Influence of interfacial elasticity on liquid entrainment in thin foam films

The influence of interfacial elasticity on the rate of liquid drainage from gas-liquid interfaces is a subject that has encouraged prolific scientific work on coalescence and film stability. Elucidating this relationship is important for the design of surfactant mixtures where the amount of liquid content of the foam is critical for the aesthetics and/or effectiveness of the product. However, contradictory theoretical predictions exist with regard to how surface elasticity may influence thin-film dynamics. In this work, interferometric studies are performed to measure the liquid film entrainment between a bubble and an air-liquid interface in response to systematic variations of the surface elasticity. The surface elasticity is varied by adjusting the age of the interface or by adjusting the bulk concentration of a surface-active molecule known to form highly elastic surface layers. Surprisingly, the results indicate the absence of a strong relationship between the surface shear elasticity and the entrainment of liquid in foam films. In addition, qualitative differences are observed between the shapes of foam films with differences in interfacial shear viscosity, with no net effect on liquid entrainment under the conditions studied.