Surface tension will affect the fluids aeration (air release), foaming and demulsability (water separation) – all very important for the ultimate lube performance characteristics. Strictly speaking, demulsability is affected by interfacial tension between oil and water rather than surface tension between oil and air – but the two go hand-in hand.

Surface tension can be thought of as the tightness or tension of the interface between air, for example a bubble, and the bulk fluid – a bit like the tension in the rubber of an inflated balloon.

Generally, finished lube surface properties are inferior to the base oil from which they are blended because of the additives, so base oil surface property characteristics usually indicate the best case performance that can be achieved. Ideally we want to use base oils that have the highest possible surface tension since any additive or contaminant will always lower the surface tension from the pure base oil reference.

Additives, whether dispersants, detergents, antioxidants or zincs, will always have surface activity, and this means they will tend to locate at air-oil or water-oil interfaces – and hence lower the relevant tension. It does not mean they go there exclusively, but their concentrations are higher there.

So lets think of some consequences of this tendency to concentrate at interfaces. The movement of machinery and lubricant creates bubbles in the bulk fluid. If the bubbles are big, they will float to the surface of the oil rapidly and burst. If they are small, they will float very slowly, and a long bubble residence time makes the fluid compressible – not good in a hydraulic fluid, which should transmit physical movements precisely.

We said any additive or particle contaminant will always lower surface tensions, and this always makes bubbles smaller, never bigger. So the bubbles in finished lubes or contaminated base oils rise more slowly, giving poorer air release performance. This is the reason there is no additive fix for the aeration of a base oil or lube. Foams on the surface of the bulk fluid, as opposed to bubbles in the bulk, are a different matter and do have additive fixes, which we will discuss later.

Returning to aeration, API Group I base oils have high levels of aromatics, some of which act like surface active molecules or additives to lower surface tension. Since Group II and III base oils usually have only trace levels of aromatics, they will always have superior surface properties, grade for grade, compared to Group I. We have to compare similar grades since bubble movement will be determined by both the bulk viscosity of the fluid as well as the bubble size.

Consider some other features of base oils and lubes, such as demulse performance. Ideally we would like any water in the oil to form a separate phase, that is, not be incorporated as an emulsion droplet. Again, high interfacial tensions between oil and water will ensure the water separates quickly, but additive and particles will limit the rate of separation due to tension lowering effects.

Yet another property controlled by surface or interfacial tensions is foaming of the fluid. High tensions allow foams to be free-draining, meaning the bulk liquid between bubbles drops out under gravity so that the foam collapses quickly – much like a champagne foam. Low tensions from additives or particles tend to stabilize foams by allowing the fluid to remain stuck between neighboring bubbles. Unlike with aeration, there are a number of additives that can assist the draining or collapse of foam and the coalescing of small foam bubbles into bigger ones. Aeration and foaming operate by quite different mechanisms, even though they both involve bubbles, and this is why there is an additive fix for foaming (antifoams) but not for aeration.

So how do we manage surface properties of finished lubes, accepting the fact that we will normally need additives in a lubricant? Where demulse and air release are critical in applications such as industrial turbine and hydraulic fluids, we minimize additive treat rates and nowadays use Group II or III base oils when possible, rather than Group I.

Where high additive treats are absolutely necessary such as in crank-case engine lubes, we must ensure that antifoams are effective, and if not, change them for ones that are. Demulsability is not such a problem in engines since high operating temperatures will usually purge water, and consequences of incorporated water, such as rusting, can be dealt with by corrosion inhibitors.

As regards aeration and engines, we have to accept theres not too much we can do apart from monitoring. In engines with hydraulically activated valve trains we ensure that air incorporation is within acceptable limits, for example, through the engine test for oil aeration in hydraulic-electronic unit injectors that is part of the API CH-4 specification for heavy-duty diesel engine oils.

In summary, keep base oils clean in the supply chain, and where surface properties are critical, dont over-additize lubricants. Avoid Group I base oils for best surface property performance.