Soret driven convection in a colloidal solution heated from above at very large solutal Rayleigh number

Convection in a colloidal suspension with a large negative separation ratio is studied experimentally by heating from above. Shadowgraph observation at very large solutal Rayleigh numbers are reported as a function of time. Fast relaxation oscillations are reported for the root mean square value of the shadowgraph intensity. While pure fluids exhibit a transition to turbulent convection for Rayleigh number of about 10^6, stable spoke-pattern planform with up and down columnar flows are observed up to solutal Rayleigh numbers of the order of 10^9. It is suggested that the surprising stability of the planform against turbulence is due to nonlinear focusing arising from the concentration dependence of the diffusion coefficient.Comment: 10 pages, 4 figure

Antropología, ciudad y jóvenes. Un diálogo con Teresa Caldeira

Teresa Pires do Rio Caldeira, Ph.D. en antropología por la Universidad de California, Berkeley, es profesora asociada del Departamento de Antropología de la Universidad de California en Irvine; ha dictado clases en la Universidad de Campiñas, Brasil; fue investigadora del Centro Brasileño de Análisis y Planificación (CEBRAP) y profesora invitada de la Universidad de São Paulo. Sus investigaciones se centran principalmente en lo urbano: las relaciones sociales y la vida en las ciudades. La ciudad de São Pablo ha sido del centro fundamental de referencia en sus análisis sobre discriminación social, segregación espacial y cambio urbano; violencia urbana; ciudadanía, democracia y derechos individuales; movimientos sociales y políticas de participación popular; relaciones entre géneros y culturas jóvenes en el mundo neoliberal

Equilibrium and non-equilibrium concentration fluctuations in a critical binary mixture

When a macroscopic concentration gradient is present across a binary mixture, long-ranged non-equilibrium concentration fluctuations (NCF) appear as a consequence of the coupling between the gradient and spontaneous equilibrium velocity fluctuations. Long-ranged equilibrium concentration fluctuations (ECF) may be also observed when the mixture is close to a critical point. Here we study the interplay between NCF and critical ECF in a near critical mixture aniline/cyclohexane in the presence of a vertical concentration gradient. To this aim, we exploit a commercial optical microscope and a simple, custom-made, temperature-controlled cell to obtain simultaneous static and dynamic scattering information on the fluctuations. We first characterise the critical ECF at fixed temperature $T$ above the upper critical solution temperature $T_{c}$, in the wide temperature range $T-T_{c}\in[0.1,30]$ $^{o}$C. In this range, we observe the expected critical scaling behaviour for both the scattering intensity and the mass diffusion coefficient and we determine the critical exponents $\gamma$, $\nu$ and $\eta$, which are found in agreement with the 3D Ising values. We then study the system in the two-phase region ($T<T_{c}$). In particular, we characterise the interplay between ECF and NCF when the mixture, initially at a temperature $T_{i}$, is rapidly brought to a temperature $T_{f}>T_{i}$. During the transient, a vertical diffusive mass flux is present that causes the onset of NCF, whose amplitude vanishes with time, as the flux goes to zero. We also study the time dependence of the equilibrium scattering intensity $I_{eq}$, of the crossover wave-vector $q_{co}$ and of the diffusion coefficient $D$ during diffusion and find that all these quantities exhibit an exponential relaxation enslaved to the diffusive kinetics.Comment: 11 pages, 4 figure

Dark Field Differential Dynamic Microscopy enables the accurate characterization of the roto-translational dynamics of bacteria and colloidal clusters

Micro- and nanoscale objects with anisotropic shape are key components of a variety of biological systems and inert complex materials, and represent fundamental building blocks of novel self-assembly strategies. The time scale of their thermal motion is set by their translational and rotational diffusion coefficients, whose measurement may become difficult for relatively large particles with small optical contrast. Here we show that Dark Field Differential Dynamic Microscopy is the ideal tool for probing the roto-translational Brownian motion of shape anisotropic particles. We demonstrate our approach by successful application to aqueous dispersions of non-motile bacteria and of colloidal aggregates of spherical particles

Quantitative optical microscopy of colloids : The legacy of Jean Perrin

When the discontinuous structure of matter was yet an intriguing hypothesis, Jean Perrin performed a set of elegant and pioneering experiments that marked the birth of what today we consider quantitative optical microscopy. Picking up the baton from Perrin, today microscopists face incredible challenges, aiming to extract quantitative information from the increasingly content-rich and complex images made available by modern microscopy techniques. Here, I provide an overview of these challenges and describe the solutions adopted to succeed in this complex task when investigating colloidal systems or systems in which colloidal particles are embedded as microrheological probes

Dynamic scaling for the growth of non-equilibrium fluctuations during thermophoretic diffusion in microgravity

