Measuring lubricant viscosity in‐situ using ultrasound: impact of temperature, shear rate, and pressure.

Drinks, sunscreen, blood, oils in engines: fluids are everywhere and carry essential functions across many industries -from food and cosmetics to pharmaceuticals and transport- where their viscosity plays a critical role. Viscosity affects product texture, drug delivery, and lubrication, and is highly dependent on operating conditions such as temperature, pressure, and shear rate. Conventional viscometers can seldom perform in-situ measurements, leading to a lack of experimental study of lubricant viscosity under realistic tribological conditions.

This study explored the use of shear ultrasonic waves for in-situ viscosity measurement. Ultrasonic viscometry, a relatively new technique, analysed the reflection coefficient at the interface between the test piece and the fluid. However, the influence of temperature, shear rate, and pressure on this method was not yet fully understood.

Several test rigs were instrumented with ultrasonic transducers, and a data processing method was developed. Temperature effects were studied from 20°C to 60°C. Results evidenced that a single cross-temperature calibration was sufficient to calibrate the ultrasonic viscometer, thus greatly reducing calibration time while improving measurement accuracy. Shear measurements revealed the ultrasonic viscosity was consistently lower than the low-shear viscosity, indicating that the ultrasonic viscometer acted as a high-shear device. Ultrasonic shear rates were found to range from 10^4 s−1 to 10^7 s−1, with parameters such as formulation, degradation, and frequency affecting the results. High-pressure measurements were conducted up to 600 MPa, and results were found to be in good agreement with high-pressure and high-shear literature values. In parallel, several models were developed to simulate ultrasonic wave propagation through multiple media. The results were compared to the experimental data and showed qualitative agreement.

This work advanced the development of the ultrasonic viscometer and aimed to make it more accessible for industrial in-situ viscosity measurements.