Experimental evidences of the Gardner phase in a granular glass

Analyzing the dynamics of a vibrated bi-dimensional packing of bidisperse granular discs below jamming, we provide evidences of a Gardner phase deep into the glass phase. To do so we perform several independent compression cycles within the same glass and show that the particles select different average vibrational positions at each cycle, while the neighborhood structure remains unchanged. We compute the mean square displacement as a function of the packing fraction and compare it with the average separation between the cages obtained for different compression cycles. Our results are fully compatible with recent numerical observations obtained for a mean field model of glass as well as for hard spheres in finite dimension. We also characterize the distribution of the cage order parameters. Here we note several differences from the numerical results, which could be attributed to activated processes and cage heterogeneities.Comment: 4 pages, 3 figure

Shear modulus and Dilatancy Softening in Granular Packings above Jamming

We investigate experimentally the mechanical response of a monolayer of bi-disperse frictional grains to an inhomogeneous shear perturbation across the jamming transition. We inflate an intruder inside the packing and use photo-elasticity and tracking techniques to measure the induced shear strain and stresses at the grain scale. We quantify experimentally the constitutive relations for strain amplitudes as low as 0.001 and for a range of packing fractions within 2% variation around the jamming transition. At the transition strong nonlinear effects set in : both the shear modulus and the dilatancy shear-soften at small strain until a critical strain is reached where effective linearity is recovered. The dependencies of the critical strain and the associated critical stresses on the distance from jamming are extracted via scaling analysis. We check that the constitutive laws, when applied to the equations governing mechanical equilibrium, lead to the observed stress and strain profiles. These profiles exhibit a spatial crossover between an effective linear regime close to the inflater and the truly nonlinear regime away from it. The crossover length diverges at the jamming transition.Comment: 5 pages, 5 figure

Dense granular flow around a penetrating object: Experiments and hydrodynamic model

We present in this Letter experimental results on the bidimensional flow field around a cylinder penetrating into dense granular matter together with drag force measurements. A hydrodynamic model based on extended kinetic theory for dense granular flow reproduces well the flow localization close to the cylinder and the corresponding scalings of the drag force, which is found to not depend on velocity, but linearly on the pressure and on the cylinder diameter and weakly on the grain size. Such a regime is found to be valid at a low enough "granular" Reynolds number.Comment: 5 pages, 4 figure

Elasticity of granular packings close to Jamming. Elasticit\'e des empilements granulaires proche de la transition de blocage

We investigate experimentally the mechanical response to shear of a 2D packing of grains across the jamming transition. First, we develop a dedicated experimental setup, together with tracking and photoelastic techniques in order to prepare the packing in a controlled fashion and to quantify the stress and strain tensors at the grain scale. Second, we install a inflating probe (a 2D "balloon"), which shears the packing with a cylindrical symmetry. We probe experimentally stresses and strains for strain amplitudes as low as $10^{-3}$ and for a range of packing fractions within $2\%$ variation around the jamming transition. We observe not only that shear strain induces shear stresses, but also normal stresses. Moreover, we show that both shear and normal stresses behave nonlinearly with the shear strain. Finally, we show by scaling analysis that the constitutive laws are controlled by the Jamming transition.Comment: 10 pages, 8 figures, in frenc

Shear softening above jamming

We investigate experimentally the mechanical response of a monolayer of frictional grains to an inhomogeneous shear perturbation across the jamming transition. We inflate an intruder inside the packing and use photoelasticity and tracking techniques to measure the induced shear strain and stresses at the grain scale. We quantify experimentally the constitutive relations for strain amplitudes as low as 10–3 and for a range of packing fractions within 2% variation around the jamming transition. At the transition strong nonlinear effects set in. The dependencies of the critical strain and the associated critical stresses on the distance from jamming are extracted via scaling analysis. We check that the constitutive laws, when applied to the equations governing mechanical equilibrium, lead to the observed stress and strain profiles. These profiles exhibit a spatial crossover between an effective linear regime close to the inflater and a truly nonlinear regime away from it. The crossover length diverges at the jamming transition

Horse males became over-represented in archaeological assemblages during the Bronze Age

International audienceHighlights 1. We generated low-coverage DNA sequence data for 19 new Upper Paleolithic horses from Goyet and Trou Magritte, Belgium. 2. We determined their molecular sex together with that of 249 previously-published horse archaeological remains spanning the last 40,000 years across Eurasia. 3. Osseous archaeological and paleontological assemblages showed balanced male:female sex ratios up until ~3,900 years cal. BP. 4. Archeological assemblages post-3,900 years cal. BP became strongly biased towards males (approximately ~79%), underlining a strong shift in animal gender representation that occurred during the Bronze Age

Dense flow around a sphere moving into a cloud of grains

A bidimensional simulation of a sphere moving at constant velocity into a cloud of smaller spherical grains without gravity is presented with a non-smooth contact dynamics method. A dense granular “cluster” zone of about constant solid fraction builds progressively around the moving sphere until a stationary regime appears with a constant upstream cluster size that increases with the initial solid fraction ϕ0 of the cloud. A detailed analysis of the local strain rate and local stress fields inside the cluster reveals that, despite different spatial variations of strain and stresses, the local friction coeffcient μ appears to depend only on the local inertial number I as well as the local solid fraction ϕ, which means that a local rheology does exist in the present non parallel flow. The key point is that the spatial variations of I inside the cluster does not depend on the sphere velocity and explore only a small range between about 10−2 and 10−1. The influence of sidewalls is then investigated on the flow and the forces

Force de traînée dans un milieu granulaire ""chaud""

Nous étudions expérimentalement le déplacement à vitesse constante d'un disque intrus dans un empilement 2D dense de plus petits disques constituant un milieu granulaire modèle. Nous mesurons la force de traînée sur l'intrus, en variant sa vitesse et sa taille ainsi que la ""température"" du milieu par vibration. Deux régimes sont observés: à basse température (faibles vibrations), la force de traînée ne dépend pas de la vitesse de l'objet tandis qu'à haute température (fortes vibrations), la force de traînée présente une dépendance linéaire avec cette vitesse

Origin of the Broad Lifetime Distribution of Localized Excitons in InGaN/GaN Quantum Dots

We derive an energy-dependent decay-time distribution function from the multi-exponential decay of the ensemble photoluminescence (PL) of InGaN/GaN quantum dots (QDs), which agrees well with recently published single-QD time-resolved PL measurements. Using eight-band k.p modelling, we show that the built-in piezo- and pyroelectric fields within the QDs cause a sensitive dependence of the radiative lifetimes on the exact QD geometry and composition. Moreover, the radiative lifetimes also depend heavily on the composition of the direct surrounding of the QDs. A broad lifetime distribution occurs even for moderate variations of the QD structure. Thus, for unscreened fields a multi-exponential decay of the ensemble PL is generally expected in this material system.Comment: 5 pages, 4 figures. accepted at Physica Status Solid