Diffusion processes are widespread in biological and chemical systems, where they play a fundamental role in the exchange of substances at the cellular level and in determining the rate of chemical reactions. Recently, the classical picture that portrays diffusion as random uncorrelated motion of molecules has been revised, when it was shown that giant non-equilibrium fluctuations develop during diffusion processes. Under microgravity conditions and at steady-state, non-equilibrium fluctuations exhibit scale invariance and their size is only limited by the boundaries of the system. In this work, we investigate the onset of non-equilibrium concentration fluctuations induced by thermophoretic diffusion in microgravity, a regime not accessible to analytical calculations but of great relevance for the understanding of several natural and technological processes. A combination of state of the art simulations and experiments allows us to attain a fully quantitative description of the development of fluctuations during transient diffusion in microgravity. Both experiments and simulations show that during the onset the fluctuations exhibit scale invariance at large wave vectors. In a broader range of wave vectors simulations predict a spinodal-like growth of fluctuations, where the amplitude and length-scale of the dominant mode are determined by the thickness of the diffuse layer.Comment: To appear in Scientific Report

Quantitative optical microscopy of colloids : The legacy of Jean Perrin

When the discontinuous structure of matter was yet an intriguing hypothesis, Jean Perrin performed a set of elegant and pioneering experiments that marked the birth of what today we consider quantitative optical microscopy. Picking up the baton from Perrin, today microscopists face incredible challenges, aiming to extract quantitative information from the increasingly content-rich and complex images made available by modern microscopy techniques. Here, I provide an overview of these challenges and describe the solutions adopted to succeed in this complex task when investigating colloidal systems or systems in which colloidal particles are embedded as microrheological probes

Correcting artifacts from finite image size in Differential Dynamic Microscopy

Differential Dynamic Microscopy (DDM) analyzes traditional real-space microscope images to extract information on sample dynamics in a way akin to light scattering, by decomposing each image in a sequence into Fourier modes, and evaluating their time correlation properties. DDM has been applied in a number of soft-matter and colloidal systems. However, objects observed to move out of the microscope's captured field of view, intersecting the edges of the acquired images, can introduce spurious but significant errors in the subsequent analysis. Here we show that application of a spatial windowing filter to images in a sequence before they enter the standard DDM analysis can reduce these artifacts substantially. Moreover, windowing can increase significantly the accessible range of wave vectors probed by DDM, and may further yield unexpected information, such as the size polydispersity of a colloidal suspension

El liberalismo y la burguesía golpista en la Argentina: las propuestas políticas, económicas y sociales del Consejo Empresario Argentino y de la Comisión de Enlace, 1966-1976

Este artículo analiza las propuestas políticas, económicas y sociales de las corporaciones empresarias que representaron los intereses de la burguesía agropecuaria e industrial en la Argentina de las décadas de 1960 y 1970. Nos centraremos en dos organizaciones poco estudiadas, el Consejo Empresario Argentino y la Comisión de Enlace. Mediante un análisis comparado de documentos y testimonios representativos de ambos grupos, buscamos detectar los puntos en que sus propuestas coincidían y aquellos donde divergían. Su análisis permitirá demostrar que, a contrapelo de lo que proponen las interpretaciones dominantes, no hay en estos grupos una coincidencia ideológica en torno a las recetas liberales. De esta forma, buscamos demostrar que la confluencia de ambos sectores en el apoyo a los regímenes dictatoriales de la etapa no responde a la existencia de un programa común.This paper analyze the political, economic and social proposals of business corporations representing the interests of the agricultural and industrial bourgeoisie in the Argentina in the 1960s and 1970s. We will focus on two organizations that have been little-studied: Consejo Empresario Argentino and Comisión de Enlace. Through a comparative analysis of representative documents and testimonies of both groups, we seek to detect the points at which its proposals agreed and those which diverged. This analysis will show that, against the dominant interpretations, these groups have no ideological coincidence around liberal ideas. In this way, we seek to demonstrate that the confluence of both sectors in supporting dictatorial regimes stage, does not respond to the existence of a common platform.Fil: Sanz Cerbino, Gonzalo Sebastian. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Saavedra 15. Centro de Estudios Urbanos y Regionales; Argentin

Otras migraciones: los Latin Kings en España en el relato de F

Este trabajo propone una discusión en torno a algunos de los problemas relacionados con la migración juvenil a España mostrándolos a través de la mirada y la experiencia vivida por F, un miembro ecuatoriano de la organización denominada los Latin Kings. Se analizarán las inadecuadas condiciones del destino que hacen posible la razón de ser y reproducción de esta organización, intentando mostrar de qué modo F, en un hábil y variado juego de adaptaciones, reinvenciones y traducciones de sus existencias en Ecuador y en España logra mantenerse y reproducirse socialmente en situaciones hostiles. A través del relato de vida de este joven ecuatoriano se logra mostrar la conflictividad y la incorporación de una subalternidad como el signo biopolítico del racismo y la discriminación.The following research proposes a discussion about the difficulties related with youth migration to Spain, through the eyes and the experience of F, an Ecuadorian member of the organization known as the Latin Kings. It will analyze the inadequate conditions of destiny, that makes possible the meaning of being and the way of reproduction of this organization, trying to show how F, in a diverse and skilled set of adaptations, translations and reinventions of his existence in Ecuador and Spain manages a way to maintain and socially reproduce in hostile situations. Through the life story of this young Ecuadorian, it is possible to reveal the conflicts and the incorporation of a bio-political sign of racism and discrimination in a subaltern organization