We present an experimental study of forced drainage through a soap foam, where a constant liquid flux at the top of a dry foam produces a downwards traveling wave with a constant velocity and uniform liquid content. The results are not consistent with existing models and we propose a new model, based upon relaxing the condition of wall rigidity throughout the network and emphasizing the importance of viscous damping in the nodes where Plateau borders meet. This model agrees well with the experimentally measured (power-law) scaling behavior of the drainage velocity, and the width of the propagating liquid front, on the imposed flow rate and bubble size.

• Received 22 December 1998

DOI:https://doi.org/10.1103/PhysRevLett.82.4232