Relation between oxidation kinetics and reactant transport in an aqueous foam

Aqueous foams are expected to constitute exquisite particularly suitable reactive medium for the oxidation of metals, since the reactant H⁺ can be supplied through the continuous liquid phase, while the reactant O₂ can be transported through the gas bubbles.

To test this hypothesis, we investigated the oxidation of a metallic copper cylinder immersed in an aqueous foam. To study the relation between the transport of these reactants and the kinetics of the chemical reaction we use a forced drainage setup which enables us to control both the advection velocity of the H⁺ ions through the foam and the foam liquid fraction.

We find experimentally that the mass of dissolved copper presents a maximum with the drainage flow rate, and thus with the foam liquid fraction. Modeling analytically the transfer of H⁺ and O₂ through the foams enables us to show that this non-monotonic behavior results from a competition between the advective flux of H⁺ ions and the unsteady diffusion of O₂ through the thin liquid films which tends to be slower as the area of the thin liquid films decreases with the drainage flow rate and the liquid fraction. This study shows for the first time how to optimize the foam structure and drainage flow in reactive foams in which the reactants are present both in the liquid and gaseous phases.