July 30, 2001
Researchers Solve Decades-Old Problem: Why Branched Hydrocarbons Make Better Lubricants Than Linear Hydrocarbons
Santa Barbara, Calif.-- Lubricants--thin films that enable one surface to slide by another--are most evident in their absence. The annoying sound of abrading surfaces called "chatter" alerts a machine operator to the possibility of incipient failure. Blisters, formed to cushion the rubbing of shoe against skin, are most apparent and most painful when the blister breaks and the lubricating fluid leaks out.
Lubricants reduce friction. And though human awareness and use of "friction" dates back millennia (at least to the making of the first fires), precise scientific investigations of the molecular-scale properties of lubricants and friction are little more than a decade old. And the resultant theory that is beginning to emerge applies alike to micromachines and earthquakes and to the sensory experiences of touch and taste affected by the apprehension of texture.
Jacob Israelachvili, UC Santa Barbara professor of chemical engineering and of materials, conducts investigations designed to explore the fundamentals of how surfaces interact. "We are looking," said Israelachvili, "at a very special state of matter." What he finds intellectually stimulating about his investigations is "how non-intuitive a lot of things are." In other words, in the world of surface interactions, often what one thinks should happen is not at all what happens.
A pair of papers by Israelachvili and his former graduate student Carlos Drummond explains one such phenomenon, which has been a long-standing problem in the field of surface interactions. Scientists have known for decades from straightforward observation that branched hydrocarbons make better lubricants between two surfaces under pressure than do linear hydrocarbons. Though this fact is evident, it would seem from the viscosity of the two hydrocarbons that the reverse should be true.
Both types of hydrocarbons are chains of molecules made up either exclusively or primarily of hydrogen and carbon. Linear hydrocarbons are like strands of straight hair. Branched hydrocarbons are like millipedes with numerous tiny side-shoots.
Linear hydrocarbons flow more readily than branched hydrocarbons and therefore have a lower viscosity. It would seem intuitively that the material that flows more readily would make a better lubricant than the more sluggish material. Or, to put it another way, it would seem that the material that is more liquid-like would retain more of its liquidity under pressure than the material that is less liquid to begin with. But that commonsense deduction is derived from contemplating the behavior of materials in the bulk. Put the same materials under the pressure of sliding surfaces, and surprising things occur.
Israelachvili and his student reported first on "Dynamic Behavior of Confined Branched Hydrocarbon Lubricant Fluids Under Shear" last year in Macromolecules (June 6, 2000). What matters are not the bulk properties of the hydrocarbon, but the precise shapes of the molecules because shape determines how the molecules pack under pressure.
It turns out that millipede-like structures cannot be neatly packed together like a stack of boards because the small side-shoots prevent the molecules from fitting together side by side. That molecularly messy structure means that the branched hydrocarbons remain liquid-like and, therefore, are good lubricants under the pressure of sliding surfaces.
The researchers next looked at linear hydrocarbons and discovered that a thin film of such material under pressure between two sliding surfaces does not just lose some of its liquidity, it loses all of it and becomes a solid. When the film is only three to four molecules thick, it turns into a solid. With the sliding of three solids against each other, the whole system grinds to a halt.
Pressure Induced Phase Transition
Israelachvili and Drummond have discovered and explained a phase transition from liquid to solid due to pressure in confined geometries. They reported that discovery in "Dynamic Phase Transitions in Confined Lubricant Fluids Under Shear," which appeared in April on-line in the publication of the American Physical Society Physical Review E.
With the linear hydrocarbons, pressure on a thin film between surfaces results in an orderly crystalline structure. With the branched hydrocarbons the resultant structure under very intense pressure is more like an amorphous solid or glass. In other words, the molecules are not actually structured in the lattice of a crystal but nonetheless begin to exhibit the properties of a solid.
The second paper also begins to explore the question of how a solid under pressure can become a liquid. Israelachvili explains, "If you have a solid between two surfaces and slide the surfaces, something has to give somewhere. Something has to crack or have dislocations. Nobody knows yet what exactly is going on. What we do know is that to get movement in that solid film, it has to change and turn into something that allows movement to occur. But what?"
Cooperative Rearrangements Take Time
Another interesting finding is that it can take a long time for a solid thin film to transform into a liquid. The molecules are so highly confined that the surfaces have to rearrange themselves cooperatively. When a solid film transforms into a liquid, long distances are required.
Israelachvili likes to imagine the rearrangement as akin to brushing long hair that is badly tangled. Repeated movements of the brush through the hair are required. Knots cannot be undone with a single pass because each individual hair in the knot has to be rearranged with respect to every other individual hair in the knot to undo the tangle.
There's little question from the way Israelachvili talks about his work that what motivates him is a keen intellectual curiosity. But he is also aware that the applications for the smooth operation of machines are enormous. Moving parts in close proximity require lubricants, which are key both to conserving mechanisms and making them more efficient.
This fundamental understanding of lubricants will enable the design of new lubricants. Most lubricants are now oil-based. But one goal is to design water-based lubricants that will be environmentally more benign than the oil-based ones in present use.
The Smaller, the More Important Surface
Perhaps even more intriguing are the ramifications of thin film research for the micro- and nanoscale devices of the future. In the world of the very small, the role of surfaces matters even more than in the macro world of turbines and engines. There the ratio of surface to volume increases. In addition, Israelachvili points out, in the nano and micro worlds surfaces tend to be smoother so issues of friction in operation become bigger.
Israelachvili's research on friction is funded by EXXON, the U.S. Department of Energy, and the Office of Naval Research; and its relevance to earthquakes and seismology, by the Keck Foundation. The research reported in these two papers was also supported by the Venezuelan National Oil Company, which made provision for Drummond to come from Venezuela to the United States to do a Ph.D. in chemical engineering under Israelachvili's supervision. Drummond now works in Bordeaux where his research is being funded in part by the French oil company ELF.
Note: Professor Israelachivili can be reached at 805-893-8407 or firstname.lastname@example.org.
Media ContactTony Rairden