Emmanuelle Rio

On the origin of the remarkable stability of aqueous foams stabilised by nanoparticles: link with microscopic surface properties

We have performed a quantitative study of the coarsening of foams stabilised by partially hydrophobic silica nanoparticles. We have used a variety of techniques: optical and electron microscopy, microfluidics, and multiple light scattering. Using earlier studies of planar particle monolayers, we have been able to correlate the interfacial properties and the macroscopic temporal evolution of the foam. This has shed light on the origin of the absence of coarsening of particle-stabilised foams. Such particle-stabilised foams appear to be the only known foam system where coarsening is inhibited by surface elasticity.

Aqueous foams stabilized solely by particles

Foams are dispersions of bubbles in liquids, often water. They are frequently stabilized by surfactant or polymer, but like Pickering emulsions, they can be also stabilized solely by particles. If the particles have a moderate hydrophobicity, the foams can be extremely stable (lifetimes of the order of years). Due to technical preparation difficulties, very few studies can be found in the literature to date. We will discuss the origin of these difficulties. We will also describe experiments using partially hydrophobic fumed silica particles, in which the foam properties were correlated with the properties of spread and adsorbed layers of these particles at the air–water interface. These combined experiments allowed us to define the conditions necessary to stop bubble disproportionation.

Unusually stable liquid foams.

Obtaining stable liquid foams is an important issue in view of their numerous applications. In some of these, the liquid foam in itself is of interest, in others, the liquid foam acts as a precursor for the generation of solid foam. In this short review, we will make a survey of the existing results in the area. This will include foams stabilised by surfactants, proteins and particles. The origin of the stability is related to the slowing down of coarsening, drainage or coalescence, and eventually to their arrest. The three effects are frequently coupled and in many cases, they act simultaneously and enhance one another. Drainage can be arrested if the liquid of the foam either gels or solidifies. Coalescence is slowed down by gelified foam films, and it can be arrested if the films become very thick and/or rigid. These mechanisms are thus qualitatively easy to identify, but they are less easy to model in order to obtain quantitative predictions. The slowing down of coarsening requests either very thick or small films, and its arrest was observed in cases where the surface compression modulus was large. The detail of the mechanisms at play remains unclear.

Viscoelastic properties of silica nanoparticle monolayers at the air-water interface

We have investigated the rheological behaviour of silica nanoparticle layers at the air-water interface. Both compressed and deposited layers have been studied in Langmuir troughs and with a bicone rheometer. The compressed layers are more homogeneous and rigid, and the elastic response to continuous, step and oscillatory compression are similar, provided the compression is fast enough and relaxation is prevented. The deposited layers are less rigid and more viscoelastic. Their shear moduli deduced from the oscillatory uniaxial compression are much smaller than those deduced from pure shear deformation suggesting that the effective shear rate is smaller than expected in the compression measurements.

On the origin of the stability of foams made from catanionic surfactant mixtures

Using mixtures of the anionic myristic acid (C13COOH) and the cationic cetyl trimethylammonium chloride (C16TA+Cl−) in aqueous solutions at a 2:1 ratio, we show that the outstanding stability of foams generated from sufficiently concentrated “catanionic” surfactant mixtures can be explained by a synergy effect between two fundamentally different mechanisms. Applying a multi-scale approach, in which we link static and dynamic properties of the bulk solutions, isolated gas/liquid interfaces, thin liquid films and foams, we identify these two mechanisms to be as follows: firstly, cationic mixtures create tightly packed surfactant layers at gas/liquid interfaces, which are strongly viscoelastic and also confer high disjoining pressures when two interfaces are approaching each other to form a thin liquid film. Foams created with such kind of interfaces tend to be extremely stable against coalescence (film rupture) and coarsening (gas exchange). However, typical time scales to cover the interfaces are much longer than typical foaming times. This is why a second mechanism plays a key role, which is due to the presence of micron-sized catanionic vesicles in the foaming solution. The bilayers of these vesicles are in a gel-like state, therefore leading to nearly indestructible objects which act like elastic micro-spheres. At sufficiently high concentrations, these vesicles jam in the presence of the confinement between bubbles, slowing down the drainage of liquid during the initial foaming process and therefore providing time for the interfaces to be covered. Furthermore, the tightly packed vesicles strongly reduce bubble coalescence and gas transfer between bubbles.

On the long-term stability of foams stabilised by mixtures of nano-particles and oppositely charged short chain surfactants

We have studied the foaming properties of aqueous dispersions containing mixtures of silica nano-particles (Ludox TMA) and a short-chain amphiphile (n-amylamine). By combining standard hand shaking methods and microfluidic techniques we show that stable foams can be obtained at amine concentrations above approximately 0.5 wt%, which appears to be a critical concentration for cooperative association between particles and amine. In contrast to foams stabilised solely by nano-particles, these foams suffer from slow coarsening due to gas exchange between bubbles. “Superstable” foams for which coarsening is inhibited can only be produced at sufficiently high particle and amine concentrations (typically 10 and 3 wt%, respectively) for which the dispersions also gel in the continuous phase of the foam. We combine investigations of the static and dynamic properties of the particle-laden air–water interfaces in an attempt to elucidate some of the key mechanisms which control the observed behaviour.

Dual gas and oil dispersions in water: production and stability of foamulsion

In 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.

The role of surface rheology in liquid film formation

The role of surface rheology in fundamental fluid dynamical systems, such as liquid coating flows and soap film formation, is poorly understood. We investigate the role of surface viscosity in the classical film-coating problem. We propose a theoretical model that predicts film thickening based on a purely surface-viscous theory. The theory is supported by a set of new experimental data that demonstrates slight thickening even at very high surfactant concentrations for which Marangoni effects are irrelevant. The model and experiments represent a new regime that has not been identified before.

Influence of the contact angle of silica nanoparticles at the air–water interface on the mechanical properties of the layers composed of these particles

We have studied the properties (surface pressure, compression and shear moduli, texture) of silica nanoparticle layers at the air–water interface. Particle hydrophobicity or, equivalently, the contact angle between particles, air and water, is the main factor that influences surface organization and surface elastic moduli. The surface layers are denser for particles of higher hydrophobicity. The compression and shear moduli, as well as the yield and melt strains, present a maximum for contact angles around 90°. The dependence of the mechanical properties on particle hydrophobicity is closely related to the foamability and stability of the foams made from dispersions.

Liquid dispersions under gravity: volume fraction profile and osmotic pressure

Many physical properties of concentrated dispersions of immiscible fluids are captured by the concept of an osmotic pressure, which measures how much energy is required to deform the bubbles or drops upon compaction. This pressure has a strong impact on the flow and drainage behavior of dispersions. Nevertheless, theoretical models describing its variation with the volume fraction ϕ of the continuous phase are so far available only in the limits of low or high ϕ and experimental data are scarce. We report an experimental study of osmotic pressure in foams and emulsions, showing how the effects of ϕ, disorder, grain size, polydispersity and interfacial tension can all be captured by a single law which satisfies previously established theoretical constraints. Building on this result, we propose the first equation which accurately describes the variation of the volume fraction with the height of a fluid dispersion under gravity.