The stability of pneumatic foam plays a crucial role in foam fractionation and froth flotation. It has been observed that the humidity gradient in the free board of a foam fractionation column is a controlling factor of the dynamic foam stability (Li, X. et al, Int. J. Miner. Process. 2010, 94, (1-2), 14-19.), however, the underlying physics as to how the humidity gradient affects the foam stability has not been completely understood.
Experiments have been carried out to study the effect of environment humidity on the apparent stability of static foam, coupled with direct experimental observations of Marangoni flow induce by non-uniform evaporation of thin liquid films in the presence of surfactant. Generally speaking, the Marangoni effect stabilise the foam against stretching caused by shearing and mechanical vibration. However, it accelerates the collapse of the foam under evaporative conditions. Preliminary results showed that the foam collapses not only because the foam films become more susceptible to rupture because they lose liquid inventory due to evaporation, but the non-uniform evaporation actually causes a surface-tension gradient on the bubble surface, which pulls liquid from the free surface via the Marangoni effect.