A new pressure sensor array for normal stress measurement in cSomplex fluids

A new pressure sensor array, positioned on the bottom plate of a standard torsional rheometer, is presented. It is built from a unique piezo-capacitive polymeric foam and consists of 25 capacitive pressure sensors (of surface 4.5 x 4.5 mm(2) each) built together in a 5 x 5 regular array. The sensor array is used to obtain a mapping of the normal stresses in complex fluids, which dramatically extends the capability of the rheometer. We demonstrate this with three examples. First, a pressure profile is reconstructed in a polymer solution, which enables the simultaneous measurement of the first and the second normal stress differences N-1 and N-2, with a precision of 2 Pa. In a second part, we show that negative pressures can also be measured. Finally, we focus on the normal stress fluctuations that extend both spatially and temporally in a shear-thickening suspension of cornstarch particles. We evidence the presence of a unique heterogeneity rotating very regularly. In addition to their low cost and high versatility, the sensors show here their potential to finely characterize the normal stresses in viscosimetric flows. (C) 2021 The Society of Rheology.