Viscosity of cohesive granular flows.

Cohesive granular materials such as wet sand, snow, and powders can flow like a viscous liquid. However, the elementary mechanisms of momentum transport in such athermal particulate fluids are elusive. As a result, existing models for cohesive granular viscosity remain phenomenological and debated. Here we use discrete element simulations of plane shear flows to measure the viscosity of cohesive granular materials, while tuning the intensity of inter-particle adhesion. We establish that two adhesion-related, dimensionless numbers control their viscosity. These numbers compare the force and energy required to break a bond to the characteristic stress and kinetic energy in the flow. This progresses the commonly accepted view that only one dimensionless number could control the effect of adhesion. The resulting scaling law captures strong, non-Newtonian variations in viscosity, unifying several existing viscosity models. We then directly link these variations in viscosity to adhesion-induced modifications in the flow micro-structure and contact network. This analysis reveals the existence of two modes of momentum transport, involving either grain micro-acceleration or balanced contact forces, and shows that adhesion only affects the latter. This advances our understanding of rheological models for granular materials and other soft materials such as emulsions and suspensions, which may also involve inter-particle adhesive forces

Homogénéité des caractères sédimentologiques des sables ogoliens entre Nouakchott (Mauritanie) et Mbour (Sénégal)

L'étude de l'ensemble dunaire ogolien (20.000-12.000 ans BP) le long du littoral de la Mauritanie méridionale et du Sénégal permet de montrer la très grande homogénéité des caractères sédimentologiques de la nappe sableuse. Les études ultérieures permettront de préciser les limites de cette couverture sableuse ogolienne et d'expliquer les mécanismes de son homogénéisatio

Simple Model for Wet Granular Materials with Liquid Clusters

We propose a simple phenomenological model for wet granular media to take into account many particle interaction through liquid in the funicular state as well as two-body cohesive force by a liquid bridge in the pendular state. In the wet granular media with small liquid content, liquid forms a bridge at each contact point, which induces two-body cohesive force due to the surface tension. As the liquid content increases, some liquid bridges merge, and more than two grains interact through a single liquid cluster. In our model, the cohesive force acts between the grains connected by a liquid-gas interface. As the liquid content increases, the number of grains that interact through the liquid increases, but the liquid-gas interface may decrease when liquid clusters are formed. Due to this competition, our model shows that the shear stress has a maximum as a function of the liquid-content.Comment: 6 pages, 8 figures. Discussion is updated. Accepted for publication in EP

Internal relaxation time in immersed particulate materials

We study the dynamics of the solid to liquid transition for a model material made of elastic particles immersed in a viscous fluid. The interaction between particle surfaces includes their viscous lubrication, a sharp repulsion when they get closer than a tuned steric length and their elastic deflection induced by those two forces. We use Soft Dynamics to simulate the dynamics of this material when it experiences a step increase in the shear stress and a constant normal stress. We observe a long creep phase before a substantial flow eventually establishes. We find that the typical creep time relies on an internal relaxation process, namely the separation of two particles driven by the applied stress and resisted by the viscous friction. This mechanism should be relevant for granular pastes, living cells, emulsions and wet foams

On the role of chirality in structure-odor relationships

The influence of chirality on odors was studied on 16 enantiomeric pairs according to the dispersion/hydrogen bonding theory of receptor-odorant interaction. Comparisons of molecular structures were made by superimposition of optimized conformations, using the Alchemy II package. The quality of fit was assessed using the RMS parameter included in Alchemy II and a new index for hydrogen bonding: the angle between H-bonds in the two molecules. In the case of camphoraceous odorants where an interaction model was already known superimposition according to the model led to correct predictions of the high similarity of odors observed in enantiomeric pairs. For several urinous odorants comparisons were made using d-androstenone as a reference compound for the urinous odor. Correct predictions were obtained for l-androstenone, both enantiomers of androsta-4, 16-dienone, and (+)-2-methyl-4-(5,5,6-exo-trimethy1-2-exonorbornyl)-cyclobexane. The (−) enantiomer of the latter compound was correctly predicted only if it was assumed that its weak intensity is due to a partial interaction with the hydrophobic zone of the receptor. For ambergris odorants which have a complex odor (−)-Ambrox was selected as the reference compound. The odors of (+)-Ambrox and enantiomers of four other compounds (ambergris or woody) were correctly predicted by superimposition. For nootkatone and three derivatives which have a grapefruit note for one enantiomer and a woody note for the other no models or reference compounds were available. The superimpositions were made between grapefruit enantiomers, on the one hand, and woody enantiomers on the other hand. Woody and grapefruit characters were correctly predicted in all cases. The limits of this approach based on molecular modelling are discusse

Dense flows of cohesive granular materials

International audienceUsing molecular dynamic simulations, we investigate the characteristics of dense flows of model cohesive grains. We describe their rheological behavior and its origin at the scale of the grains and of their organization. Homogeneous plane shear flows give access to the constitutive law of cohesive grains which can be expressed by a simple friction law similar to the case of cohesionless grains, but intergranular cohesive forces strongly enhance the resistance to the shear. Then we show the consequence on flows down a slope: a plugged region develops at the free surface where the cohesion intensity is the strongest. Moreover, we measure various indicators of the microstructure within flows which evidence the aggregation of grains due to cohesion and we analyze the properties of the contact network (force distributions and anisotropy). This provides new insights into the interplay between the local contact law, the microstructure and the macroscopic behavior of cohesive grains

Thermal transients and convective particle motion in dense granular materials.

The mechanism of dry granular convection within dense granular flows is mostly neglected by current analytical heat equations describing such materials, for example, in geophysical analyses of shear gouge layers of earthquake and landslide rupture planes. In dry granular materials, the common assumption is that conduction by contact overtakes any other mode of heat transfer. Conversely, we discover that transient correlated motion of heated grains can result in a convective heat flux normal to the shear direction up to 3-4 orders magnitude larger than by contact conduction. Such a thermal efficiency, much higher than that of water, is appealing and might be common to other microscopically structured fluids such as granular pastes, emulsions, and living cells