Film tension and dilational film rheology of a single foam bubble

Abstract

The balance of normal and tangential forces in the liquid film of a foam bubble yields equations which describe the shape of bubbles under gravity conditions and the variation of local characteristics of the film, such as thickness, tension, surfactant concentration, elasticity, etc. The conservation of surfactant molecules within the local elements in the film is taken into account. Upon stretching of non-uniform films the local expansion is larger in parts of smaller local elasticity. Therefore the stability of non-uniform films depends on their local characteristics. The rheological properties of liquid films are very sensitive to the parameters of the surface equation of state and adsorption isotherm. In particular, the intrinsic (2D) compressibility of adsorption layers can influence the properties and, therefore, the stability of liquid films.