Foamability and foam stability of oily mixtures

Lubricating oils consist of base oil, containing different hydrocarbons, and modifying components (additives) which improve the application performance. Some of these additives are able to stabilize entrained air bubbles, potentially causing serious problems for engines, transmissions and hydraulic systems. Here we evaluate the foamability and foam stability of model mineral oils (hexadecane, light oil, heavy oil and their mixtures) in the presence and in the absence of nanoparticles as additives, at several temperatures. The results allow us to categorize the systems studied into three groups: (1) Oils unable to entrap any air during the stirring period; (2) Oils able to entrap air during stirring, but unable to retain it after stopping the stirring; (3) Oils which form stable bubbles and foams. Hexadecane, with and without nanoparticles, falls into the first group. Heavy oil in the presence of nanoparticles falls into the third group, whereas all other mixtures are in the second group. The inability of hexadecane to entrain air is related to its low viscosity and very low foam film stability which leads to instantaneous coalescence of the bubbles formed. The increased foamability of heavy and light oils and their mixtures is explained by: (1) their higher viscosity as compared to hexadecane which leads to slower foam film thinning and (2) the presence of long chain alkanes in these oils which create weak steric repulsion between the foam film surfaces. The addition of nanoparticles increases the foamability and the foam stability of heavy oil, without changing significantly the foam properties of the light oil and hexadecane. The latter effect is explained by the inability of the particles to attach to the light oil-air and hexadecane-air interfaces, whereas the same particles adsorb on the heavy oil-air interface and create additional steric repulsion between the air bubbles, thus allowing the formation of stable foam.