Arches and avalanches in particle-laden interfaces

The project Arches and Avalanches in Particle-Laden Interfaces focuses on liquid interfaces stabilized by a dense layer of micro-particles which adsorb onto them. This adsorption and the resulting stabilization are driven by capillarity i.e. by the fact that at micro- and milli- metric scales the shape of a liquid portion is fixed by the minimization of its surface energy. While largely unknown from most of us, these interfaces are common both in nature - in the form for example of rain drops covered by pollen, dust or sand - and in industrial products such as emulsions and ultra-stable foams Yet, to date, the relation between the macroscopic properties of these interfaces and the attributes of the individual particles which constitute them remains poorly understood. The lack of knowledge is especially problematic for large and fast deformations which indeed correspond to the typical conditions encountered during industrial processes. The present project aims to fill this gap following an original experimental approach. The latter is based on two aspects: the careful engineering of particles showing different and controlled properties and the observation of the interfacial compression/decompression at the micro, meso and macro levels simultaneously. To do so an interferometer will track the position of individual particles with respect to the liquid interface, high speed imaging will record the changes in particle packing and density at the level of tens to hundreds of particles while changes of shape and pressure will be measured at the interface level. The results will be used to improve existing models which mainly fail because they rely on a continuous approach considering the interfaces as elastic membranes. Our data should enable us to account for their granular character and therefore render unexplained phenomena such as arches and avalanches.

Microparticles can get trapped at the interface between two fluids and build there dense assemblies. The latter reduce the surface energy and stabilize the covered interfaces by further preventing direct contact between the armored fluid and its surrounding. These microparticles therefore appear as promising materials to replace surfactants. These molecules, essential to the production or foam or emulsions cannot be easily recovered and are toxic for most living organisms. Yet, in contrast to surfactants, microparticles are only found at the interface and cannot build reservoirs in the fluid to close holes that could open at the interface. To better evaluate these limitations, we studied how compressed particle rafts relax under the sudden and local release of the compression. We showed that in contrast to continuous media such as isotropic solids or fluids, the answer of these granular monolayer strongly depends on the direction of compression. We explained these results by the development of a network made of particle chains that transmit the stress with the entire system. Thus, for holes opening on the side from which compression was applied, keystone particles are removed and an avalanche develops providing many particles, which quickly heal the interface. In contrast, if the hole is found between branches of these chains, particles remain jammed cannot fill the hole in the coverage. Our project also demonstrated the importance of the hole size, of the initial compression level of the raft, of its initial length which fixes the distance to the hole to be filed and unexpectedly to the "age" of the particles. Here "age" must be understood as the movements they were subjected to while being at the interface. Fresh particles were simply sprinkled while old ones have been mixed for several minutes. We interpret these findings considering the possible ageing of the contact line that develops between each particles. The latter may influence the capillary lateral interactions of these particles as well as the friction between them and thus modify the assembly cohesion and its self-healing ability. Finally, to characterize the dynamics of the contact lines found around the particles, we developed a novel interferometric method offering very high temporal and spatial resolutions. The method can be used to investigate interfacial deflections regardless their origin and should therefore be of interest for other applications.