A SOLUBLE MODEL FOR FOAM DRAINAGE

A model is set up which is believed to contain the essential physics o f foam drainage. Mathematically, the model leads to a non-linear parti al differential equation, which we were able to solve exactly by analy tical methods for the physical boundary conditions relevant for recent experiments in which a dry foam is wetted by a steady addition of liq uid. The solution represents a density wave of constant profile (or so litary wave) in qualitative accord with these experiments. The observe d dependences of liquid fraction and wet/dry foam boundary velocity on input flow are reproduced by the model.