows down, and neutral-salt adsorption by the diffuse part of the
double layer couples to bulk diffusion at the time scale, . In the
``strongly nonlinear'' regime (controlled by a dimensionless parameter
resembling the Dukhin number), this effect produces bulk concentration
gradients, and, at very large voltages, transient space charge. The article
concludes with an overview of more general situations involving surface
conduction, multi-component electrolytes, and Faradaic processes.Comment: 10 figs, 26 pages (double-column), 141 reference
Reconstruction of Cell Lineage Trees in Mice
The cell lineage tree of a multicellular organism represents its history of cell divisions from the very first cell, the zygote. A new method for high-resolution reconstruction of parts of such cell lineage trees was recently developed based on phylogenetic analysis of somatic mutations accumulated during normal development of an organism. In this study we apply this method in mice to reconstruct the lineage trees of distinct cell types. We address for the first time basic questions in developmental biology of higher organisms, namely what is the correlation between the lineage relation among cells and their (1) function, (2) physical proximity and (3) anatomical proximity. We analyzed B-cells, kidney-, mesenchymal- and hematopoietic-stem cells, as well as satellite cells, which are adult skeletal muscle stem cells isolated from their niche on the muscle fibers (myofibers) from various skeletal muscles. Our results demonstrate that all analyzed cell types are intermingled in the lineage tree, indicating that none of these cell types are single exclusive clones. We also show a significant correlation between the physical proximity of satellite cells within muscles and their lineage. Furthermore, we show that satellite cells obtained from a single myofiber are significantly clustered in the lineage tree, reflecting their common developmental origin. Lineage analysis based on somatic mutations enables performing high resolution reconstruction of lineage trees in mice and humans, which can provide fundamental insights to many aspects of their development and tissue maintenance
Diffuse charge and Faradaic reactions in porous electrodes
Porous electrodes instead of flat electrodes are widely used in electrochemical systems to boost storage
capacities for ions and electrons, to improve the transport of mass and charge, and to enhance reaction rates.
Existing porous electrode theories make a number of simplifying assumptions: (i) The charge-transfer rate is
assumed to depend only on the local electrostatic potential difference between the electrode matrix and the pore
solution, without considering the structure of the double layer (DL) formed in between; (ii) the charge-transfer
rate is generally equated with the salt-transfer rate not only at the nanoscale of the matrix-pore interface, but also
at the macroscopic scale of transport through the electrode pores. In this paper, we extend porous electrode theory
by including the generalized Frumkin-Butler-Volmer model of Faradaic reaction kinetics, which postulates charge
transfer across the molecular Stern layer located in between the electron-conducting matrix phase and the plane
of closest approach for the ions in the diffuse part of the DL. This is an elegant and purely local description of the
charge-transfer rate, which self-consistently determines the surface charge and does not require consideration of
reference electrodes or comparison with a global equilibrium. For the description of the DLs, we consider the
two natural limits: (i) the classical Gouy-Chapman-Stern model for thin DLs compared to the macroscopic pore
dimensions, e.g., for high-porosity metallic foams (macropores >50 nm) and (ii) a modified Donnan model for
strongly overlapping DLs, e.g., for porous activated carbon particles (micropores <2 nm). Our theory is valid
for electrolytes where both ions are mobile, and it accounts for voltage and concentration differences not only on
the macroscopic scale of the full electrode, but also on the local scale of the DL. The model is simple enough to
allow us to derive analytical approximations for the steady-state and early transients. We also present numerical
solutions to validate the analysis and to illustrate the evolution of ion densities, pore potential, surface charge,
and reaction rates in response to an applied voltage
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