Controlling foam ageing in viscoelastic media

Foams can be defined as dispersions of gas bubbles in a continuous medium. They exhibit a typical cellular structure, where the gas bubbles are tightly packed together, so that the liquid phase is spread in a network of thin films and channels between bubbles. Thanks to their lightness and their peculiar mechanical properties, foams are suitable for several industrial applications. Foams are intrinsically unstable. During their life, they can undergo three different mechanisms of destabilisation: drainage, coalescence and coarsening. These processes alter the foam structure and can be a strong limitation in industrial processes. The desired longevity of a foam clearly depends on the specific applications, which call for a thorough understanding of foam stabilisation. The purpose of the present thesis is to investigate how the mechanical properties of the continuous phase impact on liquid foam coarsening, in conditions where drainage and coalescence can both be neglected. In order to do that, we use emulsions as the liquid phase, because of their viscoelasticity. The study of foams requires the use of non-invasive methods. Thus, foamed emulsions are studied by employing diffusing-wave spectroscopy and imaging, to get information about the evolution of the mean bubble size and the gas permeability of the liquid films between bubbles. What we observed is that the emulsion elasticity does influence the foam coarsening either in two or in three dimensions, showing a peculiar behaviour that lays the groundwork for further investigations.