2,232 research outputs found
Asymptotics from scaling for nonlinear wave equations
We present a scaling technique which transforms the evolution problem for a
nonlinear wave equation with small initial data to a linear wave equation with
a distributional source. The exact solution of the latter uniformly
approximates the late-time behavior of solutions of the nonlinear problem in
timelike and null directions.Comment: 14 pages; minor changes (notation, typos
Equilibrium properties of highly asymmetric star-polymer mixtures
We employ effective interaction potentials to study the equilibrium structure
and phase behavior of highly asymmetric mixtures of star polymers. We consider
in particular the influence of the addition of a component with a small number
of arms and a small size on a concentrated solution of large stars with a high
functionality. By employing liquid integral equation theories we examine the
evolution of the correlation functions of the big stars upon addition of the
small ones, finding a loss of structure that can be attributed to a weakening
of the repulsions between the large stars due to the presence of the small
ones. We analyze this phenomenon be means of a generalized depletion mechanism
which is supported by computer simulations. By applying thermodynamic
perturbation theory we draw the phase diagram of the asymmetric mixture,
finding that the addition of small stars melts the crystal formed by the big
ones. A systematic comparison between the two- and effective one-component
descriptions of the mixture that corroborates the reliability of the
generalized depletion picture is also carried out.Comment: 26 pages, 9 figures, submitted to Phys. Rev.
Structure and thermodynamics of colloid-polymer mixtures: a macromolecular approach
The change of the structure of concentrated colloidal suspensions upon
addition of non-adsorbing polymer is studied within a two-component,
Ornstein-Zernicke based liquid state approach. The polymers' conformational
degrees of freedom are considered and excluded volume is enforced at the
segment level. The polymer correlation hole, depletion layer, and excess
chemical potentials are described in agreement with polymer physics theory in
contrast to models treating the macromolecules as effective spheres. Known
depletion attraction effects are recovered for low particle density, while at
higher densities novel many-body effects emerge which become dominant for large
polymers.Comment: 7 pages, 4 figures; to be published in Europhys. Let
Possible horizontal transfer of two subclasses of Mutator-like elements within Poaceae
Dissertação de mestrado em Gestão da Formação e Administração Educacional (Organizações Educativas e Gestão Escolar), apresentada à Faculdade de Psicologia e de Ciências da Educação da Universidade de CoimbraA presente dissertação é resultado de uma investigação realizada num Agrupamento de
Escolas na zona Centro que obteve uma melhoria entre o primeiro e o segundo ciclo de
avaliação externa de escolas. Centra-se na liderança praticada nas organizações escolares e
na relação entre a melhoria das escolas com o Programa de Avaliação Externa de Escolas,
com vista a compreender o papel das lideranças na melhoria de uma organização. A
avaliação externa de escolas tem-se tornado indispensável para a criação de mecanismos de
regulação que, de forma integrada com o processo de autoavaliação realizado pelas escolas
desenvolvem novas práticas e, consequentemente, implementam processos de melhoria,
visĂveis pelas tomadas de decisĂŁo dos lĂderes das escolas.
Com vista a verificar o que as lideranças do agrupamento têm feito para melhorar os
resultados dos alunos, a prestação do serviço educativo e a capacidade de autoavaliação e
melhoria desde a primeira avaliação externa, optámos pela metodologia de estudo de caso,
procurando conjugar técnicas de natureza quantitativa e qualitativa. Neste sentido,
recorremos à análise dos documentos estruturantes do Agrupamento de Escolas (Projeto
Educativo, Regulamento Interno, Plano Anual de Atividades, Plano de Melhoria,
Relatórios de Avaliação externa e Contraditório), à realização de entrevistas com o Diretor
e com a Coordenadora da Equipa de Autoavaliação e, por fim, à elaboração e
administração de um questionário para conhecer as perspetivas dos docentes sobre o
funcionamento e o desempenho do Agrupamento de Escolas, bem como sobre as
mudanças ocorridas na sequência da avaliação externa de escolas (AEE).
