Pressure threshold for inhibition of dense granular film opening.

Controlling the stability of a granular film is essential in a wide range of industrial applications, from aerated building materials to recovering ore by flotation and treating wastewater. We therefore carry out experiments of granular film opening where particles of hundred of micrometers above random close packing zip the two interfaces of a soap film which liquid pressure is controlled. We create a hole at the center of this dense granular film and, surprisingly, we observe that the opening is not always inhibited. Different behaviours are identified: total bursting of the granular film, intermittent opening and jammed state for which the hole does not evolve. The liquid pressure drives the transition from one opening behaviour to another. Lower is the liquid pressure, more jammed is the system. The critical pressure transition scales as the surface tension over the particle size until the finite size of the granular film is only few tens of the particle size. Ultimately we evidence that spontaneous hole in thin film between particle do not lead to the granular film failure.

Daisy-shaped liquid bridges in foam-filled granular packings.

HYPOTHESIS: Many applications of liquid foams use them to fill the porosity of various granular media. How is the liquid distributed in such foam-filled systems, in which the geometry of the bubble assembly can be strongly constrained by pore confinement? EXPERIMENTS: We study how the liquid is distributed in a grain packing filled with liquid foam, as a function of both liquid content and bubble-to-grain size ratio. Moreover, Surface Evolver simulations are carried out at the scale of a single bubble confined into a tetrahedral pore. FINDINGS: We reveal that foam-filled granular assemblies exhibit a robust pendular-like regime, which is reminiscent of the pendular regime in unsaturated media. The main difference is that here the liquid bridges are daisy-shaped, i.e. with a liquid core bounded by bubbly petals. A simple theoretical model is proposed to describe the foam liquid bridges between contacting grains. In the case of large bubbles, the model is compared with the Surface Evolver simulation. The model is also applied to the case of wall liquid bridge, which is compared with the experimental observation. Beyond their geometrical characteristics, the presence of these liquid bridges, which can represent almost 25% of the liquid contained in the porosity, makes it possible to imagine a new approach (binder foam-based) to bind granular assemblies and turn them to solid materials.

The drainage of foamy granular suspensions.

Foam-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 the particles can be accounted by two parameters: the volume fraction of particles in the suspension (φp) 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 φp-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 (φp,λ) leading to the arrest of the drainage process.

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 an effective-medium model developed by Johnson [J. Appl. Mech. 65, 380 (1998)] for the oedometric test, 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 does not 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.

Origin of the slow linear viscoelastic response of aqueous foams.

We have studied the slow linear viscoelastic response of wet aqueous foams by macroscopic creep compliance measurements, combined to a diffusing-wave spectroscopy investigation of the local dynamics. The data strongly suggest that this rheological response arises from two distinct relaxation mechanisms: The first is due to the coarsening induced bubble rearrangements and governs the steady-state creep; the second results from the interplay between surface tension and surface viscosity of the gas-liquid interfaces and gives rise to a transient relaxation.