My last post focused on G-Oil, a new motor oil that uses beef tallow as a base, thus decreasing America's dependence on foreign energy sources. One thing I didn't address in that blog is the role of nanotechnology in oils -- which is because I had to learn some things first.
The G-Oil website has a section on nanoparticles, mostly focusing on the fact that the surface area to volume ratio of a nanoparticle is quite large. This is true, but it doesn't tell you why that has anything to do with making oil perform better.
The words "NANO GEODESIC BEARINGS" attracted my interest not just because they were in all caps, but because anytime you hear 'nano' and 'geodesic' together, you have to think buckyballs, those carbon structures resembling soccer balls that spent way too much time in the dryer and shrunk to about a nanometer in diameter.
Incidentally in my search for buckyballs, I stumbled across a toy called buckyballs that is unfortunately named. It's a collection of 216 magnetic ball bearings that technaBob describes as "fun, powerful choking hazards". You can make 3D structures like cubes, pyramids, etc. But as a magnetician, I'm a little miffed that the magic of magnetism has been ascribed to carbon. Carbon has its strengths, but magnetism really isn't one of them.
Buckyballs can form a solid very similarly to the way atoms form a solid. The crystal structure is face-centered-cubic (fcc), so imagine a repeating structure of a cube with a buckyball at each corner, plus one in the very middle of each face of the cube, like the photograph at left (which is made of they toy "buckyballs").
Buckyballs aren't the only molecules that can do this. There is a whole class of crystals called molecular crystals that use molecules instead of atoms as their building block. Most atomic crystals have an atom at each lattice site and the atoms are connected by very strong bonds (usually ionic or metallic). The bonds holding molecular crystals together are much weaker, so the molecules have a little more freedom of motion than the atoms. In some molecular crystals, the molecules can change orientation while maintaining their location in the crystal. When buckyballs form a solid, they can spin in place. Imagine the individual balls in the picture rotating while always maintaining the shape of the cube.
That suggests the idea of buckyballs as tiny ball bearings isn't such a far-fetched idea, especially since the buckyball's second cousin once removed, graphite, is an outstanding lubricant. That idea doesn't really hold water. Graphite lubricates because it is made of sheets of carbon atoms that separate when you apply force. The separation releases minute quantities of gas that act like atomic ball bearings.
Sure enough, most of the people who tried using buckyballs on surfaces like aluminum and stainless steel found that high concentrations of buckyballs resulted in a compacted mess with extremely high friction and low concentrations of buckyballs produced no difference in behavior compared with the bare surfaces. The problem, it would seem, is that you need to have the buckyballs held in one location with the freedom to rotate. And if you think about it, that's pretty much how ball bearings work anyway.
In 1996, Jacob N. Israelachvili and co-workers at UC-San Diego showed that adding buckyballs to organic solvents allowed the liquid to flow more freely against mica surfaces. The UCSD researchers explained the increased lubricity by suggesting that very small numbers of buckyballs layers deposit themselves along the mica surfaces. The bonds between the buckyballs and the surface are not very strong, which leaves them free to rotate as they do in the solid phase. The buckyball rotation allows the liquid molecules to move past the mica surfaces much more smoothly than they could otherwise.
I thought perhaps this was the mechanism that explained the buckyballs in G-Oil, but the UCSD work was organic solvents against mica at room temperature. That doesn't necessarily translate to motor oil against metal surfaces at high temperature, the conditions found in an engine.
I found a fair number of number of technical articles claiming that buckyballs do not markedly enhance the lubricity of motor oil. They do, however, improve the thermal conductivity of the oil and that makes the oil better at carrying heat away from the engine. Overheating is one of the more severe problems in racing because it's hard to go fast when your pistons are starting to melt.
More searching led me to a patent application that combines nanoparticles and microparticles in oil to improve fuel efficiency. The nanoparticles are made from hard materials, like alumina, diamond, ceria and titania -- the type of materials you'd use to polish things. The microparticles (which are 100-1000 times bigger than the nanoparticles) are softer materials like zinc oxide, graphite, talc, and copper oxide.
One of the problems with friction in your engine is that you lose energy because pieces have to move through the viscous oil. Another problem is that two surfaces that rub against each other at high heat are constantly exchanging atoms, so your parts go from a flat surface to a surface that has microscopic dips and bumps. Even though you can't see the imperfections with your bare eyes, those little divots and bumps impede the motion of the parts against each other.
In the patent application, the nanoparticles are made from hard materials because part of their job is to polish the insides of your engine and keep them nicely smooth. The soft microparticles are designed to embed themselves into materials that have developed microsized defects. The combination of the two types of particles, the patent submitter claims, can decrease the coefficient of friction from 0.22 to 0.01. (The coefficient of friction ranges from 0 to 1 for abrasive friction: a passenger car tire on asphalt is about 0.7-0.8.)
Another role I know about for nanoparticles in oil came from a discussion with Professor Shah at the University of Delaware a couple of years ago. He told me about putting alumina or titania nanoparticles in oil to increase the heat capacity of the oil. This initially confused me because both alumina and titania are thermal insulators. Copper is an excellent thermal conductor, so I would have guessed a metal of some type would be a good bet to improve heat conduction.
And I would have been wrong. Temperature is the motion of molecules. Solids are much better at conducting heat than liquids or gases. In the alumina nanoparticles in oil, Professor Shah told me that the oil forms a semi-crystalline layer coating the nanoparticles, and that semi-crystalline layer is better at dispersing heat than the liquid oil.
So I'm guessing that the primary role of buckyballs, or any carbon additives is primarily in polishing the surfaces of mating parts. Oil has to deal with very high pressure, as its whole purpose is creating a very thin film between two pieces so that they slip smoothly past each other. The current additives used for pressure are sulfur and/or phosphorous additives, which can actually encourage the parts to corrode, so using buckyballs for oil used in gearboxes, for example, would make sense.
It's sort of funny that so many manufacturers use 'nanotechnology' in their advertisements, as if just the fact that they put something in their product that is really small should be enough to sell you on their product. I'd really rather know exactly what's in the product and why -- especially if it's something I'm going to put on my face.