Abstract
Ninety years of high-pressure measurements with many different types of viscometers have shown that faster-than-exponential (super-Arrhenius) pressure dependence of viscosity is universal for glass-forming liquids and, therefore, all typical liquid lubricants. Dielectric spectroscopy at elevated pressure also yields super-Arrhenius response in the dependence on pressure of the primary relaxation time. In contrast, classical elastohydrodynamic lubrication (EHL) has gone to great lengths to ignore this phenomenon, including fictional accounts of the results of viscometry. As a result of this, classical EHL is unable to quantitatively account for one of the most important properties affecting friction at low sliding velocity, the low-shear viscosity. Differences in friction between similar liquids at low sliding velocity can be explained by their different inflection pressures. Some observed liquid response to shear stress at high pressure can be explained with the measured super-Arrhenius pressure dependence. It should be clear that, had classical EHL employed realistic pressure dependence of viscosity from its beginning, the field would have been in a better position today to solve engineering problems which involve the differences among molecular structures.
Highlights
The pressure dependence of viscosity was necessary to explain the presence of a film sufficiently thick to separate the roughness features of engineering surfaces in 37 Page 2 of 10Tribol Lett (2016) 63:37 elastohydrodynamic lubrication (EHL) [1]
This behavior has surprisingly been absent from the dialog of classical EHL since its beginning, and this absence is at least partly responsible for the spectacular failure of the field to provide an understanding of EHD friction using the thermophysical properties of the liquid
The low pressure response is most often slower-than-exponential, resulting in an inflection point in a log viscosity versus pressure plot. The absence of this real behavior from classical EHL has had serious consequences and is at the heart of the failure to understand the mechanism of friction
Summary
Tribol Lett (2016) 63:37 elastohydrodynamic lubrication (EHL) [1]. piezoviscous response is at the foundation of the field. Faster-than-exponential, or super-Arrhenius pressure dependence of viscosity (in reference to the dependence of a material property which relies on an Arrhenius law), has been a feature of accurate measurements of viscosity at high pressure for 90 years [2]. This behavior has surprisingly been absent from the dialog of classical EHL since its beginning, and this absence is at least partly responsible for the spectacular failure of the field to provide an understanding of EHD friction using the thermophysical properties of the liquid. In this second installment [3] of a series differentiating classical from quantitative EHL, it will be shown that, missing from the classical approach, super-Arrhenius piezoviscous response is the natural behavior of glass-forming liquids and that it is indispensable for understanding the behavior of liquids at high pressure and high shear stress
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