Pressure sensor enables normal stress measurements in non-Newtonian fluids

Non-Newtonian fluids can act in very unusual ways. A normal stress can result in the fluid swelling or climbing up a rod. Understanding normal stresses in these fluids has been a fundamental challenge that limits their industrial applications.

Gauthier et al. developed a pressure sensor array that can characterize normal stresses on non-Newtonian fluids at the millimeter scale. Unlike previous pressures sensors using piezoresistive arrays, this pressure sensor uses a piezo-capacitive polymeric material, which can be made in a standard lab using simple ingredients.

“We demonstrate in our manuscript is that this simplicity of fabrication does not come at the detriment of precision,” said author Anaïs Gauthier. “Our sensors do not drift in time and can measure pressures as low as a few pascals, with a frequency of typically 100 hertz, which is much higher than the rate of most rheological measurements.”

With a polymeric foam, the authors built a square array of 25 sensors. They tested the device with a classic polymer flow, finding they could detect very small normal stresses, which will allow characterizing extremely dilute polymer solutions.

Further experimentation showed the sensor can also make local measurements of negative pressures in a flow and characterize heterogenous flows. Most notably, the authors measured the rotation of a single normal stress wave that appeared in the shear thickening regime of a non-Newtonian cornstarch mixture – something that had never been measured before.

“We have been really excited by the results obtained on the cornstarch measurement,” Gauthier said. “It could explain the problems encountered in industry when mixing shear thickening fluids.”

Source: “A new pressure sensor array for normal stress measurement in complex fluids,” by Anaïs Gauthier, Mickaël Pruvost, Olivier Gamache, and Annie Colin, Journal of Rheology (2021). The article can be accessed at https://doi.org/10.1122/8.0000249 .