Emulsions like mayonnaise, foams like shaving cream or polymer microgels like hair gels are jammed assemblies of soft deformable elements (bubbles, droplets or polymer blobs) in a liquid matrix. At the macroscopic scale, they behave as complex yield stress fluids, intermediate between solids and liquids : elastic-like at rest, they flow like liquids when the exerted stress is higher than a yield stress.
Beside this complex mechanical behaviour in bulk, such fluids also exhibit significant slip at the walls, because it may be easier to shear a liquid lubrication layer than to deform the soft elements. Beyond the prediction of flow properties, which is crucial in applications like oil extraction, cosmetics or food processing, measuring and understanding the corresponding friction laws at the wall provides us some information on the dissipation mechanisms at play in such systems, which couple hydrodynamics and the elastic or capillary deformation of the microstructure.
During the seminar, I will first present experimental results on wall friction in both foams and polymer microgels, where we show that the slip behaviour is controlled by how "jammed" are the soft elements in the fluid. Second, I will discuss the influence of the wall roughness in a model experiment at the single bubble level.
Marie LE MERRER travaille à l’Institut Lumière Matière (ILM)/UCBL en tant que Chargée de Recherche CNRS.