Elasticity of an interfacial particle raft

We study the collective behaviour of a close packed monolayer of non-Brownian particles at a fluid-liquid interface. Such a particle raft forms a two-dimensional elastic solid and can support anisotropic stresses and strains, e.g. it buckles in uniaxial compression and cracks in tension. We characterise this solid in terms of a Young's modulus and Poisson ratio derived from simple theoretical considerations and show the validity of these estimates by using an experimental buckling assay to deduce the Young's modulus.Comment: 7 pages, 5 figure

Scale-free channeling patterns near the onset of erosion of sheared granular beds

Erosion shapes our landscape and occurs when a sufficient shear stress is exerted by a fluid on a sedimented layer. What controls erosion at a microscopic level remains debated, especially near the threshold forcing where it stops. Here we study experimentally the collective dynamics of the moving particles, using a set-up where the system spontaneously evolves toward the erosion onset. We find that the spatial organization of the erosion flux is heterogeneous in space, and occurs along channels of local flux $\sigma$ whose distribution displays scaling near threshold and follows $P(\sigma)\sim J/\sigma$, where $J$ is the mean erosion flux. Channels are strongly correlated in the direction of forcing but not in the transverse direction. We show that these results quantitatively agree with a model where the dynamics is governed by the competition of disorder (which channels mobile particles) and particle interactions (which reduces channeling). These observations support that for laminar flows, erosion is a dynamical phase transition which shares similarity with the plastic depinning transition occurring in dirty superconductors. The methodology we introduce here could be applied to probe these systems as well.Comment: 8 pages, 6 figure

Surface free energy and microarray deposition technology

Microarray techniques use a combinatorial approach to assess complex biochemical interactions. The fundamental goal is simultaneous, large-scale experimentation analogous to the automation achieved in the semiconductor industry. However, microarray deposition inherently involves liquids contacting solid substrates. Liquid droplet shapes are determined by surface and interfacial tension forces, and flows during drying. This article looks at how surface free energy and wetting considerations may influence the accuracy and reliability of spotted microarray experiments

The Shape and Motion of a Ruck in a Rug

The motion of a ruck in a rug is used as an analogy to explain the role of dislocations in the deformation of crystalline solids. We take the analogy literally and study the shape and motion of a bump, wrinkle or ruck in a thin sheet in partial contact with a rough substrate in a gravitational field. Using a combination of experiments, scaling analysis and numerical solutions of the governing equations, we first quantify the static shape of a ruck on a horizontal plane. When the plane is inclined, the ruck becomes asymmetric and moves by rolling only when the the inclination of the plane reaches a critical angle. We find that the angle at which this first occurs is larger than the angle at which the ruck stops, i.e. static rolling friction is larger than dynamic rolling friction. Once the ruck is in motion, it travels at a constant speed proportional to the sine of the angle of inclination, a result that we rationalize in terms of a simple power balance. We conclude with a simple implication of our study for the onset of rolling motion at soft interfaces.Comment: 4 pages, 4 figure

Discharge flow of a granular media from a silo: effect of the packing fraction and of the hopper angle

International audienceSilos are widely used in the industry. While empirical predictions of the flow rate, based on scaling laws, have existed for more than a century (Hagen 1852, translated in [1]-Beverloo et al. [2]), recent advances have be made on the understanding of the control parameters of the flow. In particular, using continuous modeling together with a mu(I) granular rheology seem to be successful in predicting the flow rate for large numbers of beads at the aperture (Staron et al.[3], [4]). Moreover Janda et al.[5] have shown that the packing fraction at the outlet plays an important role when the number of beads at the apeture decreases. Based on these considerations, we have studied experimentally the discharge flow of a granular media from a rectangular silo. We have varied two main parameters: the angle of the hopper, and the bulk packing fraction of the granular material by using bidisperse mixtures. We propose a simple physical model to describe the effect of these parameters, considering a continuous granular media with a dilatancy law at the outlet. This model predicts well the dependance of the flow rate on the hopper angle as well as the dependance of the flow rate on the fine mass fraction of a bidisperse mixture

