| Description |
The rotational rheometer (cone-and-plate or parallel plates rheometer) is one of the most effective devices for measuring rheological properties of the viscoelastic liquid: the viscosity (?), the first normal stress difference (N?). However, it has been found practically that some errors were potentially associated with this type of rheometer: The "axial compliance error" is due to the use of linear-variable-displacement-transducer (LVDT) for first normal stress (N?) measurement, and it is potentially significant in the time-dependent material response measurement. Secondly, the low natural frequencies of sensitive LVDT springs fail in recording the high frequency response of a material. Lastly, misalignment of the sample holder (cone and plate) will change the geometry of the sample. These errors were quantified by performing rheology studies with the LVDT detached and a novel device fabricated with Micro-Electronic-Machining-System (MEMS) technique. The device is a pressure sensor plate of 25mm in diameter. It contains eight miniature capacitive pressure sensors, allowing measurements of the radical pressure profile, from which both the first normal stress (N?) and the second normal stress (N?) can be calculated. The apparent response time of N? to start-up of NIST-1490 shear flow was measured. The apparent response time was longer being measured with the LVDT than being measured with the pressure sensor plate, indicating that significant axial compliance errors were present during LVDT measurements. The natural frequency of the LVDT was lower than the high frequency behavior of the tested fluid NIST-1490. A slight cone-plate misalignment, smaller than the manufacturer?s suggested limit, developed a sinusoid-shaped radical pressure profile of the Poly(dimethylsiloxane) (PDMS), corresponding to the axial plane of the tilt. However, this misalignment error can be reduced significantly by averaging the pressure profiles over clockwise and counterclockwise rotation manners. With the pressure sensor plate, the normal stress ratio, ¥= - N?, was measured to be 0.189 for PDMS. ? N? |
