Serge Mora

Surface instability of soft solids under strain

Using a uniaxial deformation setup, we show that the free surface of an homogeneous elastic material is unstable under compression: parallel grooves nucleate orthogonally to the direction of compression when a characteristic stretch ratio a* is reached. We measure experimentally the variation of a* as well as the wavelength of the grooves as a function of the thickness h0 of the material. All data collapse on single curves when normalizing h0 by a characteristic length which is the ratio of the surface tension to the shear modulus of the material. This length scale acts as a regularization parameter for the system. We introduce a theoretical model that captures well the features of the instability. The observed nucleation-like process for the grooves development suggests that the instability is subcritical.

Fractures in complex fluids: the case of transient networks

We present a comprehensive review of the current state of fracture phenomena in transient networks, a wide class of viscoelastic fluids. We will first define what is a fracture in a complex fluid and recall the main structural and rheological properties of transient networks. Secondly, we review experimental reports on fractures of transient networks in several configurations: shear-induced fractures, fractures in Hele–Shaw cells, and fracture in extensional geometries (filament-stretching rheometry and pendant drop experiments), including fracture propagation. The tentative extension of the concepts of brittleness and ductility to the fracture mechanisms in transient networks is also discussed. Finally, the different and apparently contradictory theoretical approaches developed to interpret fracture nucleation will be addressed and confronted to experimental results. Rationalized criteria to discriminate the relevance of these different models will be proposed.

Structure and rheological properties of model microemulsion networks filled with nanoparticles.

Abstract.Model microemulsion networks of oil droplets stabilized by non-ionic surfactant and telechelic polymer C18 -PEO(10k)- C18 have been studied for two droplet-to-polymer size ratios. The rheological properties of the networks have been measured as a function of network connectivity and can be described in terms of simple percolation laws. The network structure has been characterised by Small Angle Neutron Scattering (SANS). A Reverse Monte Carlo (RMC) approach is used to demonstrate the interplay of attraction and repulsion induced by the copolymer. These model networks are then used as matrix for the incorporation of silica nanoparticles (R = 10 nm), individual dispersion being checked by scattering. A strong impact on the rheological properties is found for silica volume fractions up to 9%.

Propagation of a brittle fracture in a viscoelastic fluid

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Gravity driven instability in elastic solid layers.

During pendant drop experiments, a model physical gel made from oil in water microemulsion droplets reversibly linked together by triblock copolymers, exhibit a very peculiar filament rupture corresponding to highly brittle failure of a viscoelastic fluid. The fracture propagation has been tracked by high speed videomicroscopy. Analysis of the time evolution of the fracture profile shows that the fracture is purely elastic and reversible without any significant bulk and interfacial viscous dissipation. However, since the elastic moduli of such complex fluids are low, hyper elastic corrections have to be taken into account for a quantitative analysis of the fracture profile. This brittle behavior is well explained by a hyperelastic generalization of the viscoelastic trumpet model of de Gennes. The velocity of the fractures propagation is measured and compared to the predictions of a simple microscopic model.

The kinetic approach to fracture in transient networks

We study the structure and dynamics of a transient network composed of droplets of microemulsion connected by telechelic polymers. The polymer induces a bridging attraction between droplets without changing their shape. A viscoelastic behaviour is induced in the initially liquid solution, characterized in the linear regime by a stretched-exponential stress relaxation. We analyse this relaxation in the light of classical theories of transient networks. The role of the elastic reorganizations in the deformed network is emphasized. In the non-linear regime, a fast relaxation dynamics is followed by a second one having the same rate as that in the linear regime. This behaviour, in step strain experiments, should induce a non-monotonic behaviour in the elastic component of the stress for a constant shear rate. However, we obtain in this case a singularity in the flow curve very different from the one observed in other systems, that we interpret in terms of fracture behaviour.

Microemulsion nanocomposites: phase diagram, rheology and structure using a combined small angle neutron scattering and reverse Monte Carlo approach

We demonstrate the instability of the free surface of a soft elastic solid facing downwards. Experiments are carried out using a gel of constant density ρ, shear modulus μ, put in a rigid cylindrical dish of depth h. When turned upside down, the free surface of the gel undergoes a normal outgoing acceleration g. It remains perfectly flat for ρgh/μ<α* with α*≃6, whereas a steady pattern spontaneously appears in the opposite case. This phenomenon results from the interplay between the gravitational energy and the elastic energy of deformation, which reduces the Rayleigh waves celerity and vanishes it at the threshold.

Stress relaxation in model transient networks: percolation and rearrangement of the crosslinks

We introduce a model describing failure in transient networks. The two main ingredients are (i) the lifetime of a non connecting bond is far smaller than that of an active bond, (ii) the lifetime of an active bond is a decreasing function of the force carried by this bond. We show that these assumptions are sufficient to predict a failure threshold. Below this threshold the bonds distribution is driven by a diffusion equation. The bonds are redistributed homogeneously in the sample, leading to its self-adhesive feature. Beyond this threshold, the diffusion coefficient is negative, causing a catastrophic amplification of any heterogeneity. The final stage is the fracture. Finally, we give an interpretation of delayed fractures for these kinds of materials.

Off-equilibrium surface tension in colloidal suspensions.

The effect of silica nanoparticles on transient microemulsion networks made of microemulsion droplets and telechelic copolymer molecules in water is studied, as a function of droplet size and concentration, amount of copolymer, and nanoparticle volume fraction. The phase diagram is found to be affected, and in particular the percolation threshold characterized by rheology is shifted upon addition of nanoparticles, suggesting participation of the particles in the network. This leads to a peculiar reinforcement behaviour in such microemulsion nanocomposites, the silica influencing both the modulus and the relaxation time. The reinforcement is modelled based on nanoparticles connected to the network via droplet adsorption. Contrast-variation small angle neutron scattering coupled to a reverse Monte Carlo approach is used to analyse the microstructure. The rather surprising intensity curves are shown to be in good agreement with the adsorption of droplets on the nanoparticle surface.