|
In this contribution a feasibility study on resonating sensors for rheologic properties such as e.g., viscosity
facilitating measurements at tunable frequencies is presented. For the concepts presented in this work, sample
liquids are subjected to time harmonic shear stresses induced by a resonating wire and a suspended resonating
platelet, respectively. From the resulting frequency response the liquid's rheological properties can be deduced by
fitting the parameters of an appropriate closed-form model representing the physical behavior of the sensors. To
allow large penetration depths of the shear waves being imposed by the resonating mechanism into the test liquid,
it is desired to have oscillators with resonance frequencies in the low kilohertz range. Large penetration depths
become important when examining complex liquids such as multi-phase systems as, e.g., emulsions. For the
investigation of liquids showing shear thinning (or thickening) or viscoelastic behavior, it is necessary to record
the liquid's characteristics not only at one single frequency but in a range of different frequencies, which in the best
case should cover several decades of resonance frequencies. For this purpose, especially in the case of resonating
microsensors, it is desired to have devices, which can be operated at tunable frequencies without changing their
geometries. For the two concepts presented in this work, the ability of tuning the sensor's resonance frequency
is based on varying the normal stresses within tungsten wires. The use of appropriate materials and different
micro-fabrication techniques are discussed and the applicability of the devices for rheological measurements are
outlined. The models are compared to measurement results and the capability of the particular resonator for
accurate and reliable sensing is discussed.
|
