Non-aqueous foams in lubricating oil systems

We have investigated the foaming properties of mixtures of a hydrocarbon oil solvent with various low molar mass, polymeric and overbased detergent particle additives used in lubricating oils. Foam formation occurs only when the additive concentration and temperature are such that the single-phase mixtures are close to a phase separation boundary when solvent affinity for the solute is low. This is likely to maximise the tendency of the solute to adsorb at the liquid–air surface and thereby promote foam formation. Multi-phase mixtures either maintain or suppress the foamability of the single-phase mixtures. The foams formed contain spherical, polydisperse gas bubbles and liquid volume fractions greater than about 20 vol.% indicating they are “wet”, transient foams in which the adsorbed solute films at the liquid–air surface can affect liquid drainage but are incapable of resisting coalescence between contacting bubbles. Consistent with this model, the foam half-life is found to scale with the kinematic viscosity of the liquid.