Coupling between hydrodynamics and surfactant transport at interfaces: experimental and numerical challenges. – HydroSurfDyn

The main idea has been to consider the different levels of coupling between transport and flow, and to focus specifically on the dynamics of surfactants, both from a theoretical, numerical or experimental point of view. Numerically, we developed a flow simulation in which the surfactants transport, their adsorption to interfaces and their effect on the local surface tension are explicitly taken into account. Theoretically, we have revisited the relevant approximations that should be made to understand the thin liquid film dynamics. Finally, experimentally, we developped an innovative set up to measure, in one experiment, the interfacial velocities and the film thickness as a function of space and time.

We contributed to the understanding of the expansion dynamic of a thin liquid film stabilized by surfactants, which has a direct impact on the bubble deformation dynamics in a liquid foam (theory). We studied the dynamics of a soap film impacted by an acoustic wave (theory and experiments). We also evidenced the fine dynamics of the gravitationnal drainage of a film, which directly impacts its life time (experiments).

The project will finish on nov. 2015, as the PI obtained a job in the industry.

3 publications
2 master thesis
1 preprint
6 conferences

Numerous industrial and everyday life processes rely on surfactants, whose first role is to decrease surface tension. However this equilibrium characteristic is often of secondary relevance in the out of equilibrium situations in which they are most often used. Instead, these dynamical situations are governed by the coupling between hydrodynamics and surfactant transport within the fluid, and to the interface.
Indeed, the rate at which surface-active species replenish interfaces controls the generation of surface tension stresses, which provide the boundary conditions for the underlying flow. The complexity of this coupling arises from the fact that this rate is not an intrinsic property of the molecules, but depends instead on the entire set of transport phenomena (diffusion, convection, adsorption), and is thus highly dependent on the details of the hydrodynamics and the geometry of the flow. It is the reason why most situations featuring hydrodynamics and surfactants are still not fully understood.
The knowledge of the local surface stresses would be of great importance to determine the relevant physical processes and pose the correct assumptions, yet existing experimental techniques are not able to measure these local quantities. Thus, the first (experimental) part of the present projects is to develop a novel set of techniques to measure the local interfacial velocity and surfactant concentration.
In addition, the conceptual complexity of this coupling, and the resulting challenges posed by the interpretation of experimental data prompted us to develop numerical simulation algorithms and the associated theory to accurately model the hydrodynamics in the presence of interfaces and surfactants, which we intend to release to the scientific community under an open-source licence.
These experimental and theoretical advances will be used to revisit a series of simple, well posed and poorly understood experimental configurations exhibiting in an exemplary manner the aforementioned coupling between hydrodynamics and surfactant transport. We will in particular strive to explicit and predict quantitatively the relation between surfactants dynamical properties and the observed interfacial rheologies.