Vertex corrections in the theory of foam drainage

The experimental results of Koehler and co-workers have indicated that vertices (nodes) can in some cases play a dominant role in the dissipative process that controls foam drainage. We present calculations of numerical constants that can express the effect of the vertices, in a first approximation. Two limiting cases are treated: Poiseuille flow, and free-boundary flow (in the sense that the stress at the surface of the Plateau borders is everywhere zero). Consequences for the relationship between average flow velocity and volume flow rate in steady drainage are indicated.