Este estudo salienta que o papel de uma liderança efetiva e reconhecida é parte
fundamental no desenvolvimento de uma organização e que a Avaliação Externa de
Escolas é sentida e vivenciada pela organização em estudo, e ao ser conjugada com os
dispositivos de reflexão interna e de autoavaliação, incentiva ao aperfeiçoamento dos
planos de melhoria, encaminhando o Agrupamento para a melhoria contĂnua da qualidade
do seu serviço educativo.The following dissertation is the result of an investigation in a School’s Division from
center area of Portugal that has improved consederably its results in the first and second
level of the External Evaluation of Schools. Its main focus is on the leadership of the
Schools Organizations and the relationship between the Schools Improvement and the
External Evaluation Program from Schools, taking in consideration the role of leadership
in the improvement of an Organization. The external evaluation of schools has become,
without doubt, necessary to create regulamentation mechanisms in order that schools could
develop new cope ways and therefore, to implement and establish netter working
processes, acknowledged by the schools chairmen and directors.
With the main interes of checking the School’s Division’s Leadership and their work to
improve the capability of self-evaluation, we opted to user a methodological study over
quality and quantity. In this matter we chose to study and analyse the School’s Division
documents, such as Educative Projects, Internal Regulation, Activities Annual
Planification, Effort Planification and External Evaluation Reports. We also took action in
interviewing School Directors, the self-evaluation Team Coordinator and foremost to
create and practice a questionnaire to acknowledge the teacher’s perspectives about
School’s Division maintenance and development, as well as the School’s External
Evaluation .
This study case empathizes the role of an effective and recognised leadership. It’s essential
and crucial to the welfare and development of an organisation and the External School’s
Evaluation is taken in great consideration by our study case.
Moreover, dued to the self-evaluation system and combined with internal reflection
devices it enhances the leadership’s accuracy in the quality improvement.La dissertation suivante est le résultat d'une investigation réalisée sur un ensemble d'écoles
de la région centre du pays dont on a observé une amélioration entre le premier et le
second cycle de l'Ă©valuation externe des Ă©coles. Elle porte sur la direction des organisations
scolaires et le lien avec l'amélioration apportée aux écoles participant au programme
d'Ă©valuation externe des Ă©coles. Elle vise ainsi Ă comprendre le rĂ´le des directions dans la
progression d'une institution. L'Ă©valuation externe des Ă©coles est devenue indispensable Ă
la création de mécanismes de régulation, qui, intégrés au processus d'auto évaluation
réalisée par l’école, permettent de développer de nouvelles méthodes. C'est ainsi par
conséquent que sont mis en place des systèmes d'amélioration, rendus visibles grâce aux
prises de décision des responsables désignés de l’établissement scolaire.
Dans le but de vérifier la stratégie adoptée par les directions dans l’amélioration des
résultats scolaire des élèves, la qualité du service éducatif et la capacité d’auto-évaluation
depuis la première évaluation externe, nous choisirons de présenter ce travail à travers une
étude de cas, cherchant ainsi à concilier des méthodes à la fois quantitative et qualitative.
Ainsi, nous avons recours à une analyse de documents d’ordre organisationnel de
l’ensemble scolaire (projet éducatif, règlement intérieur, plan annuel d’activités, plan
d’amélioration, comptes rendus de l’évaluation externe et contradictoire), à la réalisation
de rencontres avec le directeur et la coordinatrice de l’équipe d’autoĂ©valuation et, enfin, Ă
l’élaboration d’un questionnaire visant à connaitre les projets à venir dans le futur
fonctionnement et l’investissement de l’ensemble d’écoles, tout comme dans les
changements réalisés dans la séquence d’évaluation externe des écoles.
