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Séminaires MSC
"Matière et Systèmes Complexes"

                      

Mardi 13 mai 2008 à 11h30
Bâtiment Condorcet, 6ème étage, salle 646 A.


Martin van Hecke
(Leiden University, Netherlands)


Rate Dependence and Non-Affine Flows of Foams
 
When left unperturbed, foams jam into a metastable state corresponding to a local minimum of the surface energy. Under forcing, foam bubbles overcome these local minima, the foam starts to flow, and the viscous dissipation that arises in the thin fluid films that surround the gas bubbles becomes important. Here we will address the surprising and non-trivial role of disorder for foam flows, by experiments on the rheology of 2D foams both at the coarse grained and at the bubble level. Our central finding is that for disordered foams the effective viscosity scales differently from the local bubble drag. Monodisperse foams do not exhibit this anomalous behavior, which points to the role of non-affine, swirling motion in setting the coarse grained flow features. The effective viscosity also exhibits nontrivial scaling with wetness --- both anomalous scaling laws are reminiscent of the behavior of the shear modulus
near jamming.
We then argue that since bubble interactions are probably simpler than those of, e.g., frictional grains, and are similar to those of the soft spheres without static friction that have been studied extensively in the context of jamming, foams are eminently suited for fundamental studies of the flow of disordered media.