Micelle enrichment in liquid foams

Surfactant enrichment is central to foam-based separation processes, yet the composition of the foam liquid is widely assumed to remain uniform due to efficient dissipation of local concentration gradients. Using contrast-matched small-angle neutron scattering, we directly quantify micelle concentrations within the liquid channels of a steady-state foam over time and height, and distinguish them from surfactant adsorbed at the gas-liquid interface. Despite nearly constant macroscopic structure, the micellar concentration in Plateau borders increases steadily with both foam age and height, reaching more than two orders of magnitude above the initial solution concentration. This study provides the first direct measurement of persistent surfactant accumulation within the foam liquid. The observed gradients are fully accounted for by upward interfacial transport driven by bubble aging, while downward drainage and other equilibration mechanisms play a negligible role. These results overturn the common assumption of rapid surfactant homogenization within foam liquids and imply that micellar gradients must be considered when analyzing separation processes, foam stability, and, more broadly, any phenomenon involving exchange between interfaces and the foam liquid.