"Matière et Systèmes Complexes"
13 mai 2008 à 11h30
Bâtiment Condorcet, 6ème
étage, salle 646 A.
Rate Dependence and Non-Affine Flows of Foams
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
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.