In-situ ultrasonic viscometry of lubricants under high-pressure and high-shear

Abstract Viscosity affects lubricant film thickness and the separation of machine parts. It is thus a major parameter to ensure adequate lubrication and machine operation. Viscosity is dependent on operating conditions, especially pressure, which is known to vary up to several GPa in tribological contacts. Few viscometers are capable of performing in-situ measurements, and replicating the combined extreme operating conditions outside the contact zone is difficult. This work employs ultrasound to enable in-situ viscosity measurements under high-pressure and high-shear. The low-shear viscosity behaviour under pressure of distilled water, octane, squalane (SQL), squalane + polyisoprene (SQL+PIP), diisodecylphthalate (DidP), and polyalphaolefin 100 (PAO100) was derived from the literature using the Williams-Landel-Ferry-Yasutomi (WLF-Yasutomi) model. Combined with shear-thinning models from the literature, the viscosity under high-pressure and high-shear ( $$4.5 \times 10^6 \,{\text {s}^{-1}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>4.5</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mn>6</mml:mn> </mml:msup> <mml:mspace/> <mml:msup> <mml:mtext>s</mml:mtext> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> ) was determined. An ultrasonic viscometer was instrumented onto a high-pressure test cell. Several fluids were used to calibrate the ultrasonic viscometer under pressure. The ultrasonic viscosities of SQL+PIP and PAO100 were computed at $$40 \,^{\circ }\text {C}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>40</mml:mn> <mml:mmultiscripts> <mml:mspace/> <mml:mrow/> <mml:mo>∘</mml:mo> </mml:mmultiscripts> <mml:mtext>C</mml:mtext> </mml:mrow> </mml:math> , from ambient pressure up to 600 MPa, and compared with literature data. This work contributes to a better understanding of the ultrasonic in-situ viscometry technique. Such insight is crucial to apply this technique to challenging environments. The ultrasonic viscometer also holds significant potential to advance the understanding of complex fluids under high-pressure and high-shear conditions, where conventional measurement methods often fall short. Moreover, the ultrasonic viscometry technique has strong potential for industrial application, where there is a growing need for real-time, in-situ monitoring of fluid properties under varying operating conditions. This can lead to improved process control, safety, and efficiency across a range of industries.