Oils that go solid
R. David Whitby | TLT Worldwide November 2010
A host of interactions occur when two spheres collide and pressure increases.
Researchers used an apparatus similar to Newton’s Cradle to investigate collisions between liquid-coated spheres.
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In June C.M. Donahue, C.M. Hrenya andR. H. Davis of the University of Colorado in Boulder, Colo., submitted a paper in
Physical Review Letters in which they describe investigations of three-body collisions between liquid-coated spheres.
The researchers used a Stokes’ Cradle, which is similar to the Newton’s Cradle (the popular desktop toy named after Sir Issac Newton) for their experimental work. The Stokes’ Cradle concerned flows involving liquid-coated grains that occur in nature (pollen capture and avalanches) and in industry (air filtration and pharmaceutical mixing).
The Stokes’ Cradle has just three spheres. Newton’s Cradle has five steel balls that are suspended from strings in a line. When a ball at one end is lifted and released, it collides with the others and stops while the sphere at the far end of the line lifts away from the group. The sphere then swings back and the process repeats itself.
The researchers coated the three balls in the Stokes’ Cradle with oil and discovered that when one end was lifted and allowed to fall slowly, all three balls stuck together. If swung harder, the incoming ball rebounded while the others moved off together. When swung really hard, all three balls separate on collision. The changing oil pressures between adjacent spheres were responsible for this behavior. For example, when two spheres collide and pressure increases, the oil behaves as a solid.
Many oils exhibit an increase in viscosity as pressure rises. This is a desirable quality of lubricants in achieving effective elastohydrodynamic lubrication, which is a characteristic defined by the oil’s pressure-viscosity coefficient.
Only at pressures of 3,000 psi (20.7 Mpa) and higher does pressure begin to cause a significant increase in the viscosity of mineral oils. At 5,000 psi (34.5 Mpa), the viscosity of a typical mineral oil is approximately double its value at atmospheric pressure. As pressure increases, the viscosity accelerates as mineral oils cease to behave like liquids and eventually change their state to that of a waxy solid.
Viscosity increases with pressure because the molecules are squeezed together forcing greater interaction. In an EHL contact, where the pressure can be 2.1 Gpa (300,000 psi), the viscosity is so high that the oil is considered a pseudoplastic solid. Viscosity at high pressures is measured by flow through pressurized capillary tubes or a ball falling down a pressurized tube. A pressure-viscosity coefficient also can be measured from oil film thickness and other parameters from a transparent disk-on-ball apparatus.
The pressure-viscosity coefficient is an important parameter in tribology. The pressure-viscosity property of liquid lubricants was at the foundation of the discovery of EHL by Duncan Dowson and G. R. Higginson in 1966. In EHL contacts, oil film thickness is directly proportional to the 0.74 power of the pressure-viscosity coefficient. Another important parameter is the adiabatic bulk modulus, which is calculated using the velocity of sound in the lubricating oil. It has been found that the pressure-viscosity coefficient increases exponentially with the adiabatic bulk modulus, and the relationship depends on the molecular structure of the oil.
The pressure-viscosity coefficient refers to the relationship between the load placed on the oil film (pressure) at the dynamic load zone and the thickness of the oil film (viscosity) at that load when all other factors (i.e., material, temperature, geometry, speed, load) are constant. The pressure-viscosity coefficient gives fixed values for lubricant film thickness in the EHL regime based on a mathematical estimation, as noted in the American Gear Manufacturers Association Information Sheet, AGMA 925-A03.
The actual unit of measure (mm
2/N) is less useful than the percentage differences between different types of mineral oils and synthetic fluids. For equivalent viscosity oils at a given temperature, pressure-viscosity coefficients are ranked naphthenic oils>paraffinic oils>polyal phaolefins>polyalkylene glycols. This means that naphthenic oils go solid at high pressures that are lower than for the other fluids. Synthetic oils are not necessarily best all the time.
David Whitby is chief executive of Pathmaster Marketing Ltd. in Surrey, England. You can reach him at pathmaster@dial.pipex.com.