FOAMING, FOAM FILMS, ANTIFOAMING AND DEFOAMING

A general introduction to foams, the initial stages in the production of foams in aqueous solution, foam structures and the classification o f bulk foams according to their lifetimes and stability are presented. Fundamental studies on horizontal and vertical isolated foam lamellae with emphasis on drainage and stability are reviewed. For freshly pre pared foams containing fairly thick lamellae, the mechanical-dynamical properties of the surface adsorbed layers (surface tension gradients) are decisive for retaining stability. Important parameters to be take n into consideration are the surface elasticity, viscosity (bulk and s urface), gravity drainage and capillary suction. Also the film should exhibit low permeability to gases. Providing the stability of a foam f ilm (containing dilute surfactant) is retained during the initial dyna mic drainage process, then eventually a static (equilibrium) situation will be reached at film thicknesses < 100 nm. In this region, interfa cial interactions dominate and the stability of the film must be discu ssed in terms of the intermolecular forces (electrostatic double layer repulsion, dispersion force attraction and steric forces). This may l ead to the formation of common black and Newton black films and these structures have been shown to be resilient to rupture and have low gas transfer characteristics. At high surfactant concentrations (> c.m.c. ) stabilization of films and foams can occur by a micellar laying mech anism (stratification). Antifoaming and defoaming theories are present ed, together with the mechanisms of heterogeneous antifoaming agents ( non-polar oil, hydrophobic solid particles or mixtures of both) includ ing recent theories describing the role of the emulsion and pseudo-emu lsion film in the stability of foams containing oil droplets. Finally, defoaming by ultrasonic waves is briefly reviewed.