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Lubricant rheology governs the friction in any lubricated contact. In hydrodynamic lubrication, the genesis of friction is well-understood. However, when asperity contacts occur, the situation becomes more complex. In this “mixed” lubrication regime, it is known that lubricants can deviate dramatically from Newtonian behavior, but the source of this effect has not been identified. In particular, the question arises as to whether the non-Newtonian behavior of the lubricant is due to the extreme thinness of the film or to the very large shear rates to which it is subjected. In the current work, we analyze friction force measurements in the magnetic slider/disk interface to help resolve this question. Because of its precision geometry, the slider/disk interface is ideal for such an investigation. Results of the analysis indicate (1) the lubricant retains its bulk viscosity in films as thin as 11–12 molecular diameters; (2) the rheological state of the lubricant is determined by a parameter we introduce here as the “Newtonian” shear stress, and (3) the rheology of the lubricant at high Newtonian shear stress may indicate a newly discovered property of liquids.

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