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Of the fields for which the piezoviscous property of liquids is of interest, elastohydrodynamics is unique in deriving its existence from this property. Even recent publications in the field, however, have based the description of the pressure variation of viscosity on myths rather than empirical data. The author offers a description of the construction and operation of an easy-to-use viscometer capable of accurate measurements to a pressure of 1 GPa. The classic measurements of Nobel laureate P. W. Bridgman are repeated and examples of measurements on lubricants are given.
KEY WORDS
Elastohydrodynamic; High-Pressure; Viscometer; Viscosity; Traction; Film Thickness; Pressure-Viscosity Coefficient
INTRODUCTION
The limiting low-shear viscosity of organic liquids at high pressures is of major interest to the study of elastohydrodynamic lubrication (EHL). The pressure-viscosity behavior of liquids is essential for the calculation of film thickness and traction in lubricated concentrated contacts. Fine details of chemical structure can produce significant differences in viscosity at gigaPascal pressure (Bair (1)). Also, viscosity mixing rules can be pressure dependent so that some additives in small concentration may produce significant changes in viscosity at high pressure.
It may be surprising then that the descriptions used by the EHL community for the variation of viscosity with temperature and pressure are generally idealizations that have little connection with the behavior of real liquids. This present situation within EHL may be attributed to the widespread use (Hamrock (2)) and even promotion (Evans and Johnson (3)) of the Roelands viscosity model, which was based upon inappropriately truncated data sets from the literature (Bair (4)).
The viscometer that is the subject of this article is well suited to the measurement of limiting low shear viscosity as a function of temperature and pressure. Viscosity varies with shear stress (or shear rate), as well, in a manner that is known (Tanner (5)) to follow approximately a power law at high shear stress. Shear-dependent viscosity may be measured to high pressure using capillary or circular Couette instruments or by oscillatory shear, and a review of experimental techniques (Bair (6)) has been published. As shear stress (or shear rate) is reduced, the viscosity tends asymptotically to a value known as the limiting low shear viscosity, μ. This property is important to the film-forming capability of lubricants in the...