Evaluation of a steady-state test of foam stability

We have evaluated a steady-state test of foam stability, based on the steady-state height of a foam produced by a constant velocity of gas flow. This test is mentioned in the book by Bikerman Foams, Springer, Berlin, 1973 and an elementary theory was developed for it by Verbist et al. J. Phys. Condens. Matter 8 (1996) p. 3715. For the study, we used an aqueous solution of the cationic surfactant dodecyl trimethylammonium bromide, C12TAB, at a concentration of two times the critical micelle concentration (2 cmc). During foam generation, bubbles collapse at the top of the column which, in turn, eventually counterbalances the rate of bubble production at the bottom. The resulting balance can be described mathematically by an appropriate solution of the foam drainage equation under specified boundary conditions. Our experimental findings are in agreement with the theoretical predictions of a diverging foam height at a critical gas velocity and a finite foam height in the limit of zero velocity. We identify a critical liquid fraction below which a foam is unstable as an important parameter for characterizing foam stability. Furthermore, we deduce an effective viscosity of the liquid which flows through the foam. Currently unexplained are two experimental observations, namely sudden changes of the steady-state foam height in experiments that run over several hours and a reduction in foam height once an overflow of the foam from the containing vessel has occurred.