Vortex distribution in a confining potential

We study a model of interacting vortices in a type II superconductor. In the weak coupling limit, we constructed a mean-field theory which allows us to accurately calculate the vortex density distribution inside a confining potential. In the strong coupling limit, the correlations between the particles become important and the mean-field theory fails. Contrary to recent suggestions, this does not imply failure of the Boltzmann-Gibbs statistical mechanics, as we clearly demonstrate by comparing the results of Molecular Dynamics and Monte Carlo simulations

Isothermal adsorption of polyampholytes on charged nanopatterned surfaces

We investigate the adsorption of neutral polyampholytes on charged nanopatterned surfaces. The surfaces have charged domains but are overall neutral. To perform efficient simulations, we use an approach which combines the explicit form of the interaction potential between the polyampholyte monomers and the surface with a 3d Ewald summation method. We observe that the amount of adsorption and the structure of the adsorbed polyampholytes depend strongly on the surface pattern, the relative size of the surface domains, and the charge distribution along the polyampholyte backbone

Effects of the dielectric discontinuity on the counterion distribution in a colloidal suspension

We introduce a new method for simulating colloidal suspensions with spherical colloidal particles of dielectric constant different from the surrounding medium. The method uses exact calculation of the Green function to obtain the ion-ion interaction potential in the presence of a dielectric discontinuity at the surface of the colloidal particle. The new method is orders of magnitude faster than the traditional approaches based on series expansions of the interaction potential

Weak and Strong Coupling Theories for Polarizable Colloids and Nano-Particles

A theory is presented which allows us to accurately calculate the density profile of monovalent and multivalent counterions in suspensions of polarizable colloids or nano-particles. In the case of monovalent ions, we derive a weak-coupling theory that explicitly accounts for the ion-image interaction, leading to a modified Poisson-Boltzmann equation. For suspensions with multivalent counterions, a strong-coupling theory is used to calculate the density profile near the colloidal surface and a Poisson-Boltzmann equation with a renormalized boundary condition to account for the counterion distribution in the far-field. All the results are compared with the Monte Carlo simulations, showing an excellent agreement between the theory and the simulations

Simulations of Coulomb systems with slab geometry using an efficient 3D Ewald summation method

We present a new approach to efficiently simulate electrolytes confined between infinite charged walls using a 3d Ewald summation method. The optimal performance is achieved by separating the electrostatic potential produced by the charged walls from the electrostatic potential of electrolyte. The electric field produced by the 3d periodic images of the walls is constant inside the simulation cell, with the field produced by the transverse images of the charged plates canceling out. The non-neutral confined electrolyte in an external potential can be simulated using 3d Ewald summation with a suitable renormalization of the electrostatic energy, to remove a divergence, and a correction that accounts for the conditional convergence of the resulting lattice sum. The new algorithm is at least an order of magnitude more rapid than the usual simulation methods for the slab geometry and can be further sped up by adopting a particle–particle particle–mesh approach

Aplicação do modelo de distribuição bidispersa na investigação da densidade superficial de carga estrutural em coloides magnéticos

