The rheology of foam

Foam rheology has arrived at an interesting stage of active debate, some of it focused on two dimensional systems. Theorists are ambitious to formulate comprehensive models and experimentalists want to look more critically and constructively at the techniques of rheometry. Are there grounds for optimism, that the difficulties of this topic may be overcome? Whatever the eventual extent of success, we argue that liquid foam should stand as the appropriate prototype for that wide class of substances that exhibit elasticity and a yield stress, above which they flow. We focus mainly on the constitutive law for steady shear and the recent debate over shear localization in two dimensions.