International audienceWhen a small volume of pure water – typically a drop – is injected within an aqueous foam, it locally triggersthe rupture of foam films, thus opening an empty cavity in the foam's bulk. We consider the final shape ofthis cavity and we quantify its volume as a function of the volume of injected water, the diameter of thebubbles and the liquid fraction of the foam. We provide quantitative understanding to explain how andwhen this cavity appears. We epitomize the dilution of surfactants at the water–air interfaces as the maincause lying behind the coalescence process. We identify a new coalescence regime for which a criticalsurfactant concentration rules the stability of the foam