Dissertação (mestrado)—Universidade de Brasília, Faculdade UnB Planaltina, Programa de Pós-Graduação em Ciência de Materiais, 2013.A densidade superficial de carga é um parâmetro essencial para o monitoramento da estabilidade coloidal de EDL-MF, pois promove uma interação repulsiva capaz de contrabalançar as interações atrativas do tipo van der Waals e dipolar magnética. Recentemente, foi referenciada na literatura a influência do tamanho médio das nanopartículas sobre a o valor de saturação da carga superficial de EDL-MF. Nesse sentido, este trabalho teve como objetivo geral a investigação da influência da polidispersão sobre a densidade superficial de carga de EDL-MF. Foram analisadas amostras de coloides magnéticos aquosos à base de nanopartículas core-shell do tipo MnFe2O4@Fe2O3 com três tamanhos médios distintos. Os histogramas em carga superficial foram ajustados por uma função de distribuição do tipo sigmoidal dupla assimétrica, o que possibilitou a proposição de um modelo de distribuição bidispersa em densidade superficial de carga. A partir da modelização foi possível estimar as peculiaridades de um sistema polidisperso em carga superficial considerando-se apenas duas frações de nanopartículas. De forma geral, os valores de saturação médios da carga superficial determinados para as partículas de maiores tamanhos estão em excelente acordo com os valores obtidos por métodos tradicionais. Para as partículas de menores tamanhos, o modelo proposto não foi satisfatório devido ao reduzido tamanho cristalino das nanopartículas e à maior polidispersão, que impossibilitaram a subdivisão da população de nanopartículas em apenas duas frações. Conclui-se, portanto, que apesar de a polidispersão ser uma característica que influencia várias propriedades dos sistemas coloidais magnéticos, no caso das amostras investigadas, ela não foi determinante para o valor da densidade superficial de carga. _______________________________________________________________________________________ ABSTRACTThe surface charge density is a key parameter for monitoring the colloidal stability on EDL-MF since it promotes a tuning repulsive interaction against attractive interactions among nanoparticles, like van der Waals and magnetic dipolar. It has recently been referenced in the literature the size dependence of the nanoparticles mean size on the saturation value of the surface charge of EDL-MF. In this sense, the main goal of this work concerns the investigation of the influence of polydispersity on the surface charge density of EDL-MF. Samples of electrostatic stabilized magnetic colloids based on core-shell type nanoparticles (MnFe2O4@-Fe2O3) with three different mean sizes were analyzed. The histograms of surface charge distributions were fitted by an asymmetric double sigmoidal function, which allowed the proposition of a bidisperse distribution model in surface charge density. From the modeling, it was possible to estimate the peculiarities of a polydisperse system in surface charge considering only two fractions of nanoparticles. In general, the average of the charge saturation values determined for the nanoparticles of larger sizes are in excellent agreement with values obtained by using well established methods. For the sample of smaller particles, the proposed model was not satisfactory due to the small crystalline size of nanoparticles and their high polydispersity. Finally, although the size polydispersity is a feature that influences many properties of magnetic colloidal systems, in the case of the samples investigated here, it was not critical on the value of the surface charge density

LiveXtend : broadcast events anytime, anywhere and in real-time

Estágio realizado na ClusterMedia Labs, Ld.ªTese de mestrado integrado. Engenharia Informátca e Computação. Faculdade de Engenharia. Universidade do Porto. 200

Simulations of electrolyte between charged metal surfaces

We present a new method for simulating ungrounded charged metal slabs inside an electrolyte solution. The ions are free to move between the interior and exterior regions of the slab–electrolyte system. This leads to polarization of both sides of each slab, with a distinct surface charge induced on each surface. Our simulation method is based on the exact solution of the Poisson equation using periodic Green functions. To efficiently perform the calculations, we decouple the electrostatic energy due to surface polarization from that of purely Coulomb interaction between the ions. This allows us to combine a fast 3D Ewald summation technique with an equally fast calculation of polarization. As a demonstration of the method, we calculate ionic density profiles inside an electrolyte solution and explore charge neutrality violation in between charged metal slabs

HOMOSSEXUALIDADES FEMININAS E EDUCAÇÃO: UMA REFLEXÃO SOBRE O “ARMÁRIO”

Este artigo de revisão propõe tecer uma discussão acerca dos estudos de gênero e do percurso histórico da sexualidade como construções sociais. Apontamos para compreensão da sexualidade, permeada de valores sociais contingentes. Indicamos o modo como a mulher foi colocada em uma posição de subalternidade a partir de discursos que legitimavam os corpos dos homens como sendo normais, plenos e saudáveis, em detrimento dos corpos das mulheres. Partimos da problemática que a sexualidade como construção histórica e social, pode ser apropriada pelos discursos da educação para a afirmação e/ou reconhecimento da mulher homossexual? E ao final propomos tecer diálogos sobre o dispositivo do “armário” e suas implicações na educação como forma de resistência contra a hegemonia centrada nos mecanismos de opressão