4. Surface characteristics

The surface characteristics of lubricating oils are linked to their surface-active and solvent properties, and concern the following three types of interface:

• liquid-gas, e.g. oil-air, with aeration and foaming phenomena;

• liquid-liquid, for example oil-water, with the phenomena of oil-in-water emulsion (direct emulsion) and water-in-oil emulsion (inverse emulsion);

• liquid-solid, e.g. oil on friction surfaces, which plays an important role in boundary lubrication and detergency, and oil on solid impurities, which plays an important role in dispersivity.

Surface tension between oil and air is linked to foaming and aeration phenomena, which tend to worsen as surface tension decreases, while interfacial tension between oil and an immiscible...