Impact of the Gas Phase on the Properties of Foams

While the vast majority of everyday foams are made out of air, there is an increased interest in foaming using specific gases and/or mixtures, with multiple applications in food and beverages, pharmaceutical or energy industries. Generating a pure gas foam instead of a compressed air one can help intensifying a process, or resolving material compatibility issues (e.g. avoid the presence of oxygen within a specific process). When selecting a gas phase to generate a foam, it is also necessary to consider the interaction between this gas phase and the foam liquid phase (e.g. aqueous or oil-based), as well as with the surfactant molecules (anionic, cationic, non-ionic, amphoteric, etc.). Previous studies explored this theme most notably the strong stability of nitrogen foams compared to carbon dioxide ones. CO2 foams are notably unstable at standard conditions due to the high solubility of CO2 in water. Acidification is also pointed as a cause of micelles destabilization when anionic surfactant is used. However, at higher pressure, the roles are reversed, and CO2 foams have been proven more stable. This result is most simply explained by noting that the density of CO2 at high pressure is ~0,75 that of water at this pressure, whereas that of N2 is < 0.1 of water’s density. Thus the usual gravitational driving force for foam separation is much reduced in the case of high pressure CO2, and indeed the CO2-water mixture might be called an emulsion rather than a foam when CO2 is closer to supercritical conditions. 

Our present work aims at extending these empirical observations through the use of (i) additional gases and (ii) mixture of gases. As shown in previous studies, the choice of gas strongly influences the foam properties. In particular, we evaluated how the choice of gas impacts coarsening, coalescence and drainage phenomena which together determines the ability of the foam to remain stable. A better understanding of the impact of gas on foam can enable to design a foam system with desirable properties for each application. 

In the experiment part, foam properties, such as the  foamability index, foam height, bubble size/structure and its distribution evolution with time, were measured through the use of a foam analysis device for multiple gases, including air, nitrogen, argon, and carbon and extended to mixtures of gases. This leads to multiple key findings: i) At atmospheric conditions, previous studies about the relative instability of nitrogen and carbon dioxide foams are validated and extended to other gases / mixtures of gases, ii) We did not observe a significant gas-specific impact of the porous media used to generate foams and iii) To help better understand the instability caused by CO2, we also investigated foam properties using different surfactants.

In the theoretical part, the influence of gas properties, including surface tension, viscosity and gas solubility, on foam topology behavior was theoretically and numerically analyzed. These results were also compared with the experimental observations obtained using the foam analysis device.