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, visíveis pelas tomadas de decisão dos líderes 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 contínua 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'évaluation externe des écoles. Elle porte sur la direction des organisations scolaires et le lien avec l'amélioration apportée aux écoles participant au programme d'évaluation externe des écoles. Elle vise ainsi à comprendre le rôle des directions dans la progression d'une institution. L'évaluation externe des écoles est devenue indispensable à 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’autoévaluation et, enfin, à 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 file