Nonconventional screening of the Coulomb interaction in FexOy clusters: An ab-initio study

From microscopic point-dipole model calculations of the screening of the Coulomb interaction in non-polar systems by polarizable atoms, it is known that screening strongly depends on dimensionality. For example, in one dimensional systems the short range interaction is screened, while the long range interaction is anti-screened. This anti-screening is also observed in some zero dimensional structures, i.e. molecular systems. By means of ab-initio calculations in conjunction with the random-phase approximation (RPA) within the FLAPW method we study screening of the Coulomb interaction in FexOy clusters. For completeness these results are compared with their bulk counterpart magnetite. It appears that the onsite Coulomb interaction is very well screened both in the clusters and bulk. On the other hand for the intersite Coulomb interaction the important observation is made that it is almost contant throughout the clusters, while for the bulk it is almost completely screened. More precisely and interestingly, in the clusters anti-screening is observed by means of ab-initio calculations

Correlation effects and orbital magnetism of Co clusters

Recent experiments on isolated Co clusters have shown huge orbital magnetic moments in comparison with their bulk and surface counterparts. These clusters hence provide the unique possibility to study the evolution of the orbital magnetic moment with respect to the cluster size and how competing interactions contribute to the quenching of orbital magnetism. We investigate here different theoretical methods to calculate the spin and orbital moments of Co clusters, and assess the performances of the methods in comparison with experiments. It is shown that density functional theory in conventional local density or generalized gradient approximations, or even with a hybrid functional, severely underestimates the orbital moment. As natural extensions/corrections we considered the orbital polarization correction, the LDA+U approximation as well as the LDA+DMFT method. Our theory shows that of the considered methods, only the LDA+DMFT method provides orbital moments in agreement with experiment, thus emphasizing the importance of dynamic correlations effects for determining fundamental magnetic properties of magnets in the nano-size regime

Riemann-Hilbert problem, integrability and reductions

The present paper is dedicated to integrable models with Mikhailov reduction groups $G_R \simeq \mathbb{D}_h.$ Their Lax representation allows us to prove, that their solution is equivalent to solving Riemann-Hilbert problems, whose contours depend on the realization of the $G_R$-action on the spectral parameter. Two new examples of Nonlinear Evolution Equations (NLEE) with $\mathbb{D}_h$ symmetries are presented.Comment: 19 pages, 3 figures, Dedicated to Darryl Holm's 70th birthda

Reatores Aeróbicos para o Biotratamento de Águas Poluídas e Efluentes Sintéticos Contendo Ácido 3-Clorobenzóico

The degradation of 3-chlorobenzoic acid in polluted waters and synthetic effluents by a previously isolated indigenous strain of Pseudomonas putida was studied. Batch biodegradation assays were performed using a 2 L microfermentor at 28 °C with agitation. To simulate polluted water, 100 mg.L–1 of 3-chlorobenzoic acid were added to surface river water. Continuous‑flow assays were performed in an aerobic up-flow fixed-bed reactor constructed from PVC employing hollow PVC cylinders as support material. Synthetic wastewater was prepared by dissolving 3-chlorobenzoic acid in non-sterile groundwater. Biodegradation was evaluated by spectrophotometry, chloride release, gas chromatography and microbial growth. In batch experiments the indigenous strain of Pseudomonas putida degrades 100 mg.L–1 of 3-chlorobenzoic acid in 28 hours with a removal efficiency of 92.2 and 87.2%, expressed as compound and chemical oxygen demand removal, respectively. In the continuous-flow reactor the removal of an average influent concentration of 98.6 mg.L–1 reached 91.7% of compound and 88.9% of COD removal. The process efficiency remained approximately constant despite changes in the influent flow, compound concentration and temperature. The absence of metabolites was determined by gas chromatography performed at the end of the batch process and at the effluent of the continuous reactor. The ability of the isolated indigenous strain to degrade 3-chlorobenzoic acid in both batch and continuous reactors represents a promising feature to improve the treatment of effluents.Foi estudada a degradação do ácido 3-clorobenzóico em águas poluídas e efluentes sintéticos por uma cepa nativa previamente isolada de Pseudomonas putida. Ensaios de biodegradação de lotes foram realizados usando um microfermentador de 2 L a 28 °C sob agitação. Para simular água poluída, 100 mg.L–1 de ácido 3-clorobenzóico foram adicionados a água superficial de rio. Foram realizados ensaios de fluxo contínuo num reator aeróbico de fluxo ascendente de leito fixo construído em PVC, utilizando cilindros de PVC como material de suporte. Água sintética residuária foi preparada dissolvendo ácido 3-clorobenzóico em água subterrânea não estéril. A biodegradação foi avaliada por espectrofotometria, liberação de cloreto, cromatografia gasosa e crescimento microbiano. Nos experimentos em lote, a cepa nativa de Pseudomonas putida degrada 100 mg.L–1 de ácido 3-clorobenzóico em 28 horas com uma eficiência de remoção de 92,2 e 87,2%, expressada como remoção de composto e demanda de oxigênio químico, respectivamente. No reator de fluxo contínuo, a remoção de uma concentração média de afluente de 98,6 mg.L–1 atingiu 91,7% do composto e 88,9% da remoção de COD. A eficiência do processo permaneceu aproximadamente constante apesar das mudanças de fluxo, concentração do composto e temperatura. A ausência de metabólitos foi determinada por cromatografia gasosa realizada no final do processo em lote e no efluente do reator continuo. A capacidade da cepa nativa para degradar o ácido 3-clorobenzóico em ambos os lotes e reatores contínuos representa uma função promissora para melhorar o tratamento de efluentes.Fil: Gallego, A.. Universidad de Buenos Aires. Facultad de Farmacia y Bioquímica. Departamento de Sanidad Nutrición Bromatología y Toxicología; ArgentinaFil: Rossen, Ariana Altair. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad de Buenos Aires. Facultad de Farmacia y Bioquímica. Departamento de Sanidad Nutrición Bromatología y Toxicología; ArgentinaFil: Gemini, V.. Universidad de Buenos Aires. Facultad de Farmacia y Bioquímica. Departamento de Sanidad Nutrición Bromatología y Toxicología; Argentina. Instituto Nacional de Tecnología Industrial; ArgentinaFil: Fortunato, M. S.. Universidad de Buenos Aires. Facultad de Farmacia y Bioquímica. Departamento de Sanidad Nutrición Bromatología y Toxicología; ArgentinaFil: Rossi, S.. Universidad de Buenos Aires. Facultad de Farmacia y Bioquímica. Departamento de Sanidad Nutrición Bromatología y Toxicología; ArgentinaFil: Planes, E.. Instituto Nacional de Tecnología Industrial; ArgentinaFil: Korol, S.. Universidad de Buenos Aires. Facultad de Farmacia y Bioquímica. Departamento de Sanidad Nutrición Bromatología y Toxicología; Argentin

