Speed of crack propagation in dry aqueous foam

Applying pressure driving to a single layer of aqueous foam bubbles induces a void propagation that is a surprisingly close analog of dynamic crack propagation. Depending on the rate of applied stress, both a ductile and a brittle mode of propagation are observed, the latter at much higher propagation speeds. A pronounced velocity gap is found, with a well-defined upper limit to the ductile crack speed and a well-defined lower limit to the brittle propagation speed. Both limits can be quantitatively explained by analyzing processes on the scale of single bubbles and single films, respectively, confirming the importance of the microscopic (single-bubble) scale for the overall description of these fracture phenomena. We find that the brittle crack velocity is limited by the speed of wave propagation in the foam, so that the brittle mode can be understood as a supersonic crack.