The stability of emulsions : Dedicated to Professor J. Th. G. Overbeek on the occasion of the 25th anniversary of his appointment as a Professor of Physical Chemistry.

Recent work on the film thinning behavior during the coalescence of oil drops in water is summarized. In the experimental work, color movies were taken of the light interference patterns produced by the thin film of water trapped between the rising drop and the bulk oil phase. The drops were relatively small (0.2–0.32 cm diam.); sufficient KCl was added to minimize double layer repulsion. Concentrations of 10−6-10−2 g l−1 of sodium lauryl was used. The oils used were toluene, anisole, cyclohexanol and a mixture of anisole and cyclohexane. Five different patterns of film behavior were observed. The mechanisms to explain these patterns are based on the concepts of dynamic immobility (when the distribution of surfactant along the interface gives an interfacial tension gradient that everywhere balances the interfacial shear stress) and of steric immobility. The mechanisms include rapid approach (when both interfaces are mobile), dimple formation (when the interfacial tension gradient exceeds the interfacial shear stress), even drainage (when the interfaces are dynamically ir sterically immobile) and uneven drainage (when there is local immobility). Reasons for uneven drainage are discussed. The different theoretical models developed to describe film thinning are reviewed. The emphasis is on the assumptions and the relationship between the equations describing flow in the film and those describing surface behavior. The predictions of the different models are compared but the emphasis is on the success of more recent models based on the above mentioned mechanisms to describe symmetrical film drainage