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.

In an attempt to elucidate the remarkable stability of foams generated from dispersions of partially hydrophobic nanoparticles (fumed silica), we present investigations into the static and dilational properties of the gas–liquid interfaces of such dispersions. By relating the dynamic surface tension γ(t) and the dilational elasticity E measured using an oscillating bubble device, we confirm that the Gibbs stability criterion E > γ/2 against foam coarsening is fulfilled. We complement these studies using ellipsometry and Brewster angle microscopy, which provide evidence for a pronounced adsorption barrier for the particles and a network-like structure in the interface at sufficiently high concentrations. We observe this structure also in freely suspended films drawn from the same particle dispersions.

We have investigated the active motion of self-propelled colloids confined at the air–water interface and explored the possibility of enhancing the directional motion of self-propelled Janus colloids by slowing down their rotational diffusion. The two dimensional motion of micron-sized silica–platinum Janus colloids has been experimentally measured by particle tracking video-microscopy at increasing concentrations of the catalytic fuel, i.e. H₂O₂. Compared to the motion in the bulk, a dramatic enhancement of both the persistence length of trajectories and the speed has been observed. The interplay of colloid self-propulsion, due to an asymmetric catalytic reaction occurring on the colloid, surface properties and interfacial frictions controls the enhancement of the directional movement. The slowing down of the rotational diffusion at the interface, also measured experimentally, plays a pivotal role in the control and enhancement of active motion.