Multimodal Torsional Vibrations For The Characterization Of Complex Fluids
Free (open access)
191 - 199
T. Brack & J. Dual
A fast and dynamic method to determine the viscosity of a Newtonian fluid can be achieved by measuring the damping value of a torsional resonator immersed in the fluid. For the investigation of complex fluids, the viscous as well as the elastic behaviour of the sample have to be observed. Since these properties vary with shear rate, several frequencies have to be analysed. An approach is presented that allows to measure the viscoelastic parameters of a complex fluid via the resonance frequencies and damping values of a torsional rod vibrating at several vibration modes, i.e. at several frequencies. The measurement is performed with a phase-locked loop (PLL) based control loop, which uses the characteristic value of the phase difference between excitation of the sensor and response at resonance (e.g. 90°) to track the resonance frequencies. The slope of the phase curve at resonance, which can be identified via two different frequencies near resonance (e.g. at 90° ) is related to the damping. The focus of this work lies on the derivation of explicit, analytic equations that relate the vibrational parameters of the whole system (resonator + fluid) with the properties of the surrounding fluid for the case of low viscosity and elasticity. Keywords: resonance sensor, vibration control, complex fluids, rheology, phaselocked loop.
resonance sensor, vibration control, complex fluids, rheology, phaselocked loop.