Strengthening of foamed composite materials

We investigate the shear elastic modulus of soft polymer foams loaded with hard spherical particles and we show that, for constant bubble size and gas volume fraction, strengthening is strongly dependent on the size of those inclusions. Through an accurate control of the ratio $\lambda$ that compares the particle size to the thickness of the struts in the foam structure, we evidence a transition in the mechanical behavior at $\lambda \approx 1$. For $\lambda < 1$, every particle loading leads to a strengthening effect whose magnitude depends only on the particle volume fraction. On the contrary, for $\lambda > 1$, the strengthening effect weakens abruptly as a function of $\lambda$ and a softening effect is even observed for $\lambda \gtrsim 10$. This transition in the mechanical behavior is reminiscent of the so-called "particle exclusion transition" that has been recently reported within the framework of drainage of foamy granular suspensions [Haffner B, Khidas Y, Pitois O. The drainage of foamy granular suspensions. J Colloid Interface Sci 2015. In Press.]. It involves the evolution for the geometrical configuration of the particles with respect to the foam network, and it appears to control the mechanics of such foamy systems

The drainage of foamy granular suspensions

International audienceFoam-based materials are promising micro-structured materials with interesting thermal and acoustical properties. The control of the material morphology requires counteracting all the destabilizing mechanisms during their production, starting with the drainage process, which remains to be understood in the case of the complex fluids that are commonly used to be foamed. Here we perform measurements for the drainage velocity of aqueous foams made with granular suspensions of hydrophilic monodisperse particles and we show that the effect of particles can be accounted by two parameters: the volume fraction of particles in the suspension (φ_p) and the confinement parameter (λ), that compares the particle size to the size of passage through constrictions in the foam network. We report data over wide ranges for those two parameters and we identify all the regimes and transitions occurring in the φ_p-λ diagram. In particular, we highlight a transition which refers to the included / excluded configuration of the particles with respect to the foam network, and makes the drainage velocity evolve from its minimal value (fully included particles) to its maximal one (fully excluded particles). We also determine the conditions (φ_p,λ) leading to the arrest of the drainage process

Capture-induced transition in foamy suspensions

International audienceWe investigate the drainage behaviour of foamy granular suspensions. Results reveal large fluctuations in the drainage velocity as bubble size, particle size and gas volume fraction are varied for a given particle volume fraction. Particle capture is proved to control the overall drainage behaviour through the parameter lambda, which compares the particle size to the size of passage through constrictions within the foam pore space. lambda highlights a sharp transition: for lambda 1 particles are trapped and the resulting drainage velocity is strongly reduced. A phenomenological model is proposed to describe this behaviour

Critical size effect of particles reinforcing foamed composite materials

International audienceWe investigate the shear elastic modulus of soft polymer foams loaded with hardspherical particles and we show that, for constant bubble size and gas volume fraction,strengthening is strongly dependent on the size of those inclusions. Through anaccurate control of the ratio λ that compares the particle size to the thickness of thestruts in the foam structure, we evidence a transition in the mechanical behavior atλ ≈ 1. For λ 1,the strengthening effect weakens abruptly as a function of and a softening effect iseven observed for λ ≳ 10. This transition in the mechanical behavior is reminiscent ofthe so-called “particle exclusion transition” that has been recently reported within theframework of drainage of foamy granular suspensions [Haffner B, Khidas Y, Pitois O.The drainage of foamy granular suspensions. J Colloid Interface Sci 2015;458:200-8]. Itinvolves the evolution for the geometrical configuration of the particles with respect tothe foam network, and it appears to control the mechanics of such foamy systems

Anisotropic nonlinear elasticity in a spherical bead pack: influence of the fabric anisotropy

Stress-strain measurements and ultrasound propagation experiments in glass bead packs have been simultaneously conducted to characterize the stress-induced anisotropy under uniaxial loading. These measurements, realized respectively with finite and incremental deformations of the granular assembly, are analyzed within the framework of the effective medium theory based on the Hertz-Mindlin contact theory. Our work shows that both compressional and shear wave velocities and consequently the incremental elastic moduli agree fairly well with the effective medium model by Johnson et al. [J. Appl. Mech. 65, 380 (1998)], but the anisotropic stress ratio resulting from finite deformation does not at all. As indicated by numerical simulations, the discrepancy may arise from the fact that the model doesn't properly allow the grains to relax from the affine motion approximation. Here we find that the interaction nature at the grain contact could also play a crucial role for the relevant prediction by the model; indeed, such discrepancy can be significantly reduced if the frictional resistance between grains is removed. Another main experimental finding is the influence of the inherent anisotropy of granular packs, realized by different protocols of the sample preparation. Our results reveal that compressional waves are more sensitive to the stress-induced anisotropy, whereas the shear waves are more sensitive to the fabric anisotropy, not being accounted in analytical effective medium models.Comment: 9 pages, 8 figure

