| MadSci Network: Physics |
Hi Jane, the job of a lubricant is reduce wear between two moving surfaces as far as possible. With engines, which usually generate a lot of heat, you not only have to consider the viscosity of the oil but also how this viscosity varies with temperature - viscosity falls as temperature increases. In most cases, viscosity will alter significantly over temperature so lubrication oil in engines contain a mixture of oils of different viscosities to give ideal lubrication in all weathers and when the engine is just turned on and after it's warmed up. Anyway, I've digressed slightly. With regards different vegetable oils I found an interesting paper (from Slovenia of all places) http://acta.chem-soc.si/45/45-1-69.pdf which looks at the different viscosities of a number of vegetable oils. You don't need to read it (unless you have a burning desire to!) but take a look at Table 2 or Figure 2 to compare the viscosities over a range of temperature. As you suggested in your comment, the chemical nature of oils does influence the viscosity and the authors report that, generally, the amount of unsaturation (i.e. amount of double instead of single carbon-carbon bonds) affects viscosity. So that, olive is the most saturated (Table 1) and equally the most viscous. The reason is that viscosity within a liquid goes up as there are stronger forces of attraction between the individual molecules. With saturated (single C-C) bonds the molecules is more flexible, therefore the closer the molecules can approach each other. With closer proximity comes more entangelment and more effective van der Waals forces. Unsaturated (double C=C) bonds are rigid and impose kinks along the molecules so that these sections don't benefit so much from molecule to molecule attraction. Other factors which affect attraction are length of molecules (long equals more entanglement - like your hair) as well as more impurities and any partial oxidation, as seen when oils are heated for a long period of time (more oxygen might mean stronger van der Waals forces). But, to answer you original question, given the viscosity figures I don't think you would notice much difference in the lubrication qualities of different oils, however you would need to do testing for temperature performance and longevity for more demanding jobs. If you want to see if the figures in the paper correspond to observable differences in your own vegetable oils, there are viscosity experiments at http://jchemed.chem.wisc.edu/JCESoft/CCA/CCA2/MAIN/VISCLIQ/CD2R1.HTM Or there's a table of viscosity of various foodstuffs at the bottom of this page: http://www.foodtechsource.com/rcenter/tech_data/td_viscosity.htm Finally, here's a company that sells lubrication / coolants made from vegetable oil: http://www.manufacturingcenter.com/tooling/archives/1202/1202oil.asp Cheers, David
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