Beyond deformation: Foam fracture

When a flow of air is driven into a quasi-2D layer of ordered foam in a rectangular Hele-Shaw cell, the foam reacts in a fashion reminiscent of processes in both liquids and solids. On the macroscopic scale of the entire cell, an air finger analogous to that in non-Newtonian fluids develops. On the microscopic scale of single bubbles, elementary structural transitions (T1s) parallel the rearrangement of atoms around a crack tip in a solid. Above a critical air flow rate, the finger propagation abruptly changes to a fast cleavage process based on the rupture of successive films in the foam. We show that the interplay of surface tension and drag forces accounts for the transition and that this foam experiment allows for a detailed observation of both quasistatic and dynamic fracture processes.