Mixing of Non-Newtonian Fluids in Steadily Forced Systems

We investigate mixing in a viscoelastic and shear-thinning fluid—a very common combination in polymers and suspensions. We find that competition between elastic and viscous forces generates self-similar mixing, lobe transport, and other characteristics of chaos. The mechanism by which chaos is produced is evaluated both in experiments and in a simple model. We find that chaotic flow is generated by spontaneous oscillations, the magnitude and frequency of which govern the extent of chaos and mixing

Precision Measurements of Stretching and Compression in Fluid Mixing

The mixing of an impurity into a flowing fluid is an important process in many areas of science, including geophysical processes, chemical reactors, and microfluidic devices. In some cases, for example periodic flows, the concepts of nonlinear dynamics provide a deep theoretical basis for understanding mixing. Unfortunately, the building blocks of this theory, i.e. the fixed points and invariant manifolds of the associated Poincare map, have remained inaccessible to direct experimental study, thus limiting the insight that could be obtained. Using precision measurements of tracer particle trajectories in a two-dimensional fluid flow producing chaotic mixing, we directly measure the time-dependent stretching and compression fields. These quantities, previously available only numerically, attain local maxima along lines coinciding with the stable and unstable manifolds, thus revealing the dynamical structures that control mixing. Contours or level sets of a passive impurity field are found to be aligned parallel to the lines of large compression (unstable manifolds) at each instant. This connection appears to persist as the onset of turbulence is approached.Comment: 5 pages, 5 figure

Phase separation in a chaotic flow

The phase separation between two immiscible liquids advected by a bidimensional velocity field is investigated numerically by solving the corresponding Cahn-Hilliard equation. We study how the spinodal decomposition process depends on the presence -or absence- of Lagrangian chaos. A fully chaotic flow, in particular, limits the growth of domains and for unequal volume fractions of the liquids, a characteristic exponential distribution of droplet sizes is obtained. The limiting domain size results from a balance between chaotic mixing and spinodal decomposition, measured in terms of Lyapunov exponent and diffusivity constant, respectively.Comment: Minor changes - Version accepted for publication - Physical Review Letter

Spirometer-controlled cine magnetic resonance imaging used to diagnose tracheobronchomalacia in paediatric patients

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Aislamiento de Salmonella arizonae en carne molida de una carnicería de Corrientes, Argentina

En ocasión de investigar el grado de contaminación bacteriana en muestras de carne molida obtenidas en carnicerías de la ciudad de Corrientes e identificar bacterias patógenas como Staphylococcus aureus, Escherichia coli y Salmonella spp, se produjeron dos hallazgos fortuitos (años 2009 y 2011) consistentes en el aislamiento de Salmonella arizonae, agente poco habitual en carnes para consumo humano, pero frecuente en intestino de reptiles y quelonios. Pese a que S. arizonae fue detectada en períodos de tiempo distantes, llamó la atención que ambas muestras procedieran de la misma carnicería y aparentemente del mismo establecimiento frigorífico, lo que podría indicar una fuente permanente de infección y una situación de evidente riesgo sanitario

Seroprevalencia de T. cruzi en caninos de distintos tipos de viviendas de un barrio periférico de Corrientes, Argentina

La importancia de los perros como reservorio en el ciclo doméstico de Trypanosoma cruzi, agente de la enfermedad de Chagas, se ha visto reforzada por las altas prevalencias halladas en caninos de barrios periféricos, lo cual junto a la precariedad de viviendas, patios y su entorno, contribuyen a la presencia endémica de la enfermedad. Se consideró el grado de infección canina a T. cruzi y su asociación con factores de riesgo, utilizándose como indicador epidemiológico el porcentaje de animales infectados en el barrio Bañado Norte de la ciudad de Corrientes, dimensionándose los factores de riesgo surgidos de las características de las viviendas. La muestra estuvo conformada por 122 caninos, de los cuales el 58% pertenecían a viviendas tipo A (paredes de madera, piso de tierra) y el 42% a tipo B (ladrillos y cemento, respectivamente). La prevalencia total fue de 54%, es decir que resultaron positivos 66 sueros, de los cuales 41 pertenecían a caninos que habitaban casas tipo A y los 25 restantes a casas tipo B. Si bien la prevalencia hallada fue alta, para poder inferir la situación existente a nivel poblacional se debería aumentar el tamaño muestral. El valor de 1,41 del riesgo relativo calculado para asociar la presencia de infección con el tipo de vivienda, podría interpretarse como que los caninos procedentes de viviendas tipo A tienen mayor probabilidad de adquirir la infección, aunque el análisis estadístico no arrojó evidencias significativas para afirmar tal asociació

Multifractal Scaling, Geometrical Diversity, and Hierarchical Structure in the Cool Interstellar Medium

Multifractal scaling (MFS) refers to structures that can be described as a collection of interwoven fractal subsets which exhibit power-law spatial scaling behavior with a range of scaling exponents (concentration, or singularity, strengths) and dimensions. The existence of MFS implies an underlying multiplicative (or hierarchical, or cascade) process. Panoramic column density images of several nearby star- forming cloud complexes, constructed from IRAS data and justified in an appendix, are shown to exhibit such multifractal scaling, which we interpret as indirect but quantitative evidence for nested hierarchical structure. The relation between the dimensions of the subsets and their concentration strengths (the "multifractal spectrum'') appears to satisfactorily order the observed regions in terms of the mixture of geometries present: strong point-like concentrations, line- like filaments or fronts, and space-filling diffuse structures. This multifractal spectrum is a global property of the regions studied, and does not rely on any operational definition of "clouds.'' The range of forms of the multifractal spectrum among the regions studied implies that the column density structures do not form a universality class, in contrast to indications for velocity and passive scalar fields in incompressible turbulence, providing another indication that the physics of highly compressible interstellar gas dynamics differs fundamentally from incompressible turbulence. (Abstract truncated)Comment: 27 pages, (LaTeX), 13 figures, 1 table, submitted to Astrophysical Journa