Cette étude démontre que, le rôle d’une direction effective et reconnue est fondamentale
dans le développement d’une organisation; et que l’évaluation externe des écoles prend
tout son sens dans l’ensemble d’étude. De plus, l’ajout des dispositifs de réflexion interne
et d’autoévaluation, facilite grandement le perfectionnement des plans d’amélioration,
permettant ainsi l’amélioration constante et continue de la qualité du service éducatif de cet
ensemble scolaire
Accurate description of bulk and interfacial properties in colloid-polymer mixtures
Large-scale Monte Carlo simulations of a phase-separating colloid-polymer
mixture are performed and compared to recent experiments. The approach is based
on effective interaction potentials in which the central monomers of
self-avoiding polymer chains are used as effective coordinates. By
incorporating polymer nonideality together with soft colloid-polymer repulsion,
the predicted binodal is in excellent agreement with recent experiments. In
addition, the interfacial tension as well as the capillary length are in
quantitative agreement with experimental results obtained at a number of points
in the phase-coexistence region, without the use of any fit parametersComment: 4 pages, 4 figure
Entropy-induced Microphase Separation in Hard Diblock Copolymers
Whereas entropy can induce phase behavior that is as rich as seen in
energetic systems, microphase separation remains a very rare phenomenon in
entropic systems. In this paper, we present a density functional approach to
study the possibility of entropy-driven microphase separation in diblock
copolymers. Our model system consists of copolymers composed of freely-jointed
slender hard rods. The two types of monomeric segments have comparable lengths,
but a significantly different diameter, the latter difference providing the
driving force for the phase separation. At the same time these systems can also
exhibit liquid crystalline phases. We treat this system in the appropriate
generalization of the Onsager approximation to chain-like particles. Using a
linear stability (bifurcation) analysis, we analytically determine the onset of
the microseparated and the nematic phases for long chains. We find that for
very long chains the microseparated phase always preempts the nematic. In the
limit of infinitely long chains, the correlations within the chain become
Gaussian and the approach becomes exact. This allows us to define a Gaussian
limit in which the theory strongly simplifies and the competition between
microphase separation and liquid crystal formation can be studied essentially
analytically. Our main results are phase diagrams as a function of the
effective diameter difference, the segment composition and the length ratio of
the segments. We also determine the amplitude of the positional order as a
function of position along the chain at the onset of the microphase separation
instability. Finally, we give suggestions as to how this type of
entropy-induced microphase separation could be observed experimentally.Comment: 16 pages, 7 figure
Depletion forces in non-equilibrium
The concept of effective depletion forces between two fixed big colloidal
particles in a bath of small particles is generalized to a non-equilibrium
situation where the bath of small Brownian particles is flowing around the big
particles with a prescribed velocity. In striking contrast to the equilibrium
case, the non-equilibrium forces violate Newton's third law, are
non-conservative and strongly anisotropic, featuring both strong attractive and
repulsive domains.Comment: 4 pages, 3 figure
Colloidal stabilization via nanoparticle haloing
We present a detailed numerical study of effective interactions between
micron-sized silica spheres, induced by highly charged zirconia nanoparticles.
It is demonstrated that the effective interactions are consistent with a
recently discovered mechanism for colloidal stabilization. In accordance with
the experimental observations, small nanoparticle concentrations induce an
effective repulsion that counteracts the intrinsic van der Waals attraction
between the colloids and thus stabilizes the suspension. At higher nanoparticle
concentrations an attractive potential is recovered, resulting in reentrant
gelation. Monte Carlo simulations of this highly size-asymmetric mixture are
made possible by means of a geometric cluster Monte Carlo algorithm. A
comparison is made to results obtained from the Ornstein-Zernike equations with
the hypernetted-chain closure
Demixing in a single-peak distributed polydisperse mixture of hard spheres
An analytic derivation of the spinodal of a polydisperse mixture is
presented. It holds for fluids whose excess free energy can be accurately
described by a function of a few moments of the size distribution. It is shown
that one such mixture of hard spheres in the Percus-Yevick approximation never
demixes, despite its size distribution. In the
Boublik-Mansoori-Carnahan-Starling-Leland approximation, though, it demixes for
a sufficiently wide log-normal size distribution. The importance of this result
is twofold: first, this distribution is unimodal, and yet it phase separates;
and second, log-normal size distributions appear in many experimental contexts.
The same phenomenon is shown to occur for the fluid of parallel hard cubes.Comment: 4 pages, 2 figures, needs revtex, multicol, epsfig and amstex style
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