Quantum effective potential, electron transport and conformons in biopolymers

In the Kirchhoff model of a biopolymer, conformation dynamics can be described in terms of solitary waves, for certain special cross-section asymmetries. Applying this to the problem of electron transport, we show that the quantum effective potential arising due to the bends and twists of the polymer enables us to formalize and quantify the concept of a {\it conformon} that has been hypothesized in biology. Its connection to the soliton solution of the cubic nonlinear Schr\"{o}dinger equation emerges in a natural fashion.Comment: to appear in J. Phys.

Mechanisms of stability of armored bubbles: FY 1996 Final Report

Theoretical and experimental studies examine how a coating, or {open_quotes}armor,{close_quotes} of partially wetted solid particles can stabilize tiny bubbles against diffusion of gas into the surrounding liquid, in spite of the high capillary pressures normally associated with such bubbles. Experiments with polymethylmethacrylate (PNMA) beads and carbonated water demonstrate that armored bubbles can persist for weeks in liquid unsaturated with respect to the gas in the bubbles. This question is of concern regarding gas discharges from waste tanks at the Hanford reservation. The stresses on the solid-solid contacts between particles in such cases is large and could drive sintering of the particles into a rigid framework. Stability analysis suggests that a slightly shrunken bubble would not expel a solid particle from its armor to relieve stress and allow the bubble to shrink further. Expulsion of particles from more stressed bubbles at zero capillary pressure is energetically favored in some cases. It is not clear, however, whether this expulsion would proceed spontaneously from a small perturbation or require a large initial disturbance of the bubble. In some cases, it appears that a bubble would expel some particles and shrink, but the bubble would approach a final, stable size rather than disappear completely. This simplified analysis leaves out several factors. For instance, only one perturbation toward expelling a solid from the armor is considered; perhaps other perturbations would be more energetically favored than that tested. Other considerations (particle deformation, surface roughness, contact-angle hysteresis, and adhesion or physical bonding between adjacent particles) would make expelling solids more difficult than indicated by this theoretical study

Analysis of a model for foam improved oil recovery

During improved oil recovery (IOR), gas may be introduced into a porous reservoir filled with surfactant solution in order to form foam. A model for the evolution of the resulting foam front known as ‘pressure-driven growth’ is analysed. An asymptotic solution of this model for long times is derived that shows that foam can propagate indefinitely into the reservoir without gravity override. Moreover, ‘pressure-driven growth’ is shown to correspond to a special case of the more general ‘viscous froth’ model. In particular, it is a singular limit of the viscous froth, corresponding to the elimination of a surface tension term, permitting sharp corners and kinks in the predicted shape of the front. Sharp corners tend to develop from concave regions of the front. The principal solution of interest has a convex front, however, so that although this solution itself has no sharp corners (except for some kinks that develop spuriously owing to errors in a numerical scheme), it is found nevertheless to exhibit milder singularities in front curvature, as the long-time asymptotic analytical solution makes clear. Numerical schemes for the evolving front shape which perform robustly (avoiding the development of spurious kinks) are also developed. Generalisations of this solution to geologically heterogeneous reservoirs should exhibit concavities and/or sharp corner singularities as an inherent part of their evolution: propagation of fronts containing such ‘inherent’ singularities can be readily incorporated into these numerical schemes