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We have studied foams stabilised by surfactant-decorated nanoparticles adsorbed at the bubble surfaces. We show that the controlled compression of a single bubble allows one to understand the coarsening behavior of these foams. When bubbles are compressed, the particles become tightly packed in the surface layer. They lose their mobility, and the interface becomes solid-like when the jammed state is reached. Further compression leads to interfacial buckling characterised by crumpled surfaces. We find that the surface concentration of particles at which the jamming and the buckling transitions occur are independent of the surfactant concentration. This is a surprising feature. It suggests that the surfactants are mandatory to help the particles adsorb at the interface and that they change the equilibrium surface concentration of the decorated particles. But they do not affect the surface properties once the particles are adsorbed. We measured the compression elastic modulus of the surface in the jammed state and found it to be compatible with the Gibbs condition for which the spontaneous dissolution of bubbles is arrested. Due to this effect, the coarsening process of a foam composed of many close-packed bubbles occurs in two steps. In the first step, coarsening is slow and coalescence of the bigger bubbles is observed. In the second step, a number of very small bubbles remains, which exhibit crumpled surfaces and are stable over long times. This suggests that foam coarsening is arrested once the smallest bubbles become fully covered after the initial shrinking step.

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To understand the non-equilibrium behavior of colloidal particles with short-range attraction, we studied salt-induced aggregation of lysozyme. Optical microscopy revealed four regimes: bicontinuous texture, 'beads', large aggregates, and transient gelation. The interaction of a metastable liquid-liquid binodal and an ergodic to non-ergodic transition boundary inside the equilibrium crystallization region can explain our findings.

International audienceIn this study we have investigated mixtures of oil droplets and gas bubbles and show that the oil can have two very different roles, either suppressing foaming or stabilising the foam. We have foamed emulsions made from two different oils (rapeseed and dodecane). For both oils the requirement for the creation of foamulsions is the presence of surfactant above a certain critical threshold, independent of the concentration of oil present. Although the foamability is comparable, the stability of the foamed emulsions is very different for the two oils studied. Varying a few simple parameters gives access to a wide range of behaviours, indeed three different stability regimes are observed: a regime with rapid collapse (within a few minutes), a regime where the oil has no impact, and a regime of high stability. This last regime occurs at high oil fraction in the emulsion, and the strong slowing down of ageing processes is due to the confinement of packed oil droplets between bubbles. We thus show that a simple system consisting of surfactant, water, oil and gas is very versatile and can be controlled by choosing the appropriate physical chemical parameters

We studied the crystallization of lysozyme solutions by adding sodium chloride at pH = 4.5, 5.9 and 7.8. A universal crystallization boundary is found if data are scaled according to the salt concentration normalized by the square of the charge at the appropriate pH. Calculations show that this finding is consistent with recent attempts to rationalize protein crystallization using second virial coefficients.

We determined the water intake of internally structured oil-loaded monoglyceride-based dispersions. This was possible through small-angle X-ray scattering (SAXS) experiments on the corresponding bulk mesophases because the structural parameters in full hydration conditions are identical to those of the dispersed particles. From low water contents to full hydration, the bulk phases depend strongly on the amount of oil. At room temperature in excess water and with increasing oil concentration, successive bicontinuous cubic, reverse hexagonal, micellar cubic, and inverse micellar-type isotropic fluid phases are found. The solubilized water is determined as a function of the oil content for each phase, and it is found to range from 5-33 wt %.

Mixed lecithin-bile salt micelles are known to have a cylindrical or worm-like structure. We investigated their shape, length, flexibility and cross-sectional structure using small-angle neutron scattering (SANS). A broad range of sample compositions was studied varying both the total amphiphile concentration and the molar ratio of bile salt (sodium taurochenodeoxycholate, NaTCDC) to lecithin (egg yolk phosphatidylcholine, EYL). The length of the micelles was quantitatively linked to the micellar composition by introducing a simple model. The model takes into account the partitioning of lecithin and bile salt between the bulk, cylindrical parts and the end caps of the micelles. The model also sheds light on the organization of the micelles, both in their cylindrical regions and end caps.