The physics of foam drainage

Foam drainage describes the flow of liquid through a foam, driven by gravity and capillarity. It is an important factor for foam stability, and thus of great relevance to the detergent industry.

Over the last decade substantial progress has been made in understanding the relevant physics behind foam drainage and the related issue of drainage or creaming in emulsions. This was instigated largely by so-called forced drainage experiments, in which the foam is fed at the top with a supply of liquid and its spatial and temporal variation of liquid fraction is determined.

We review key experiments and theory and also address remaining questions concerning convective bubble motion in foams with high liquid content and the local flow mechanism of drainage.