Etude expérimentale du frottement et des rotations dans des milieux granulaires modernes

Nous étudions l'influence des rotations sur le comportement global d'un milieu granulaire à l'aide d'expériences sur des systèmes modèles 1D et 2D de cylindres confinés, soumis à leur base à un cisaillement.La théorie et les simulations des systèmes 1D prévoient un état stationnaire avec différents modes de rotation répartis spatialement. Les perturbations expérimentales modifient ces modes de rotations. Notre analyse détaillée des corrélations spatiales et temporelles montre que l'évolution du système est une superposition de différents états stationnaires de même nature que ceux de la théorie.Le système 2D ordonné montre une plus grande sensibilité aux perturbations, liée à l'inhomogénéité du réseau des contacts. Nous montrons l'organisation des rotations ainsi que l'influence de leur propagation dans le milieu sur la force de frottement globale mesurée.L'étude exploratoire des systèmes désordonnés polydisperses souligne l'effet drastique de la frustration de rotation. Après une réorganisation importante, le système évolue très lentement suivant un mouvement global de convection avec des glissements saccadés qui semblent jouer un rôle.RENNES1-BU Sciences Philo (352382102) / SudocSudocFranceF

Acoustic measurements of anisotropic elasticity in glass bead packings under uniaxial stress

International audienceCoherent elastic waves are employed here to haracterize the anisotropic elasticity of the dry 3D granular assembly in the oedometer test. Bulk compressional (P-) and shear (S-) waves are measured when they propagate espectively along the vertical (load-) direction and horizontal one, being polarized in different direction. The obtained results illustrate clearly the elastic anisotropy of the contact network. The pressure-dependent velocity is analyzed within the effective medium approach based on the Hertz-Mindlin contact theory. The influence of the initial texture of granular media is also examined using different sample preparation procedures. Combined with the density and stress field measurements, our acoustic measurements suggest that the anisotropic elasticity of a glass bead packing induced in the oedometer test reflects the effect of the initial anisotropic texture

Acoustic probing of the contact network dynamics in a granular medium during shear-band formation

International audienceWe investigate the quasistatic mechanical response of a glass bead pack during the shear band formation using a direct shear apparatus. Combined measurements of acoustic velocity and amplitude have been conducted to probe the microstructure evolution from the quasi-elastic regime to the steady-state flow. A clear signature of a shear band formation appears in acoustic transmission. Moreover, a correlation technique of multiply scattered waves is applied to examine the transition of granular media towards failure. The pulse-like loss of correlation is observed when approaching the yield stress, revealing an intermittent dynamics of the force network

Probing the shear-band formation in granular media with sound waves

International audienceWe investigate the mechanical responses of dense granular materials, using a direct shear box combined with simultaneous acoustic measurements. Measured shear wave speeds evidence the structural change of the material under shear, from the jammed state to the flowing state. There is a clear acoustic signature when the shear band is formed. Subjected to cyclic shear, both shear stress and wave speed show the strong hysteretic dependence on the shear strain, likely associated with the geometry change in the packing structure. Moreover, the correlation function of configuration-specific multiply scattered waves reveals an intermittent behavior before the failure of material

Yielding of complex liquid foams

International audienceFoamy complex matter is increasingly encountered in various application fields. Whereas final functional properties of hardened foams have been widely investigated, rheology of complex foamy materials has received less attention. Here, we consider two different types of complex matter to be mixed with aqueous foam and we investigate the resulting yield stress, which is known to control both static and flow properties of foams: (1) A concentrated emulsion possessing intrinsic yield stress properties and (2) granular suspensions which are known to exhibit particle size effects when incorporated into foam. Yield stress of emulsion foams is found to be governed by both the Bingham-capillary number (i.e., the emulsion yield stress scaled by the bubble capillary pressure) and the volume fraction of interstitial emulsion. Foams made with small solid particles reveal granular packings confined between foam bubbles and the resulting yield stress is shown also to be governed by the Bingham-capillary number, here based on Mohr-Coulomb criterion applied to the interstitial granular material. Yield stress of foams made with large particles is well described by the theory developed from micromechanics of particle-loaded yield stress materials, suggesting that for such size ratios, foam can be considered as a continuous matrix. Within the range of intermediate size ratio values, the normalized foam yield stress undergoes an exponential decay that seems to be related to the number of bubbles separating neighboring particles. Finally, reported results are used to estimate the thicknesses of a foamed material, such as isolating foamed mortars, that can be deposited on vertical substrates