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International audience; Laboratory study of slope stability of granular media remains a challenge for modeling, understanding, and predicting natural hazards, such as avalanches and landslides, precursory signs of which are controlled by numerous physical parameters. The present work focuses on the impact of the humidity, in the range of 40-90%, on the stability of monodisperse dense packings of spherical beads. The beads are in a transparent box that is slowly and continuously tilted and allows simultaneous top and lateral optical measurements of global displacements of grains at the surface, defined as precursors. Humidity increases the cohesion between the grains. By performing successive avalanches that destabilize deeper granular layers, we assess the role of the exposure time to the high humidity rates in the diffusion process to reach the hygroscopic equilibrium inside the packing. We highlight an increase of the stability and first precursor angles, associated to a constant angle increment between two consecutive precursors, with a dependency with both the diameter (0.2,0.5, and 0.75 mm) and the material (glass and polystyrene) of the grains.

This paper describes our findings regarding the accumulation of motile bacte-ria at the rear of a confined obstacle and the physical description of the me-chanisms at play. We found that the modification of flow due to the presence of the obstacle produces vorticity that favor the diffusion of bacteria towards the downstream stagnation point. By testing different flow rates, we deter-mined the range in which bacteria accumulate. More interestingly, we observe that hydrodynamic interaction between the bacteria and the top and bottom surface of the microfluidic chip maintain the bacteria in the region where the flow velocity is lower than their own velocity. In the case of non-motile bacte-ria, this effect is not observed because bacteria follow the streamlines as pas-sive tracers do. Fil: Miño, Gastón Leonardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro de Investigaciones y Transferencia de Entre Ríos. Universidad Nacional de Entre Ríos. Centro de Investigaciones y Transferencia de Entre Ríos; Argentina Fil: Baabour, Magali Denise. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; Argentina Fil: Chertcoff, Ricardo Héctor. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; Argentina Fil: Gutkind, Gabriel Osvaldo. Universidad de Buenos Aires. Facultad de Farmacia y Bioquímica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay; Argentina Fil: Clément, Eric. Centre National de la Recherche Scientifique; Francia Fil: Auradou, Harold. Universite de Paris Xi. Laboratoire Automatiques et Systeme Thermiques; Francia Fil: Ippolito, Irene Paula. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina

International audience; Because of the complexity of the physics that governs granular media, understanding the triggering mechanism ofgranular avalanches remains a scientific challenge. Such phenomena are controlled by many factors, including thegrain size and nature, the geometry of the granular structure, but also the environment conditions such as temperatureand moisture which influence is difficult to assess. To this end, we reproduce avalanches at the laboratoryscale under well-controlled conditions and focus our study on the moisture parameter.Recent studies show that the dynamics of a slowly inclined granular bed present three regimes of destabilization1:(1) a regime of small independent and localized rearrangements followed by (2) a regime of collective and successivemotions of grains, called ‘precursors’, which appear at quasi-periodic-tilting angles and then (3) the avalanchewhich occurs at the maximum stability angle. This avalanche angle strongly depends on the relative humidity2.The experiments consisted in recording images of the surface of the granular bed, in lateral and top simultaneousviews, while slowly and regularly tilting it from the horizontal to the avalanche angle. Image processing was thenperformed to describe the dynamics of the medium for different experimental conditions.We did experiments at different relative humidities, between 40% and 95%, on different monodisperse sphericalgrains: glass beads with diameters 0.875, 0.525 and 0.204 mm, respectively, and polystyrene beads with diameter0.14 mm. The velocity of the packing inclination was set to 0.17°/s .The two cameras acquired one image persecond, to determine the angle of the surface during the inclination and to detect the precursors, respectively. Weincline the bed of grains from the horizontal to initiate the first avalanche, and then we continue the inclination toget successive avalanches. The angle of the surface at the avalanche is controlled by the relative humidity, with atrend in agreement with Gomez et al.2.We also observed a dependency of the precursor dynamics with the relativehumidity which strongly modifies the mechanical properties of the granular pile by making it more cohesive. Otherparameters, such as grain size and nature, height of the granular bed and tilt speed, are also investigated

International audience; The possible characterization of the heterogeneity of the aperture field in rough fractures by electrical impedance measurement is demonstrated through displacement experiments using two miscible fluids of different electrical resistivity. Two model fractures have been used: their complementary rough walls are identical but have different relative shear displacements creating ``channel'' or `barrier'' structures in the void space, respectively parallel or perpendicular to the mean flow velocity $\vec{U}$. In the (``channel'') geometry, the global electrical resistance displays an initial linear variation followed by a ``tail'' part reflecting the contrast between slow and fast flow channels. In the (``barrier'') geometry, the existence of a change in the slope between two linear zones suggests the appearance of domains of different permeabilities along the fracture. These variations are well reproduced theoretically and numerically either by assuming independent flow channels parallel to $\vec{U}$ (channel case) or an aperture that varies only along the flow (barrier case). For each configuration, we present a specific inversion procedure that allows one to extract the key features of the heterogeneity from the resistance variation.

International audience; Dispersion experiments are compared for two transparent model fractures with identical complementary rough walls but with a relative shear displacement $\vec{\delta}$ parallel ($\vec{\delta}\parallel \vec{U}$) or perpendicular ($\vec{\delta} \perp \vec{U}$) to the flow velocity $\vec{U}$. The structure of the mixing front is characterized by mapping the local normalized local transit time $\bar t(x,y)$ and dispersivity $\alpha(x,y)$. For $\vec{\delta} \perp \vec{U}$, displacement fronts display large fingers: their geometry and the distribution of $\bar t(x,y)U/x$ are well reproduced by assuming parallel channels of hydraulic conductance deduced from the aperture field. For $\vec{\delta} \parallel \vec{U}$, the front is flatter and $\alpha(x,y)$ displays a narrow distribution and a Taylor-like variation with $Pe$.