Coexistence diameter in two-dimensional colloid-polymer mixtures

We demonstrate that the law of the rectilinear coexistence diameter in two-dimensional (2D) mixtures of non-spherical colloids and non-adsorbing polymers is violated. Upon approach of the critical point, the diameter shows logarithmic singular behavior governed by a term t ln(t), with t the relative distance from the critical point. No sign of a term t^2b could be detected, with b the critical exponent of the order parameter, indicating a very weak or absent Yang-Yang anomaly. Our analysis thus reveals that non-spherical particle shape alone is not sufficient for the formation of a pronounced Yang-Yang anomaly in the critical behavior of fluids.Comment: 4 pages, 4 figures, to appear in Phys. Rev. E (R

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.

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

Surface-mediated attraction between colloids

We investigate the equilibrium properties of a colloidal solution in contact with a soft interface. As a result of symmetry breaking, surface effects are generally prevailing in confined colloidal systems. In this Letter, particular emphasis is given to surface fluctuations and their consequences on the local (re)organization of the suspension. It is shown that particles experience a significant effective interaction in the vicinity of the interface. This potential of mean force is always attractive, with range controlled by the surface correlation length. We suggest that, under some circumstances, surface-induced attraction may have a strong influence on the local particle distribution

Simulation and theory of fluid demixing and interfacial tension of mixtures of colloids and non-ideal polymers

An extension of the Asakura-Oosawa-Vrij model of hard sphere colloids and non-adsorbing polymers, that takes polymer non-ideality into account through a repulsive stepfunction pair potential between polymers, is studied with grand canonical Monte Carlo simulations and density functional theory. Simulation results validate previous theoretical findings for the shift of the bulk fluid demixing binodal upon increasing strength of polymer-polymer repulsion, promoting the tendency to mix. For increasing strength of the polymer-polymer repulsion, simulation and theory consistently predict the interfacial tension of the free colloidal liquid-gas interface to decrease significantly for fixed colloid density difference in the coexisting phases, and to increase for fixed polymer reservoir packing fraction.Comment: 10 pages, 4 figure

Helical Tubes in Crowded Environments

When placed in a crowded environment, a semi-flexible tube is forced to fold so as to make a more compact shape. One compact shape that often arises in nature is the tight helix, especially when the tube thickness is of comparable size to the tube length. In this paper we use an excluded volume effect to model the effects of crowding. This gives us a measure of compactness for configurations of the tube, which we use to look at structures of the semi-flexible tube that minimize the excluded volume. We focus most of our attention on the helix and which helical geometries are most compact. We found that helices of specific pitch to radius ratio 2.512 to be optimally compact. This is the same geometry that minimizes the global curvature of the curve defining the tube. We further investigate the effects of adding a bending energy or multiple tubes to begin to explore the more complete space of possible geometries a tube could form.Comment: 10 page

Description of the fluctuating colloid-polymer interface

To describe the full spectrum of surface fluctuations of the interface between phase-separated colloid-polymer mixtures from low scattering vector q (classical capillary wave theory) to high q (bulk-like fluctuations), one must take account of the interface's bending rigidity. We find that the bending rigidity is negative and that on approach to the critical point it vanishes proportionally to the interfacial tension. Both features are in agreement with Monte Carlo simulations.Comment: 5 pages, 3 figures, 1 table. Accepted for publication in Phys. Rev. Let

Morphology of the gastric mill teeth in dotillid crabs (Crustacea: Brachyura: Dotillidae) from Indonesia

The gastric mill is a prominent structure in the digestive system of brachyuran crabs, consisting of a median tooth plate and a pair of lateral tooth plates. Among crab species that are deposit feeders, the morphology and size of the gastric mill teeth are correlated with the preferred substrate types and food spectrum. In this study, we provide a detailed description of the morphology of the median and lateral teeth of the gastric mills in eight species of dotillid crabs from Indonesia, and compare them in relation to habitat preferences and molecular phylogeny. Ilyoplax delsmani, Ilyoplax orientalis, and Ilyoplax strigicarpus have comparatively simple shapes of their median and lateral teeth, with fewer teeth on each lateral tooth plate compared to Dotilla myctiroides, Dotilla wichmanni, Scopimera gordonae, Scopimera intermedia, and Tmethypocoelis aff. ceratophora, which have more complexly shaped median and lateral teeth, with a greater number of teeth on each lateral tooth plate. The number of teeth on lateral tooth correlates with habitat preference, that is, dotillid crabs inhabiting muddy substrata have fewer teeth on the lateral tooth plate, and those inhabiting sandy substrata have a more teeth. Phylogenetic analysis using partial COI and 16S rRNA genes supports that teeth morphology is similar among closely related species. Therefore, the description of median and lateral teeth of the gastric mill is expected to contribute to the systematic study of dotillid crabs

Critical phenomena in colloid-polymer mixtures: interfacial tension, order parameter, susceptibility and coexistence diameter

The critical behavior of a model colloid-polymer mixture, the so-called AO model, is studied using computer simulations and finite size scaling techniques. Investigated are the interfacial tension, the order parameter, the susceptibility and the coexistence diameter. Our results clearly show that the interfacial tension vanishes at the critical point with exponent 2\nu ~ 1.26. This is in good agreement with the 3D Ising exponent. Also calculated are critical amplitude ratios, which are shown to be compatible with the corresponding 3D Ising values. We additionally identify a number of subtleties that are encountered when finite size scaling is applied to the AO model. In particular, we find that the finite size extrapolation of the interfacial tension is most consistent when logarithmic size dependences are ignored. This finding is in agreement with the work of Berg et al.[Phys. Rev. B, V47 P497 (1993)]Comment: 13 pages, 16 figure

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