non-linear viscoelasticity and wall slip regimes of concentrated dispersed systems : the case of aqueous foams
Résumé
Aqueous foams, as emulsions, microgels or pastes, have rich and complex, but not yet fully understood, rheological properties. In the same time, it is striking to observe very generic and similar features in the macroscopic rheological curves of all these different systems. To illustrate and to better understand some of these universal behaviors, we report here new experiments on well controlled aqueous foams (with various liquid fractions, and chemical components). First, we report results of oscillatory rheometry (frequency and amplitude sweep tests) at fixed shear rate. Usually, such sweep experiments are performed by varying frequency at fixed amplitude (or the opposite). We show here that this new mode of oscillatory measurements allows us to understand how the viscoelastic moduli depend on the oscillation frequency and amplitude. This also provides new understanding on the non-linear regimes observed at low frequencies and high amplitude. In particular, performing oscillations at controlled shear rates shift towards higher values the low frequency behavior: we can then have access to information on the slow relaxation processes, and on how these relaxation modes are actually activated by imposing a shear rate. Once experiments are performed at controlled shear rate, we show that we can recover, by these macroscopic measurements, some of the results obtained at the bubble scale on the rate of coarsening-induced rearrangements. Secondly, by steady-shear rheometry coupled to multiple light scattering, we show that different wall slip regimes can be found, depending on the bubble surface properties and on the liquid fraction. These different regimes can be explained by analyzing where and how the viscous dissipation occurs inside the foam structure as it slips. As well, we show that as the foam slips on a wall, some liquid is re-distributed inside the structure, evidencing some poroelastic effects. In all these aspects of wall slip and oscillatory shear, we also compare and discuss how far these results obtained on foams are universal and valid for other systems.