Driving modes and material stability of a vibrating polyethylene membrane viscosity sensor

In this contribution a macroscopic double membrane rheometer presented earlier is modified to a single membrane configuration in order to study further characteristics of driving modes and material properties. Excitation and readout of the membrane vibration are based on Lorentz forces induced in a static magnetic field. First, the possibility to influence on the excited vibration mode by pre-stressing the membrane is studied. It results that pre-stressing slightly increases the frequency of the resonance peaks and furthermore enables the excitation of lower order vibration modes at lower frequencies. By changing the pre-stress, the induced vibration modes and consequently the sensitivity range of the sensor can be altered. A second focus of the present work was to study the robustness of the Poly-Ethylene material and the resulting change of the vibration behavior with respect to temperature and aging. It was found that heating of the material increases the internal friction and decreases frequency and quality factor of the eigenmode. Furthermore, aging results in a considerable increase of the polymer stiffness leading to an increase of the eigenfrequency.