Anne R. Rousseau a,b,c, Alesya Mikhailovskaya d, Julien Lamolinairie e,f, Olivier Diat e, Leonardo Chiappisi f, Cécile Monteux a
a Sciences et Ingénierie de La Matière Molle, ESPCI Paris, PSL Research University, CNRS, Sorbonne Université, Paris 75005, France
b Institut de Recherche de Chimie Paris (IRCP), Chimie ParisTech, PSL Research University, CNRS, Paris 75005, France
c Agence de l’Environnement et de la Maîtrise de l’Energie (ADEME), 20 avenue du Grésillé, 49004 Angers, France
d Univ Paris Est Creteil, CNRS, Institut de Chimie et des Matériaux Paris-Est (ICMPE), UMR 7182, 94320 Thiais, France
e ICSM, Univ Montpellier, CEA, CNRS, ENSCM, Marcoule, France
f Institut Max von Laue – Paul Langevin (ILL) 71, Avenue des Martyrs – CS 20156 38042, Grenoble, cedex 9, France
Rousseau, A.; Mikhailovskaya, A.; Lamolinairie J.; Diat O.; Chiappisi L.; Monteux C. Internal structure of aqueous foams at steady-state during forced drainage. Submitted in Soft Matter
https://hal.science/hal-05531695v3
Aqueous foams consist of gas bubbles, usually air, dispersed in water. Although they may appear simple, their structure is complex, with liquid distributed between thin films separating neighbouring bubbles and thicker channels known as Plateau borders. Under gravity, liquid drains downwards while bubbles rise, progressively changing the foam structure and promoting ageing phenomena such as coarsening and coalescence. Understanding and controlling liquid drainage is therefore essential to predicting foam stability and transport properties.
A common approach to studying drainage is forced drainage, in which liquid is continuously injected at a controlled flow rate into an initially dry foam. This technique has been widely used to investigate liquid flow through foams and develop theoretical models describing their behaviour [1, 2]. However, its impact on the foam microstructure, particularly on the typical size or thickness of the thin liquid films separating bubbles, has remained poorly understood. These films are of particular interest because they can strongly influence the transport of chemical species through foams, including gaseous reactants [3, 4].
In this work, we investigated the evolution of foam microstructure during both free and forced drainage using small-angle neutron scattering (Figure). Neutron scattering provides access to structural features that are difficult to characterise in situ using conventional optical techniques, including the thickness of liquid films within opaque, three-dimensional foams. Building on a method developed by Lamolinairie et al. [5], we characterised the foam specific surface area and film thickness under different drainage conditions.
Our results show that forced drainage at low liquid flow rates provides a robust means of controlling not only the liquid fraction, but also the foam microstructure. In particular, the specific surface areas associated with the films and Plateau borders, as well as the thickness of the films, reach well-defined values under steady-state conditions. The measured foam specific surface area also agrees well with predictions from simulations of random foams, especially under forced drainage. This demonstrates that, under appropriate conditions, liquid flow does not prevent the establishment of a predictable foam structure.
Beyond structural control, forced drainage also influences the chemical composition of the liquid within the foam. While previous work has shown that micelles can accumulate during free drainage [6], our results reveal that forced drainage can rinse these micelles out of the foam, allowing its liquid composition to return to a steady state. At the same time, we observed an increase in film thickness, highlighting an unexpected consequence of forced drainage. Although these thicker films represent only a small fraction of the total liquid volume, they may significantly affect mass transfer, particularly when gaseous reactants must cross the liquid films to reach reactive sites.
This study establishes forced drainage as a powerful tool for controlling both the structure and composition of liquid foams. It also highlights the importance of measuring foam films in situ: their thickness can differ substantially from values inferred from isolated films at equilibrium. These findings provide new insights into the relationship between foam microstructure, drainage and mass transport, with implications for applications involving chemical reactions and separation processes in foams.
References
[1] Stevenson P. On the forced drainage of foam. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2007; 305: 1–9.
[2] Saint-Jalmes A. Physical chemistry in foam drainage and coarsening. Soft Matter 2006; 2: 836.
[3] Trinh P, Mikhailovskaya A, Lefèvre G, et al. Relation between oxidation kinetics and reactant transport in an aqueous foam. Journal of Colloid and Interface Science 2023; 643: 267–275.
[4] Rousseau AR, Paraison P, Lefèvre G, et al. Using Foams for the Oxidative Dissolution of Copper Particles: Transport of Reactants Is Controlled by the Foam Structure. Langmuir 2025; acs.langmuir.5c00263.
[5] Lamolinairie J, Dollet B, Bridot J-L, et al. Probing foams from the nanometer to the millimeter scale by coupling small-angle neutron scattering, imaging, and electrical conductivity measurements. Soft Matter 2022; 18: 8733–8747.
[6] Chiappisi L. Micelle enrichment in liquid foams. Phys Rev E; 113. Epub ahead of print 2026. DOI: 10.1103/1pgx-dsc4.
Figure: Scheme of the set-up used to investigate the microstructure of a foam under free and forced drainage using optical imaging and small-angle neutron scattering.