foam coarsening under steady shear : interplay between bubble rearrangements and film thinning dynamics

Aqueous foams are unstable and age by drainage and coarsening.Today, these effects are well described, as well as their impact on foam properties. For instance, the foam viscoelasticity evolve in time due to coarsening which increases the bubble size. Here, we investigate the reverse coupling, and study if and how a continuous flow of a foam can impact its dynamics of coarsening. We introduce a new protocol where brief oscillatory measurements are inserted within a steady shear, allowing us to monitor the relative variation of the bubble size with time, and as a function of an applied shear. It turns out that the coarsening rate is strongly impacted by the applied shear: it is continuously reduced above a critical shear rate, which decreases itself with the bubble size. This coarsening reduction is interpreted as the result of out-of-equilibrium and shear-dependant film thicknesses, being higher than at rest. The critical shear rate, above which films are dynamically sustained at high thickness, emerges from the competition between the rate of rearrangements and the time to drain thick films created during a rearrangement. We thus report here a first experimental proof of out-of-equilibrium film thicknesses within a sheared foam, and of the impact this has on coarsening.