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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).

We report work in which an oil film is stretched between a moving colloid sphere and a static plane surface. Under some circumstances, when the sphere and the surface are pulled apart sufficiently rapidly, an unexpected transient decrease of the sphere-surface separation is recorded. Numerical simulations indicate that cavitation bubble growth may result in the development of sufficient tension to account for this phenomenon: a finding confirmed by separate acoustic experiments and high speed microphotographic work.

This paper reports measurements of the effective tensile strength (or ‘cavitation threshold’), Fc, of Newtonian silicone oils over a range of shear viscosities, and of a multigrade oil over a range of temperatures, T, in the range 20°C ≤ T ≤ 140°C. These measurements are the first to be obtained for a lubricant under dynamic stressing by tension, over a range of temperatures representative of those encountered under its normal operating conditions. This work introduces a method of estimating Fc from direct measurements of static positive pressures: it thereby eschews the reliance in previous related work upon direct measurements of dynamic negative pressures by transducers designed for use in the range of positive pressures.