Size dependent brittle to ductile transition in bubble rafts

We report on experimental studies of the failure modes of aqueous foam under tension as a function of loading speed and system size. Foam is an example of a viscoelastic material that acts either as a solid or a fluid, depending on the details of the applied deformation. This raises the interesting question of whether foam's failure modes correspond to plastic deformation, viscous flow, or brittle fracture. Using a model foam system, a bubble raft, we observe two distinct failure modes. For high enough deformation rates and large enough ratios of the initial system width to length, brittle fracture is observed. For small deformation rates and ratios of the initial system width to length, plastic pinch-off is observed. For sufficiently small systems, there is a well-defined transition zone in which a competition between pinch-off and fracture is observed.