Your EHD Rig May Not Be As Elastohydrodynamic As You Think

The concentrated contact formed between a steel ball and a glass disc—the optical elastohydrodynamic lubrication (EHD) rig—has been the primary instrument for experimental investigations of elastohydrodynamic film thickness. It has been a source for values of pressure-viscosity coefficient, a difficult-to-define property of liquids. However, comparisons with the pressure dependence of the viscosity obtained in viscometers show little agreement. There are multiple reasons for this failure including shear-thinning and compressibility of the oil. Another reason for the poor agreement is the subject of this short note. The optical EHD rig using glass as one surface will only be in the piezoviscous-elastic (EHD) regime when the pressure-viscosity coefficient is large. For low values, it would be operating in the isoviscous-elastic regime (soft EHD).