Micromachined Viscosity Snesors

The project aims at the development of novel sensors for rheological studies on Newtonian as well as non- Newtonian fluids in a viscosity range up to 100 mPas. The concept is based on micromachined silicon structures that vibrate in the sample fluid. These structures will be excited near a mechanical resonance by means of harmonic Lorentz-forces. Depending on the properties of the sample, the resonant characteristic changes enabling the determination of the viscosity and mass density of the fluid. The technological approach opens an ample scope for systematic variations of device design parameters. Design series will be efficiently generated with in-house developed software tools to take full advantage of the possibilities of the available wafer space. Experiments with such devices will improve the theoretical knowledge by verification of analytical models and comparison with numerical calculations. This allows optimizing structures for a targeted fluid promising a deeper insight into rheological phenomena of complex liquids. The analytical and numerical models to be developed for characterizing the interaction between the fluid and the structure will in turn deliver the required information for improved sensor layouts. Furthermore, sensor designs offering viscosity measurements at different frequencies will be developed by exciting and utilizing several resonant modes. This feature is especially valuable for complex fluids that cannot be described by a single viscosity parameter. The complete micromechanical devices will be fabricated in house. The new sensors rely on specific extensions of the existing technology. The vibrating structures require an elaborate optimization of deposition and etching processes with respect to minimum residual stress. Additionally, precisely shaped side walls of the moving structures are aspired to achieve well defined in-plane vibrations of the structures. A crucial aspect of a viscosity sensor is the efficient transduction of mechanical motions into electrical signals. Preliminary studies of the proposers indicated that piezoresistive readouts exhibit promising characteristics. These readouts are not affected by the optical or dielectric properties of the sample fluid. Their fabrication is feasible with the in-house available technology. In combination with advanced electronic circuitry, the proposed readout will allow for highly sensitive measurements.

Fluids play a primary role in many industrial processes and machines but also in the technical environment of everyday life such as automobiles. It is often beneficial or even essential to monitor the actual state of the fluids in order to determine required maintenance, to monitor the process handling the particular fluid, or to indirectly obtain information about the state of the machine or plant. Very often the sensing process is actually targeting at chemical property of a liquid such as the pH-value or the presence of certain compounds. In contrast to dedicated chemical sensor, who employ a chemical interface which selectively reacts with (or adsorbs) the targeted substance, physical sensors can be used to indirectly monitor the fluid by measuring its physical properties. This concept avoids the adverse properties associated with chemical interface materials such as degradation, poisoning, poor reproducibility, etc. When it comes to physical quantities, an important class for condition monitoring applications is constitutive parameters such as conductivity, permittivity, Youngs modulus, and viscosity. The viscosity is a particularly representative parameter for liquids and commonly involves a measurement setup where a mechanically moving part interacts with the liquid. Established instruments employ precision drives and sensors to derive the viscosity from this interaction. But most often they are not suitable for integration in processes due to their lacking robustness, bulkiness, or, last but not least, price. In this project we investigated and demonstrated the feasibility of sensing concepts leading to miniaturized viscosity sensors. We employed miniaturization technologies such as micromachining and carefully evaluated the impact of a particular interaction mode with the liquid. The utilization of bearings and precision drives is avoided by using resonating devices or alternative concepts such as indirect determination by pressure drops. Besides the demonstration of the feasibility of shear-wave based devices, it was also found, that the second coefficient of viscosity is particularly suited for measuring bulk properties of fluids and can be equally well utilized for condition monitoring applications.The results provided the groundwork for a new generation of miniaturized sensors which particularly allow the online monitoring and sensing of fluids in a wide range of applications ranging from production processes to medical applications.