A model of dispersive transport across sharp interfaces between porous materials

Recent laboratory experiments on solute migration in composite porous columns have shown an asymmetry in the solute arrival time upon reversal of the flow direction, which is not explained by current paradigms of transport. In this work, we propose a definition for the solute flux across sharp interfaces and explore the underlying microscopic particle dynamics by applying Monte Carlo simulation. Our results are consistent with previous experimental findings and explain the observed transport asymmetry. An interpretation of the proposed physical mechanism in terms of a flux rectification is also provided. The approach is quite general and can be extended to other situations involving transport across sharp interfaces.Comment: 4 pages, 4 figure

Cauchy's formulas for random walks in bounded domains

Cauchy's formula was originally established for random straight paths crossing a body $B \subset \mathbb{R}^{n}$ and basically relates the average chord length through $B$ to the ratio between the volume and the surface of the body itself. The original statement was later extended in the context of transport theory so as to cover the stochastic paths of Pearson random walks with exponentially distributed flight lengths traversing a bounded domain. Some heuristic arguments suggest that Cauchy's formula may also hold true for Pearson random walks with arbitrarily distributed flight lengths. For such a broad class of stochastic processes, we rigorously derive a generalized Cauchy's formula for the average length travelled by the walkers in the body, and show that this quantity depends indeed only on the ratio between the volume and the surface, provided that some constraints are imposed on the entrance step of the walker in $B$. Similar results are obtained also for the average number of collisions performed by the walker in $B$, and an extension to absorbing media is discussed.Comment: 12 pages, 6 figure

Discrete Feynman-Kac formulas for branching random walks

Branching random walks are key to the description of several physical and biological systems, such as neutron multiplication, genetics and population dynamics. For a broad class of such processes, in this Letter we derive the discrete Feynman-Kac equations for the probability and the moments of the number of visits $n_V$ of the walker to a given region $V$ in the phase space. Feynman-Kac formulas for the residence times of Markovian processes are recovered in the diffusion limit.Comment: 4 pages, 3 figure

Collision number statistics for transport processes

Many physical observables can be represented as a particle spending some random time within a given domain. For a broad class of transport-dominated processes, we detail how it is possible to express the moments of the number of particle collisions in an arbitrary volume in terms of repeated convolutions of the ensemble equilibrium distribution. This approach is shown to generalize the celebrated Kac formula for the moments of residence times, which is recovered in the diffusion limit. Some practical applications are illustrated for bounded, unbounded and absorbing domains.Comment: 4 pages, 4 figure

Residence time and collision statistics for exponential flights: the rod problem revisited

Many random transport phenomena, such as radiation propagation, chemical/biological species migration, or electron motion, can be described in terms of particles performing {\em exponential flights}. For such processes, we sketch a general approach (based on the Feynman-Kac formalism) that is amenable to explicit expressions for the moments of the number of collisions and the residence time that the walker spends in a given volume as a function of the particle equilibrium distribution. We then illustrate the proposed method in the case of the so-called {\em rod problem} (a 1d system), and discuss the relevance of the obtained results in the context of Monte Carlo estimators.Comment: 9 pages, 8 figure

Asymptotic behavior of self-affine processes in semi-infinite domains

We propose to model the stochastic dynamics of a polymer passing through a pore (translocation) by means of a fractional Brownian motion, and study its behavior in presence of an absorbing boundary. Based on scaling arguments and numerical simulations, we present a conjecture that provides a link between the persistence exponent $\theta$ and the Hurst exponent $H$ of the process, thus sheding light on the spatial and temporal features of translocation. Furthermore, we show that this conjecture applies more generally to a broad class of self affine processes undergoing anomalous diffusion in bounded domains, and we discuss some significant examples.Comment: 4 pages, 3 figures; to be published in Phys. Rev. Let

Estudos sobre a perspectiva histórico-cultural de Vigotski

Veresk é uma publicação internacional de Psicologia que se propõe a divulgar trabalhos que examinam e aprofundam o estudo do legado da teoria de Lev Semionovitch Vigotski. Os autores são especialmente convidados a apresentarem seus textos pelos Editores Associados ou pelo Conselho Editorial. Veresk é apresentada tanto no formato impresso, quanto no eletrônico, nas versões em russo, português e inglês. Sua publicação é patrocinada pelo Instituto de Psicologia L. S. Vigotski, da Universidade Estatal de Humanidades da Rússia, pela Faculdade de Educação da Universidade Federal Fluminense, Brasil e pela Faculdade de Ciências da Saúde, do Centro Universitário de Brasília, Brasil

Manejo de buva em áreas cultivadas com milho safrinha e aveia.

A infestação de buva (Conyza spp) tem aumentado significativamente nas áreas de produção do sistema soja-trigo-milho especialmente nos estados do Paraná e Rio Grande do Sul. Foram conduzidos dois experimentos na safra 2009/2010 em uma propriedade rural do município de Campo Mourão, PR, com o objetivo de testar alternativas para o controle químico da buva, utilizando-se diferentes produtos e doses, aplicados na dessecação em pré-semeadura da soja, cultivada em áreas contíguas cobertas por palha de milho e de aveia. Foram aplicadas combinações de glifosato com diclosulan, flumioxazin, clorimuron-ethyl, imazetapyr, 2,4-0, paraquat+diuron e saflufenasil. É sabido que existe uma relação entre o tipo de cobertura e o tamanho das plantas de buva, o que pode influenciar no controle por ocasião da aplicação e que plantas menores são melhor controladas do que plantas mais desenvolvidas. Neste trabalho, 64% das plantas de buva na área com aveia estavam com. menos de 10 cm de altura e 36% entre 10 a 20 cm. Já na área com cobertura de milho safrinha apenas 20% das plantas estavam com menos de 10cm de altura, 56% entre 10 a 20 cm e outros 20% com mais de 20cm de altura. Embora não seja possível comparar os dois experimentos entre si, ficou evidente o melhor controle obtido pela maioria dos tratamentos quando aplicados sobre a cobertura de aveia, o que pode ser explicado pela diferença de tamanho das plantas por ocasião das aplicações. A combinação de glifosato com 2,4-0 ou herbicidas que possuem ação residual, complementada com aplicação sequencial que envolve um produto de contato tem mostrado ser uma importante forma de manejo químico para o controle da buva

The role of the osteoplastic flap in the endoscopic era : a retrospective multicentre experience on revision surgery

Objective. To retrospectively review the experience of five tertiary-care university hospitals on frontal sinus revision surgery with osteoplastic flap (OPF). Methods. Descriptive analysis of patients who underwent frontal sinus surgery with OPF after one or more endoscopic procedures for benign and inflammatory pathologies from 2000 to 2022. Clinical charts were reviewed for demographics, indications, clinical presen-tation, previous frontal procedures, OPF technique and outcomes. Results. Of the 124 patients who underwent an OPF procedure, 33 met inclusion criteria. With a mean of 2.1 previous endoscopic surgeries, Draf III was the most common former procedure. In 30 (91%) cases OPF was part of a combined procedure. The most common indications were inverted papilloma (61%), mucocele (9%) and chronic rhinosinusitis (6%). Frontal outflow stenosis (36%) and mucocele (9%) were the most frequent complications observed. Improvement of overall symptoms and patient satisfaction after the OPF procedure were recorded. Conclusions. Even in the endoscopic era, OPF still represents a paramount procedure that should be included in the rhino-surgeon’s armamentarium, in particular in patients with challenging pathologies and anatomy when previous endoscopic endonasal attempts have failed