108 research outputs found
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 that compares
the particle size to the thickness of the struts in the foam structure, we
evidence a transition in the mechanical behavior at . For
, every particle loading leads to a strengthening effect whose
magnitude depends only on the particle volume fraction. On the contrary, for
, the strengthening effect weakens abruptly as a function of
and a softening effect is even observed for .
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
Shear induced drainage in foamy yield-stress fluids
Shear induced drainage of a foamy yield stress fluid is investigated using
MRI techniques. Whereas the yield stress of the interstitial fluid stabilizes
the system at rest, a fast drainage is observed when a horizontal shear is
imposed. It is shown that the sheared interstitial material behaves as a
viscous fluid in the direction of gravity, the effective viscosity of which is
controlled by shear in transient foam films between bubbles. Results provided
for several bubble sizes are not captured by the R^2 scaling classically
observed for liquid flow in particulate systems, such as foams and thus
constitute a remarkable demonstration of the strong coupling of drainage flow
and shear induced interstitial flow. Furthermore, foam films are found to be
responsible for the unexpected arrest of drainage, thus trapping irreversibly a
significant amount of interstitial liquid.Comment: Published in Physical Review Letters.
http://prl.aps.org/abstract/PRL/v104/i12/e12830
Coupling of elasticity to capillarity in soft aerated materials
We study the elastic properties of soft solids containing air bubbles.
Contrary to standard porous materials, the softness of the matrix allows for a
coupling of the matrix elasticity to surface tension forces brought in by the
bubbles. Thanks to appropriate experiments on model systems, we show how the
elastic response of the dispersions is governed by two dimensionless
parameters: the gas volume fraction and a capillary number comparing the
elasticity of the matrix to the stiffness of the bubbles. We also show that our
experimental results are in good agreement with computations of the shear
modulus through a micro-mechanical approach.Comment: submitted to Soft Matte
Rheological behaviour of suspensions of bubbles in yield stress fluids
The rheological properties of suspensions of bubbles in yield stress fluids
are investigated through experiments on model systems made of monodisperse
bubbles dispersed in concentrated emulsions. Thanks to this highly tunable
system, the bubble size and the rheological properties of the suspending yield
stress fluid are varied over a wide range. We show that the macroscopic
response under shear of the suspensions depends on the gas volume fraction and
the bubble stiffness in the suspending fluid. This relative stiffness can be
quantified through capillary numbers comparing the capillary pressure to stress
scales associated with the rheological properties of the suspending fluid. We
demonstrate that those capillary numbers govern the decrease of the elastic and
loss moduli, the absence of variation of the yield stress and the increase of
the consistency with the gas volume fraction, for the investigated range of
capillary numbers. Micro-mechanical estimates are consistent with the
experimental data and provide insight on the experimental results.Comment: submitted to Journal of non Newtonian Fluid Mechanic
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
On the collapse pressure of armored bubbles and drops
International audienceDrops and bubbles wrapped in dense monolayers of hydrophobic particles are known to sustain a significant decrease of their internal pressure. Through dedicated experiments we investigate the collapse behavior of such armored water drops as a function of the particle-to-drop size ratio in the range 0.02-0.2. We show that this parameter controls the behavior of the armor during the deflation: at small size ratios the drop shrinkage proceeds through the soft crumpling of the monolayer, at intermediate ratios the drop becomes faceted, and for the largest studied ratios the armor behaves like a granular arch. The results show that each of the three morphological regimes is characterized by an increasing magnitude of the collapse pressure. This increase is qualitatively modeled thanks to a mechanism involving out-of-plane deformations and particle disentanglement in the armor
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
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
How Topological Rearrangements and Liquid Fraction Control Liquid Foam Stability
International audienceThe stability of foam is investigated experimentally through coalescence events. Instability (coalescence) occurs when the system is submitted to external perturbations (T1) and when the liquid amount in the film network is below a critical value. Microscopically, transient thick films are observed during film rearrangements. Film rupture, with coalescence and eventual collapse of the foam, occurs when the available local liquid amount is too small for transient films to be formed. Similar experiments and results are shown in the two-bubble case
Foam drainage study during plateau border mineralisation
International audienceWe investigate the drainage of a foaming solution during inorganic polycondensation by macroscopic measurements and local observations. We reveal an original mineralisation mechanism starting from Plateau border interfaces. This slow process is not able to counteract the destabilizing effects of foam drainage and we therefore propose a new strategy in which mineralisation is assisted by a biopolymer
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