Piezoelectric Rotary Vibrator (PRV) – a new oscillating rheometer for linear viscoelasticity

Abstract.

Conventional, motor controlled rheometers, allow, e.g., stress-controlled measurements of the shear viscosity up to several thousands in shear rate, but reliable oscillating viscoelastic measurements only up to f=50 Hz (ω=300 s^–1). This limitation is overcome – even if only for linear viscoelasticity – by a new piezoelectric instrument oscillating in rotary mode (PRV), covering the frequency range from 0.5 Hz up to 2 kHz , and bridging the gap to resonator methods.

The basic components of the PRV are the piezo drive, the shaft which connects the piezo drive with the lower plate, and a temperature controlled upper plate.

The piezo drive consists of a six-spokes-wheel, which is machined out of an aluminium alloy plate. On each spoke there are two piezo elements. Three spokes are exciting the vibration, which means they apply an angular momentum on the shaft and the lower plate. The other three spokes detect the angular deflection.

The PRV is operated by a LockIn-amplifier. The exciting voltage is proportional to the angular momentum, the detected voltage to the angular deflection.

To get the real – and imaginary – component of the complex shear compliance of a sample one has to carry out two measurements. The first without sample (unloaded), the second with sample (loaded).

In the paper it is shown how the complex restoring torque of the sample is revealed from the measurements, and how one gets the viscoelastic functions from the experimental data by using continuum mechanical calculations. Besides the strict solutions for a filled gap and a free sample surface, the slit approximation and the free radiation without reflection are of practical interest.

To prove the efficiency of the PRV-method some results on different materials are presented, a few of them expanded into resonator-frequencies and others compared to rheometer – and DMA – measurements.