Geometry-controlled phase transition in vibrated granular media.

We report experiments on the dynamics of vibrated particles constrained in a two-dimensional vertical container, motivated by the following question: how to get the most out of a given external vibration to maximize internal disorder (e.g. to blend particles) and agitation (e.g. to absorb vibrations)? Granular media are analogs to classical thermodynamic systems, where the injection of energy can be achieved by shaking them: fluidization arises by tuning either the amplitude or the frequency of the oscillations. Alternatively, we explore what happens when another feature, the container geometry, is modified while keeping constant the energy injection. Our method consists in modifying the container base into a V-shape to break the symmetries of the inner particulate arrangement. The lattice contains a compact hexagonal solid-like crystalline phase coexisting with a loose amorphous fluid-like phase, at any thermal agitation. We show that both the solid-to-fluid volume fraction and the granular temperature depend not only on the external vibration but also on the number of topological defects triggered by the asymmetry of the container. The former relies on the statistics of the energy fluctuations and the latter is consistent with a two-dimensional melting transition described by the KTHNY theory.

Stability of a Liquid Jet Impinging on Confined Saturated Sand.

This Letter focuses on the dynamics of a liquid jet impacting the surface of a confined, immersed granular bed. Although previous works have considered the erosion process and surface morphology, less attention has been given to the jet hydrodynamics. Based on laboratory experiments, we show that when the liquid jet forms a crater, two situations arise. For weak or no erosion and for open craters, the jet is stationary. For vertical or overhanging crater walls, the jet displays a wide range of behaviors, from quasiperiodic oscillations to symmetry breaking and exploration of different states in time. An analysis of the different system states leads to the emergence of a bifurcation diagram depending on a dimensionless parameter, J, comparing the jet impact force to the force necessary to eject a grain. The frequency of the jet oscillations depends on the inertial velocity, the jet dispersion and the ratio between the injector cross section and the confinement length.

Stability of gas channels in a dense suspension in the presence of obstacles.

We investigate experimentally the influence of a fixed obstacle on gas rising in a dense suspension. Air is injected at a constant flow rate by a single nozzle at the bottom center of a Hele-Shaw cell. Without obstacles, previous works have shown that a fluidized zone is formed with a parabolic shape, with a central air channel and two granular convection rolls on its sides. Here, we quantify the influence of the obstacle's shape, size, and height on the location and dynamics of the central air channel. Different regimes are reported: the air channel can simply deviate (stable), or it can switch sides over time (unstable), leading to two signatures not only above the obstacle, but sometimes also below it. This feedback also influences the channel deviation when bypassing the obstacle. A wake of less or no motion is reported above the largest obstacles as well as the maximum probability of gas location, which can be interesting for practical applications. The existence of a critical height h_{c}≃7 cm is discussed and compared with the existence of an air finger that develops from the injection nozzle and is stable in time. A dimensionless number describing the transition between air fingering and fracturing makes it possible to predict the channel's stability.

Polaridad de los comentarios y consistencia interna en los informes de arbitraje de artículos de investigación / Polarity of comments and internal consistency in peer review reports on scientific research articles / Polaridade dos comentários e consistência interna nos informes de arbitragem de artigos de pesquisa

El informe de arbitraje en los procesos de revisión por pares de artículos de investigación es un género clave para explicar cómo se construye colectivamente el conocimiento científico. En estos informes los evaluadores emiten, junto con una serie de comentarios, una recomendación de publicación. El análisis de la calidad del proceso de evaluación por pares se ha realizado a partir de indicadores, como las tasas de rechazo o el grado de acuerdo entre los evaluadores. Sin embargo, aún queda pendiente una evaluación cualitativa del proceso. El objetivo de este trabajo fue describir la proporción, según su polaridad (positiva, negativa y neutra), de los comentarios de 56 informes de evaluación de la Revista Onomázein y determinar si esa proporción era consistente con la recomendación de los evaluadores (Aceptado, Aceptado con enmiendas mayores o menores, y Rechazado). Del análisis de 1.472 comentarios se determinó que, independientemente de la decisión, la mayor proporción corresponde a comentarios negativos. Asimismo, podemos afirmar que los procesos analizados presentan un alto grado de consistencia. Mientras más favorable es la recomendación de los árbitros mayor es la proporción de comentarios positivos emitidos y, correspondientemente, menor es la proporción de comentarios negativos.

Peer review reports on scientific articles means a key genre to explain how scientific knowledge is collectively constructed. In these reports, reviewers write a recommendation for publication along with a series of comments. The quality analysis of the peer review process has been commonly conducted based on indicators, such as rejection rates and the agreement level among evaluators. However, a more qualitative investigation on the process still remains outstanding. This work aims at describing the polarity (positive, negative and neutral) of comments corresponding to 56 peer review reports belonging to the journal Onomázein and determining whether this proportion is consistent with the reviewers recommendation, i.e. Accepted, Accepted with major revisions, Accepted with minor revisions and Rejected). After the analysis of 1.472 comments, it was possible to determine that the highest proportion of comments is negative, independent of the decision. The analyzed processes also showed a high level of consistency. The more favorable the recommendation, the higher the proportion of positive comments, and, consequently, the less the proportion of negative comments.

