Viscosity is a very general term with various meanings. On the one hand, it refers to the viscous material behavior of fluids. On the other hand, it also refers to a set of material parameters for characterizing the dissipative energy loss during wave propagation in viscous or viscoelastic fluids. The viscosity of a fluid and its dependence on temperature as well as on the intrinsic mechanical stress of the fluid is a key parameter in many applications in medicine (e.g., hematocrit characterization of blood, characterization of synovial fluid in connection with osteoarthritis and rheumatoid arthritis), food technology (e.g., process monitoring for optimized caramelization) and cosmetics (e.g., quality control of the consistency and stability of skin emollient), as well as online condition monitoring of lubricants (e.g. oil aging quantification).
In the field of miniaturized viscosity sensors and rheometers, DISS maintains a long-lasting, intensive cooperation with the Institute for Microelectronics and Microsensor Technology (IME) of the Johannes Kepler University Linz. Over the years, we designed, modeled, fabricated, and evaluated a large number of different sensors, where a special focus has been often attributed to cost-effective and disposable components, which are essential for novel applications in medicine as well as in bio- and food technology.
Shear viscometer and rheometers
Shear viscometer (and rheometers) are used to characterize the viscous (and viscoelastic) properties of fluids in response to shear stress. The shear viscometers and rheometers developed at DISS and IME often utilize a resonant principle, where the loading of the sensor surface by the measuring fluid leads to a change in the sensor’s electromechanical resonance behavior (e.g. resonance shift, amplitude damping and phase shift), which allows for quantification of the dissipative losses in response to imposed shear stresses in the fluid. Such devices commonly operate in the medium or high frequency range and are therefore highly prone to contamination of the sensor surface.
Longitudinal viscometer and rheometers
These devices characterize the viscous (and viscoelastic) properties of fluids regarding compressive stress. The continuous measurement of the viscosity-induced damping of pressure waves is a promising principle for future fluid online-condition monitoring systems (e.g. oil quality or emulsion condition monitoring). The determining parameter in this context is the longitudinal viscosity which is a superposition of shear and dilatation viscosity. A major advantage of this sensor principle compared to resonant shear viscosity sensors is that the pressure wave to be measured penetrates deep into the measuring fluid allowing for characterization of the bulk of the fluid rather than a thin surface layer.