ABSTRACT
Foams are dispersions of air bubbles in a liquid, frequently stabilized by surfactant molecules, but polymers, proteins or particles can also be used. The stabilizer's role is to slow down the different mechanisms of foam aging: drainage, coalescence, and coarsening. In the case of stable foams, the amount of foam produced by a given device is limited by the amount of surface-active agent available to cover the bubble surfaces and to prevent coalescence. The required bulk concentration depends on the method used: this amount is less for low energy mixers than for high energy ones, because high energy mixing produces small bubbles, hence high surface area, with the amount of required surface-active agents being larger. In the case of proteins, the adsorption kinetics is slower than predicted by diffusion mechanisms. The notion of surface mobility can be related to surface rheology: the elastic moduli and viscosities will be small for a mobile surface and large for a rigid one.
