Pressure dependent friction on granular slopes close to avalanche

We investigate the sliding of objects on an inclined granular surface close to the avalanche threshold. Our experiments show that the stability is driven by the surface deformations. Heavy objects generate footprint-like deformations which stabilize the objects on the slopes. Light objects do not disturb the sandy surfaces and are also stable. For intermediate weights, the deformations of the surface destabilize the objects and generate sliding. A characteristic pressure for which the solid friction is minimal is evidenced. Applications to the locomotion of devices and animals on sandy slopes as a function of their mass are proposed

Experimental Investigation of Plastic Deformations Before Granular Avalanche

We present an experimental study of the deformation inside a granular material that is progressively tilted. We investigate the deformation before the avalanche with a spatially resolved Diffusive Wave Spectroscopy setup. At the beginning of the inclination process, we first observe localized and isolated events in the bulk, with a density which decreases with the depth. As the angle of inclination increases, series of micro-failures occur periodically in the bulk, and finally a granular avalanche takes place. The micro-failures are observed only when the tilt angles are larger than a threshold angle much smaller than the granular avalanche angle. We have characterized the density of reorganizations and the localization of micro-failures. We have also explored the effect of the nature of the grains, the relative humidity conditions and the packing fraction of the sample. We discuss those observations in the framework of the plasticity of granular matter. Micro-failures may then be viewed as the result of the accumulation of numerous plastic events

Eshelby inclusions in granular matter: theory and simulations

We present a numerical implementation of an active inclusion in a granular material submitted to a biaxial test. We discuss the dependence of the response to this perturbation on two parameters: the intra-granular friction coefficient on one hand, the degree of the loading on the other hand. We compare the numerical results to theoretical predictions taking into account the change of volume of the inclusion as well as the anisotropy of the elastic matrix

Circular differential scattering of polarized light by a chiral random medium

International audienceDifferential scattering of left- and right-handed circularly polarized waves is a general property of scattering media lacking space-inversion symmetry. We show in this paper that this scattering difference has remarkable properties in the case of an almost transparent chiral random medium. By tuning the refractive indices of the materials composing the medium, this scattering difference can be varied and even inverted. Within the limit of a dense scattering material, the difference of scattering cross sections between polarizations of opposite handedness can be largely adjusted. These properties are illustrated by measurement of the difference between the transmission of right- and left-handed polarized waves through a chiral Christiansen filter

Spatial repartition of local plastic processes in different creep regimes in a granular material

Granular packings under constant shear stress display below the Coulomb limit, a logarithmic creep dynamics. However the addition of small stress modulations induces a linear creep regime characterized by an effective viscous response. Using Diffusing Wave Spectroscopy, we investigate the relation between creep and local plastic events spatial distribution ("hot-spots") contributing to the plastic yield. The study is done in the two regimes, i.e. with and without mechanical activation. The hot-spot dynamics is related to the material effective fluidity. We show that far from the threshold, a local visco-elastic rheology coupled to an ageing of the fluidity parameter, is able to render the essential spatio-temporal features of the observed creep dynamics

Mechanical fluctuations suppress the threshold of soft-glassy solids : the secular drift scenario

We propose a dynamical mechanism leading to the fluidization of soft-glassy amorphous mate-rial driven below the yield-stress by external mechanical fluctuations. The model is based on the combination of memory effect and non-linearity, leading to an accumulation of tiny effects over a long-term. We test this scenario on a granular packing driven mechanically below the Coulomb threshold. We bring evidences for an effective viscous response directly related to small stress modulations in agreement with the theoretical prediction of a generic secular drift

Impact of a Projectile on a Granular Medium Described by a Collision Model

International audienceWe propose a model for the propagation of energy due to the impact of a granular projectile on a dense granular medium. Energy is transferred from grain to grain during binary collision events. The transport of energy may then be viewed as a random walk with a split of energy during successive collisions. There is a qualitative and quantitative agreement between this simple description and experimental results

Drying aqueous colloidal systems: Molecular interactions, self-assembly and homeostatic behavior

Evaporation is a ubiquitous process in aqueous systems, which may be advantageous for processing materials through drying or deadly for living systems. Since surfactants, polymers and particles are usually non-volatile, water evaporation will lead to the build-up of concentration gradients in the system, from the air/liquid interface into the dispersion’s bulk. These concentration gradients will in turn generate structuration gradients in the colloidal system, which lead to changes in transport properties along the gradients. We will show that such a feedback loop on water evaporation can lead to non-linear behaviors, which are crucial for land-living animals’ survival and opens new avenues in drying and filtration processes. We designed millifluidic drying cells, which consist of a small capillary attached to a large reservoir, with one tip exposed to air at a controlled relative humidity. Chemical potential boundary conditions are thus set and controlled during drying. We monitored drying with time with a combination of mapping techniques: polarized microscopy, infra-red microscopy and coherent small-angle scattering, which yields both concentration and structuration gradients. We also measured independently the evaporation rate through gravimetry. Using simple surfactant aqueous solutions, we show that the evaporation rate is nearly independent of water evaporation driving force, the air relative humidity [1]. Strikingly, this behavior is identical to that of stratum corneum, skin’s outer layer. We demonstrate that this non-linear behavior stems from the feedback loop on water transport. Dryer air should lead to a higher evaporation rate due to an increased chemical potential difference between the air and the solution. However, this variation is absorbed in a very thin and dry phase at the air/water interface. This phase corresponds to dramatically low water diffusion coefficients, which in turn efficiently decrease water evaporation [2]. Uncovering the mechanism of this homeostatic behavior opens new strategies to evaluate the impact of a formulation on skin, lung or tear films. We will also show that this mechanism becomes relevant when drying, or filtering, dispersions of interpenetrable colloids, such as microgels or “hairy” particles [3]. Indeed, large changes in water chemical potential and permeabilities will occur in the concentrated regime, in contrast to the drying of more conventional colloidal dispersions. Taking these molecular interactions into account is crucial for the processing of more complex, and thus realistic, colloidal dispersions into materials. Please click Additional Files below to see the full abstract