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In this general lecture an overview of the physics of foams will be given, with a special emphasis on how their local structure ultimately determines their macroscopic properties. First, the structure at the bubble scale (soap film, liquid channels and vertices), its equilibrium rules and the role of the liquid fraction will be described. Foams are intrinsically metastable and evolve over long time scales by three mechanisms: drainage, coarsening and coalescence. The theory of drainage, that is the gravity-induced drying of the foam, and of coarsening, that is the growth of the mean bubble size over time induced by gas exchange between bubbles, will be sketched.
I will then turn to foam rheology. Foams are complex fluids, behaving as an elastic solid below a yield stress, and flowing as a non-Newtonian fluid above. The relation between the elastic properties and yield stress, and the structure and liquid fraction, will be shown. The origins of dissipation leading to the effective viscosity, which is still a matter of debate, will be discussed. Recent issues like shear localisation under confinement and non-local rheology will be described. Finally, our recent advances in acoustic propagation through liquid foams will be reviewed, since it is a clear example where complex macroscopic properties of foams (here, the speed of sound) can be understood and rationalised from the local-scale behaviour, namely of the vibrational response of films and liquid channels. |