Parallel-Plate Geometry Correction for Transient Rheometric Experiments

Abstract

It is well known that the shear and shear rate are not uniform in the azimuthal flow within the gap between parallel concentric disks - perhaps the most versatile among the geometries used in rheometry. This flow inhomogeneity represents a disadvantage, because the data analysis becomes intricate. Typically the stress is calculated at the rim with the assumption that it varies linearly with the radial coordinate, and then a correction is applied. This correction may be very large, depending on the nature of the sample, type of test, and range of parameters. While for steady-state shear flow different methods for correcting the stress are available, for transient flows they are rather scarce and in some cases unavailable. In this work we analyze in detail the stress correction for the main rheometric experiments, and discuss when it is needed. To this end, we performed different tests with a commercial hair gel and a polyacrylamide solution. For oscillatory flows, a simple equation to correct the stress amplitude is obtained in terms of the amplitudes of the torque and shear rate.

Keywords

• rheometry
• parallel-plate geometry
• stress correction