The Surface Tension Test - Is It Worth Resurrecting?

In all of my years in oil analysis, I cannot recall a single time when data from a surface tension (ST) or interfacial tension test (IFT) appeared on a routine analytical report, aside from transformer oil analysis, for which the test is considered routine.

The reason this is so surprising is that many studies have reported that changes in an oil’s surface tension to be the earliest sign of contamination, sludge potential and oxidation. Quoting from the book “Lubrication of Industrial and Marine Machinery” by William G. Forbes, “The interfacial tension test is the most valuable single test that can be used to evaluate a turbine oil.

It is generally agreed that when the interfacial tension is between 15 and 20 dynes/cm, deposits may or may not be forming in the system. Safe practice would call for an oil change in this range.”

Figure 1. Intermolecular Forces as They Effect Interfacial Tension

What is surface tension? Consider the layer of molecules on an oil’s surface that are in contact with air.As seen in Figure 1, the molecules are more strongly attracted from below because the molecules in the air are on average further apart (the attraction is inversely proportional to the distance between the molecules).

A similar imbalance occurs between two immiscible liquids such as oil and water; in this case the phenomenon is called interfacial tension. At the air-oil interface, this imbalance creates a skin-like membrane at the oil’s surface causing the oil to want to form the smallest possible surface area.

Due to the tendency of the surface to want to contract, it behaves as if it were in a state of tension. For the same reason, a drop of water falling through air tends to be spherical because a sphere has the minimum surface-to-volume ratio.

Measurement Techniques

The most common method of measuring surface/interfacial tension involves the use of a torsion balance known as the DuNouy instrument (tensiometer), described in ASTM D971:99a and ASTM D1331.

In the test (Figures 2 and 3), the force required to lift a horizontal platinum wire ring away from the oil’s surface is measured either directly (surface tension) or at an oil/water interface (interfacial tension). The unit of measure for surface tension is dynes per centimeter (equal to mN/m).

Figure 2. How the DuNouy Method Works

Another method to determine interfacial tension is used to characterize the degradation of electrical insulating oils due to oxidation or contamination. It involves the use of a drop-weight procedure and is described in ASTM D2285:99.

With any of the prescribed methods, test accuracy is affected by a number of factors including cleanliness of the test apparatus, agitation of the fluid (required to resuspend solids) and the accidental contamination of the fluid by dirty containers and laboratory glassware. In general, good reproducibility of these tests can be difficult to obtain unless care is taken to ensure quality lab test procedures.

Interfacial Tension of Base Oils

Excepting some synthetic fluids, base oils (for example, mineral oils and polyalphaolefin) are hydrophobic (water-repelling). They are nonpolar - unlike water and most other substances in the machine’s environment. Polar dislikes nonpolar, like water running off a duck’s back. In the case of oil and water, opposites do not attract.

Pure water has high surface tension due to its high polarity. Likewise, a pure mineral base oil has a high surface tension due to its high nonpolarity.

This can be observed by simply placing a drop of pure base oil (or even turbine oil) on the surface of a glass of water. The oil will lift up on the water and assume a lens shape, because it is trying to disassociate from the water by reducing its area of contact. However, an aged and contaminated oil will exhibit greater contact area (known as the spreading coefficient) with the water and be more prone to flatten out.

This is due to the presence of surface-active hydrophilic (water-loving) impurities in the oil that adsorb to the oil-water interface. Because it can take time for hydrophilic impurities to arrive at the interface, the longer oil and water remain in contact, the lower the IFT. Testing of IFT usually calls for a 30-second aging of the interface before measurement begins.

One could say that the purity of a base oil can be characterized by its IFT because IFT is proportional to the concentration and strength of polar, surface-active impurities (for example, compounds of oxygen and sulfur, fatty acids and alcohols). Hence, a Group III mineral oil will have a higher IFT than a Group II. Likewise, a Group II will be higher than a Group I.

Increasing viscosity corresponds to increasing IFT. However, increased temperature decreases the interfacial tension between oil and water. One could say that at some elevated temperature, all substances become miscible.

The Influence of Additives on IFT, Oil-Water Emulsions and Foam

Many additives are surface-active by design and reduce IFT by default. Some are metal wetting (AW, EP and rust inhibitors for instance) and others are particle enveloping (for example, dispersants, detergents and metal deactivators). Emulsifying agents, used in some hydraulic fluids and metal working fluids, are intended to reduce interfacial surface tension to form oil-water emulsions.