O informe de arbitragem nos processos de revisão por pares de artigos de pesquisa é um gênero chave para explicar como se constrói coletivamente o conhecimento científico. Nestes informes os avaliadores emitem, junto com uma série de comentários, uma recomendação de publicação. A análise da qualidade do processo de avaliação por pares foi realizada a partir de indicadores, como as taxas de recusa ou o grau de acordo entre os avaliadores. No entanto, ainda resta pendente uma avaliação qualitativa do processo. O objetivo deste trabalho foi descrever a proporção, segundo a sua polaridade (positiva, negativa e neutra), dos comentários de 56 informes de avaliação da Revista Onomázein e determinar se essa proporção era consistente com a recomendação dos avaliadores (Aceito, Aceito com emendas maiores ou menores, e Recusado). Da análise de 1.472 comentários se determinou que, independentemente da decisão, a maior proporção corresponde a comentários negativos. Assim mesmo, podemos afirmar que os processos analisados apresentam um alto grau de consistência. Quanto mais favorável for a recomendação dos árbitros maior será a proporção de comentários positivos emitidos e, correspondentemente, menor é a proporção de comentários negativos.

Bubbles trapped in a fluidized bed: Trajectories and contact area.

This work investigates the dynamics of bubbles in a confined, immersed granular layer submitted to an ascending gas flow. In the stationary regime, a central fluidized zone of parabolic shape is observed, and the bubbles follow different dynamics: either the bubbles are initially formed outside the fluidized zone and do not exhibit any significant motion over the experimental time or they are located inside the fluidized bed, where they are entrained downwards and are, finally, captured by the central air channel. The dependence of the air volume trapped inside the fluidized zone, the bubble size, and the three-phase contact area on the gas injection flow rate and grain diameter are quantified. We find that the volume fraction of air trapped inside the fluidized region is roughly constant and of the order of 2%-3% when the gas flow rate and the grain size are varied. Contrary to intuition, the gas-liquid-solid contact area, normalized by the air injected into the system, decreases when the flow rate is increased, which may have significant importance in industrial applications.

Gas-induced fluidization of mobile liquid-saturated grains.

Gas invasion in liquid-saturated sands exhibits different morphologies and dynamics. For mobile beds, the repeated rise of gas through the layer leads to the growth of a fluidized zone, which reaches a stationary shape. Here, we present experimental results characterizing the evolution of the fluidized region as a function of the gas-flow rate and grain size. We introduce a new observable, the flow density, which quantifies the motion of the grains in the system. The growth of the fluidized zone is characterized by a spatiotemporal analysis, which provides the stabilization time, τ(s). In the stationary regime, we report two main contributions to motion in the fluidized region: the central gas rise and a convective granular motion. Interestingly, a static model with a fixed porous network accounts for the final shape of the invasion zone. We propose an explanation where the initial gas invasion weakens the system and fixes since the early stage the morphology of the fluidized zone.

Morphology of air invasion in an immersed granular layer.

We report a study of the paths formed by a finite volume of air gently injected at the base of an immersed granular material. A two-dimensional model, based on experimental observations, shows that the typical height and width of the region explored by the branched path depends not only on the injected volume V, but also on a dimensionless parameter χ which accounts for the relative effects of the gravity and capillarity. For a given injected volume V, larger gravity effects lead to taller and narrower structures; for a given χ, the typical height and width of the structure scale like V(1/2) and V(1/4), respectively, while the typical gaseous fraction in the corresponding region increases accordingly like V(1/4). Such results can be of practical importance: For instance, gas can be trapped on purpose in an underground natural container below a granular slurry. Our results can help in predicting if the gas is likely to reach the free surface and escape the system if the container presents a defect (hole or fracture).

Venting dynamics of an immersed granular layer.

Air is injected locally at the base of an immersed granular bed. The gas, which is forced to flow gently through the material, creates several paths between the grains. We observe that the latter gas venting results in the emission of bubbles in a localized region at the free surface. Additional experiments, performed in two dimensions, permit a direct visualization of the paths, and a theoretical approach shows that the typical size of the region at the free surface can be accounted for by a diffusionlike process. The diffusion coefficient is expressed as a function of the system parameters.

Dynamics of crater formations in immersed granular materials.

We report the formation of a crater at the free surface of an immersed granular bed, locally crossed by an ascending gas flow. In two dimensions, the crater consists of two piles which develop around the location of the gas emission. We observe that the typical size of the crater increases logarithmically with time, independently of the gas emission dynamics. We describe the related granular flows and give an account of the influence of the experimental parameters, especially of the grain size and of the gas flow.

Collective behavior in a granular jet: emergence of a liquid with zero surface tension.

We perform the analog to "water bell" experiments with granular jets. Rebounding from cylindrical targets, wide granular jets produce sheets or cones with shapes that mimic a zero-surface-tension liquid. The jets' particulate nature appears when the number of particles in the cross section is decreased: the emerging structures broaden, gradually disintegrating into diffuse sprays. The experiment has a counterpart in the behavior of quark-gluon plasmas generated by colliding heavy ions. There, a high collision density gives rise to collective behavior also described as a liquid.