Aqueous foams, although composed solely of gas and liquid, have a structural stability that makes them similar to solids. Because they are permeable to gases and have large exchange surfaces, foams could be used as central elements in cheap gas filtration or capture systems. However, as their structure evolves as gas exchanges take place and the alveoli deflate or expand, understanding the kinetics of gas transfer when diffusive processes combine with rearrangements of the alveoli in the foam is a fundamental step towards designing and optimizing such capture systems.

To better observe the evolution of the structure of the foam as gas exchanges take place, a team from the Laboratory of Interdisciplinary Physics in Grenoble has designed a two-dimensional experiment where the evolution of the structure can be followed in real time. The researchers chose to study more specifically the transfer kinetics of a CO2 /air mixture, a configuration that can be found, for example, at the outlet of a factory exhaust, where one would like to be able to separate the CO2 from the other components in order to store it.



Direct observation of the size of the alveoli has made it possible to develop and validate a predictive model for gas flows and for the overall size and structure of the foam, under the effect of gas diffusive flows through the liquid films. The driving force behind diffusion is the different capacity of the involved gases to dissolve in water, i.e. to cross the gas/liquid interfaces; that of CO2, for example, is around 10 times greater than that of air. Even though usual gases diffuse similarly in a homogeneous medium, whether in the liquid or gaseous phase, the team has shown that this difference in solubility is the key to gas exchange in a foam, which can be finely controlled by choosing the gases and the initial mixing conditions.

Building on this expertise, the team is currently developing an air/ CO2 separation device, using another of the foams' assets compared with exchangers based on solid materials: the possibility of inducing flows within the entire structure offers great potential for regenerating the absorbent substrate, enabling continuous filtration of a CO2 -laden gas.