The Wonderful Weirdness Of Oil Viscosity

Viscosity is the internal friction of fluids, the resistance of one fluid layer to sliding over adjacent layers. Viscosity makes lubrication possible. Viscosity allows the movement of one loaded surface over another to drag a lubricant between them, where it begins to carry at least some of the load, thereby making sliding easier. Dip your fingers in water and rub fingertips together and you feel considerable friction. Water has a low viscosity, so the pressure of your fingers quickly squeezes it out from between them. Put a drop of oil on one fingertip—cooking oil from the kitchen cruet will do—and rub again. Big difference. The greater viscosity of the oil (about 60 times that of water), combined with the motion of your fingertips, drags oil between their surfaces. This, by supporting part of the pressure on low-friction liquid films, greatly reduces the friction you feel.

This friction-reducing effect is even greater in well-designed machinery, whose bearing surfaces—that is, surfaces that bear against one another—are designed to be completely separated by a fluid film of lubricant at all times. During starting and stopping, when parts motion is too slow to establish full-film lubrication, mixed lubrication takes place; part of the load is carried by oil films and part by surface-to-surface contact. Most wear takes place at such times or in parts of the machinery where parts velocity is too low to generate full oil films.

Why is there viscosity in fluids? Most oils are constructed from long chains of carbon atoms, chemically bonded to each other and also carrying attached hydrogen atoms. Hence, like gasoline or diesel fuel, they are hydrocarbons. As in fuels, three basic structure types exist:

Viscosity is based upon the mutual attraction of these structures for one another. Pull on one molecule of an oil and its attraction to other molecules tends to pull them along with it. The longer the molecule, the more potential attracting partners it is likely to be in contact with. Why does this mutual attraction exist? All these molecules are normally electrically neutral, but there are local positive and negative regions because electrons—negative charges that make chemical bonding possible—are not uniformly distributed within molecules. A local positive zone on molecule A can therefore be attracted to a local negative zone on molecule B. Or a charged zone on one molecule can induce a local opposite charge on another.

When hydrocarbon lubricants came into wide use in the early 20th century, it wasn’t long before users noticed differences between oils from wells in Pennsylvania and oils from California, Texas, or Romania. Chemists soon learned why. California or Mideast crudes yielded oils rich in ring compounds, while Pennsylvania crude is rich in straight-chain or paraffinic structures.

Experience showed that while ring-structured oils could lubricate adequately at moderate temperatures, hotter conditions cause them to lose viscosity faster as they became hotter. Aha, this was why a given oil might lubricate crankshaft bearings perfectly well but allowed hotter parts, such as top piston rings, to wear rapidly as the hot oil became watery and more easily squeezed from between surfaces. Yet oil refined from a Pennsylvania crude did a pretty good job in both places, at 180 degrees Fahrenheit in crankshaft bearings and at 300 degrees on the surfaces of top piston rings. Why this difference?

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More research came up with a new way of characterizing oils: viscosity index, abbreviated as VI. All oils lose viscosity at higher temperatures, but research showed that oils rich in ring-shaped molecules lost more viscosity when hot than did straight-chain or branched-chain structures.

First question: Why do oils lose viscosity as their temperature rises? Temperature is the measure of average molecular energy. In a gas, this is just the velocity of molecules whizzing about, colliding with one another. But in a liquid, the molecules are in contact with each other, so this energy takes the form of vibrations and rotations (long-chain molecules are often pictured as whirling jump ropes). The more such energy is present, the less important become the forces attracting molecules to one another. As temperature rises, molecular agitation gradually overpowers the mutual attraction of molecules, until at the boiling point, molecules have enough energy to completely overcome mutual molecular attraction and fly away.

Soon the pattern was clear: Oils rich in ring structures had a low VI (zero to 30 or so), while oils containing mostly straight or branched chains had a higher VI (as high as 100). And today’s engineered oils in Groups III and IV have VIs as high as 130–140.

This made it evident that molecular shape had something to do with VI. Straight and branched chains have very mobile bonds capable of rotation, bending, and folding. Ring structures are more rigid, less mobile. Could it possibly be that, at lower temperatures, the long-chain molecules of high VI oils assume more compact shapes by folding, while ring-based molecules have pretty much the same shape at all temperatures?

That could explain why oils rich in straight- and branched-chain structures have generally higher VI. When cold, they assume more compact shapes which tend to decrease viscosity, but as they heat up, they extend by degrees, tending to increase viscosity. They still lose viscosity at higher temperatures, but they lose less because this progressive unfolding of the flexible chains works in the opposite direction, by putting each molecule in contact with more and more others. They are shape-shifters!

From there, it was just a short step to deliberately adding small amounts of highly foldable long-chain molecules to oil to increase its VI even more. Such additives, called VI improvers, were added to premium oils beginning in about 1950, and the resulting product was called an all-weather or a multi-grade oil. Before this, oils were made in single-viscosity grades such as 20, 30, and 40.

With the right engineering, a multi-grade oil could be formulated that acted like a 10w grade at Minnesota January cold-starting temperature, but which, when hot, up around the top piston rings and in the exhaust-valve guides, acted the same as a 40-grade oil. The result was an oil that acted like 10 grade when cold but like 40 grade when hot. Cold-starting was improved and wear of the hottest engine parts was reduced.

VI improver additives are long-chain molecules such as polymethacrylate. Sound maybe a bit familiar? Methacrylate is the clear plastic we know as plexiglass or Lucite.

And when it comes to molecules that fold, they are us. The complex proteins containing hundreds or thousands of atoms that make life possible also fold in ways that create specific shapes and functions.