Rheology of Multigrade Engine Lubricants in High Deformation Rate Extensional Flow

We report the results of recent experimental work involving the formation of low shear viscosity liquid filaments by the collapse of a gas bubble under a cavitation-generated shockwave. The filaments experience a significant degree of extensional deformation, at high rates of extension (typically within the range 1000 s^-1 to 5000 s^-1) which are relevant to considerations of journal bearing lubrication. The results of experiments conducted on samples of commercial multigrade engine lubricants in the SAE 15W-40 and 5W-30 viscosity grades indicate a substantially increased resistance to extensional flow when compared with the behaviour of their Newtonian counterparts. Interestingly, the technique is able to distinguish between the rheological properties of lubricants of identical SAE category (15W-40) which are formulated for use in different engines (petrol engine and diesel engine, respectively). The lubricant formulated for the latter application is found to exhibit the highest resistance to extensional flow.