Snap-off in constricted tubes and porous media

Many mechanistic models for foam in porous media rely on the concept of repeated snap-off in pore throats to explain foam generation in porous media. Repeated snap-off occurs in gas flow through singly constricted tubes, but mechanisms differ in gas flow through porous media with pores of finite volume. For instance, in a doubly-constricted tube snap-off ceases when the pore body between pore throats fills with bubbles and capillary pressure rises. Thus the mechanism of repeated snap-off, as currently formulated and applied, cannot explain steady-state foam generation in porous media.