Rheosurf - new numerical developments for interfacial rheology

Pharmaceutics, food, building and cosmetics industries use multiphasic fluids to confer texture to the final products. However, the stability of multiphase fluids such as foams for instance, is still an issue. A way to delay their undesired evolution is to carefully choose surface-active materials (whether surfactants, polymers, proteins or particles) in order to stabilise drops and bubbles by surface adsoption. Up to now, the efficiency of surfactants and polymers seems to be related to surface rheology, which impacts, on one hand, on the liquid flow at the interfaces through the boundary condition, and on the other hand, on the mechanical resistance of the interface to coarsening or rupture. Unfortunately, fundamental scientific advances in interfacial rheology are shrouded in controversy and still the subject of debate. The intrinsic difficulty comes from the quasi-impossibility to quantify one parameter without any contribution of the others, because shear and dilatational strains are often present at the same time. Moreover, surface and bulk flow are strongly coupled through the boundary conditions so that it is challenging to dissociate their respective contribution to the mechanical response of the interface. Even if several techniques exist, they are often restricted to particular surface properties and happen to be valid only for specific surfactants or polymers where surface phenomena can be uncoupled.

Over the past 10 years, numerical simulations of the dynamics and the stability under flow of microscopic elements such as droplets, vesicles, microcapsules, has made enormous progress. The challenge of the RHEOSURF project consists in the developement of new numerical codes accounting for the non linear deformations of droplets and bubbles in flows, and taking into account surface dissipation, the migration of surfactants and their kinetics of adsorption/desorption. As a second step, some experimental results will be compared to numerical simulations in the case of a spatiotemporal deformation of droplets and bubbles in simple flows, in order to determine the physical parameters which govern the deformation of the interface: Marangoni and dilatational elastic moduli, shear and dilatational surface viscosities.