Charriage de particules dans un écoulement cisaillé

Colloque avec actes et comité de lecture. Internationale.National audienceUn lit de particules soumis à un écoulement de fluide, par exemple le lit d'une rivière, se met en mouvement quand les forces hydrodynamiques deviennent supérieures à une fraction du poids apparent des particules. Nous étudions expérimentalement le transport de particules dans un tube à section rectangulaire. Nous comparons les résultats aux prédictions d'un modèle continu à deux phases, dans lequel nous utilisons une rhéologie granulaire pour la contrainte solide

Bedload Transport. Part 1 : Two-Phase Model and 3D Numerical Implementation.

GENCI- IDRIS (Grant 2010- 96212)International audienceWe have developed a three dimensional numerical model (Chauchat and Médale, 2010) based on the two-phase modeling having a Newtonian rheology for the fluid phase and Coulombtype friction for the particulate phase which has been developed by Ouriemi et al. (2009a) to study bedload transport in pipe flows. The governing equations are discretized by a finite element scheme and a penalisation method is introduced to cope with the incompressibility constraint. A regularisation technique is used to deal with the visco-plastic behaviour of the granular phase. We have performed three-dimensional computations for bedload transport in rectangular cross-section duct when the bed interface remains fixed. This numerical model which captures the complex coupling between the granular media and the fluid should enable a better understanding of the sediment transport mechanisms in these duct flows (Ouriemi et al., 2009b). The geometry corresponds to the one used in Pailha et al. (2011) where the authors investigate experimentally the behaviour of the flowing granular layer. The presented model and numerical results will be used for a detailed comparison in a near future

Investigation of the mobile granular layer in bedload transport by laminar shearing flows

International audienceThe mobile layer of a granular bed composed of spherical particles is experimentally investigated in a laminar rectangular channel flow. Both particle and fluid velocity profiles are obtained using particle image velocimetry for different index-matched combinations of particles and fluid and for a wide range of fluid flow rates above incipient motion. A full three-dimensional investigation of the flow field inside the mobile layer is also provided. These experimental observations are compared to the predictions of a two-phase continuum model having a frictional rheology to describe particle-particle interactions. Different rheological constitutive laws having increasing degrees of sophistication are tested and discussed

Liquid marbles: principles and applications

The ability of particles to adhere to a fluid–fluid interface can stabilize the formation of an emulsion. When the encapsulated fluid is a liquid and the fluid in which it is immersed is air, the object formed is called a “Liquid Marble”. Here we discuss how liquid marbles can be created, their fundamental properties and their transport and potential uses. We show how they arise naturally as an insect waste disposal system, from impact of droplets on powders and on hydrophobic soil, and in the mixing of particulate containing liquids. Our principal aim is to review research on macroscopic single marbles and their potential uses in sensors and droplet microfluidics. However, we also illustrate the similarity between liquid marbles, Pickering emulsions and “Dry Water”, and the potential application of assemblies of liquid marbles within cosmetics and pharmaceutical formulations. Finally, we discuss how modifying the surface structure of particles and providing heterogeneous surface chemistry on particles (e.g. Janus particles) might provide new types of liquid marbles and applications

Bedload Transport. Part 2: The mobile granular Layer

International audienceWe present a joint theoretical, numerical, and experimental investigation of the mobile granular layer in bedload transport conditions for pipe flows. The theoretical approach uses a twophase model having a Newtonian rheology for the fluid phase and Coulomb-type friction for the particulate phase which has been recently proposed by Ouriemi et al. (2009a). This model has been implemented into a 3D numerical code by Chauchat and Médale (2010) which can describe bedload transport in square and circular cross-section ducts. The experiments are undertaken in a rectangular duct partially filled with transparent spherical particles driven by an index-matched fluid. Direct imaging of the particles and of the fluid in a vertical slice is obtained owing to the addition of fluorescing tracers and to the illumination of the duct by a laser sheet. The main quantity that will be examined and discussed is the velocity field