Electrokinetical transport in liquid foams – E-FOAM

This fundamental project is devoted to the study of electro-osmotic transport in a class of materials where it has never been explored: liquid foams.
Though part of our daily lives and important in many industrial processes, foams are short-lived materials. Being strongly out of equilibrium, they irreversibly evolve and, as everybody has observed in a foam bath, they eventually collapse. An important mechanism at play in this process is the drainage induced by gravity, which is unavoidable. We propose here a way to cancel and reverse this gravity-driven drainage: electro-osmosis. In short, the fluid being locally charged in the vicinity of the ionic surfactant layer, it is set into motion by an external electric field. By controlling the field and the resulting flow, the foam can be maintained in a superstable state or even be rejuvenated on demand.

After more than two centuries of scrutiny, much is known about electrokinetic phenomena at solid-liquid interfaces. The case of foams, however, differs in three important ways. First, whereas man-made solid channels have a fixed geometry, soap films can vary in shape and dimensions. As many channels encountered in living world, they are deformable. Second, while in solid channels charges usually are fixed to the wall, in soap films the charges are carried by some surfactant molecules covering the liquid-gas interface, which can be mobile. Third, whereas most solid walls imply zero or small slip, the hydrodynamic boundary condition at the liquid-gas interface will involve the surface rheology and can range from zero slip to stress free condition, depending on the type of surfactants.

We will explore the consequences for foams subject to electro-osmosis. A dedicated experimental set-up will be built to characterize electro-osmosis at distinct levels: near a surfactant laden interface, in a single soap film, in the liquid channel where three films meet – the so-called Plateau border-, and finally, in the whole interconnected structure of films and Plateau borders that make up a macroscopic foam. More practically, microPIV characterization, electrokinetic and optical interferometric measurements, as well as image analysis will be performed.
On the theoretical side, complete understanding will be sought after through a combination of simulations (Molecular Dynamics), numerical solving (finite element method) and analytical approaches. Reflecting the multiscale character of the foam material, a variety of tools will be employed to bridge the scales between the dynamics of nanometric surfactant molecules and the evolution of the whole foam.

Though fundamental in nature, this project will also explore potential application with the realization of two experimental prototypes. The first is a microfluidic chip that allows stopping or activating on request the gas diffusion between two reactors. The second prototype aims at generating an everlasting foam column, whose liquid fraction is homogeneous both in space and time.