Lubricants formulated with high doses of surface-active additives will be more prone to dissolve and emulsify water during service. In fact, the higher the concentration of these additives, the higher the concentration of water that can be emulsified. For instance, when a sufficiently high concentration of surface-active agents are present in either the oil or the water phases reducing the IFT to 2 dynes/cm or less, even the slightest agitation can disrupt the interface and lead to emulsification.

As water droplet size decreases, more surface area is exposed to surface-active emulsifying agents. This high interfacial surface area effectively consumes the hydrophilic emulsifying agents.

If these polar agents are of limited supply, water droplets are more prone to stay larger and pooled together (reducing the effective interfacial area), which in turn reduces the stability of the emulsion. In a stable oil-water emulsion, the water droplet size is typically one to six microns.

Foams are essentially emulsions of air - air globules separated by thin walls of oil. These walls can be a thick as 1 mm or as thin as 0.01 micron. Highly pure liquids, such as base oils and water, do not foam.

However, the enrichment of lubricants with surface-active agents, including many additives, can negatively impact the rate at which air detrains from lubricants increasing, the tendency for foam to form when the air reaches the surfaces, and increasing the stability of foam.

Many other factors influence this as well, including viscosity, agitation, temperature and defoamant additives. Another interesting influence on IFT is dissolved air. The higher the concentration of dissolved air (and other gases), which in turn be influenced by temperature and fluid pressure, the lower the IFT and ST.

Condition Monitoring Using IFT

The health and condition of lubricating oils can, to a large degree, be measured by interfacial tension. When oils degrade and become contaminated, IFT goes down. For instance, a lubricant with an IFT similar to new oil is probably still serviceable.

However, the root cause of a downward excursion of the IFT may be hard to diagnosis because the test does not characterize the cause, only the effect. However, it at least indicates a problem exists, prompting further study, making IFT a potentially effective screening tool.

The following are oil analysis and condition-monitoring strategies for the use of IFT testing with in-service lubricants:

Oxidation Stability
When oils oxidize they form soluble and insoluble hydrophilic compounds. Many of these compounds can alter the IFT of a lubricant long before there is any noticeable change in acid number or viscosity (Figure 4).

Figure 4. Comparing Oil Oxidation Sensitivity Using Interfacial Tension and Neutralization Number

This incipient indication of oil oxidation is similar to what is observed with Rotating Pressure Vessel Oxidation Test (RPVOT), differential scanning calorimetry and cyclic voltametry. For instance, when a turbine oil’s surface tension drops to within the range of 15 to 20 dynes/cm, there is risk of sludge and deposit formation. Even photo-catalytic reactions of oil from exposure to sunlight can lower IFT.

Contamination
There are a great many contaminants that can sharply change an oil’s IFT. These include wash down detergents/soaps, terrain dust, process chemicals, antifreeze, degreasers, surfactants, etc. Many of these contaminants are invisible to other oil analysis instruments such as particle counters and spectrometers.

Cross Contamination
Lubricants that become cross-contaminated with other incompatible lubricants may show a marked change in IFT. For instance, this might occur when gear oils or motor oils are mixed with hydraulic fluids or turbine oils. Grease that has been allowed to mix with a lubricating oil can change IFT as well.

Additive Depletion
The depletion of surface-active additives (detergents, antiwear agents, rust inhibitors, etc.) by mass transfer may actually raise the IFT. In general, however, testing of used oils is best suited for those fluids with low concentrations of polar additives and a high original IFT. Examples include turbine oils, hydraulic fluids and R&O bearing oils.

Leakage
A sudden increase in oil leakage may be due to a change in IFT. In many cases, the root cause could be an increase in operating temperature. Because IFT decreases linearly with increase in temperature, high temperature can lead to leakage by a reduction in surface tension (seal meniscus, capillary flow) and a lowering of viscosity. Both can lead to sudden or increased leakage.

Field Testing of IFT

The quantitative analysis of IFT in new and used lubricants may not always be a requirement. Just as users can gain considerable knowledge of oil properties from simple field tests such as crackle test, patch test, blotter test, etc., there are ways to investigate changes in IFT as well. One method has already been mentioned - examine the spreadability of an oil drop placed on the surface of water.

Compare used oil to new oil. Even simple capillary tubes used for obtaining blood specimens can be useful. Alternatively, consider using a blender by mixing oil and water to observe the oil’s demulsifying properties. After all, tests for demulsifying properties (ASTM D1401, D2711), foam (ASTM D892) and air release (ASTM D3427) are all blood brothers to surface tension and interfacial tension tests.

Surface tension testing dates back to the early days of lubrication. Today, there may be a renewed demand for this important but scarcely deployed oil analysis tool.