Drainage of foams with regularly spaced parallel soap films

The introduction of equal soap sized bubbles into a cylindrical tube of diameter roughly equal to or smaller than the bubble diameter results in equally spaced films, arranged perpendicularly to the tube. This particular type of foam, when made with a typical surfactant solution, shows unusual drainage properties. For tube radii below 0.918l0, where l0 is the capillary length of the system, no measurable flow through the foam can be observed: if solution is poured onto the foam, it will accumulate at the top without descending through it. However, for tube radii above 0.918l0, flow occurs, whatever the flow rate. This flow is highly non-uniform, unlike what is observed in disordered bulk foams: it is confined to a channel along the tube wall, resulting in localized swelling of the vertical wetting films and breaking of the symmetry of the system. This channel disappears for a critical Plateau border cross-section. A qualitative analysis attributes the inception of flow to an instability arising in the Plateau border. A semi-quantitative analysis explains why no flow occurs for tube radii less than 0.918l0, and gives values of the critical Plateau border cross-section as a function of tube radius, in good agreement with experimental data.