Micro Resonators For Applications In Liquids And Gases

• Applicant: Professor Dr. Helmut Seidel
• Subject Area: Microsystems
• Term: from 2018 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 398351153

Monitoring the properties of liquids and gases is important in many fields of application. This includes condition monitoring of operating fluids and combustion, environmental and indoor emission surveillance as well as food and gas analysis. Sensors based on physical principles typically exhibit more stability over time than purely chemical sensors. Silicon based MEMS resonators are particularly useful for such applications. They can be immersed in liquids or are surrounded by a gas atmosphere. The fluid induced damping causes changes in the quality factor and in the resonant frequency. These changes depend on the type of resonant mode and on the surrounding geometry, particularly on the presence of a bounding plane in close vicinity (squeeze film effect and transitional zone). The fluid density and viscosity can be extracted from the damping behavior, including the presence of impurities. More specifically, the degrees of freedom of a molecule can be detected. However, the damping in liquids can be very strong, so that measuring the quality factor becomes difficult. In such cases it is the aim to find higher modes and to optimize the geometry of the resonator in such a way that energy losses of the vibration remain small, leading to reasonable quality factors. Besides such application oriented questions several scientifically relevant questions shall be addressed, including the clarification of energy loss mechanisms in different media and in different pressure regimes. Special attention is devoted to the transition regime between molecular and viscous damping (around Knudsen number 1) as well as the effect of a bounding plane positioned within a well defined gap distance of the resonator (transition from squeeze film effect to free oscillator). In this context, a thermomechanical resonance effect is under close investigation, leading to increased energy losses when the travel time of a thermal wave coincides with the period of the oscillator. A further important point is the influence of polyatomic degrees of freedom of gas molecules on the damping. As part of the ongoing project, valuable new insights on this topic were gained and a theoretical description derived. Further investigations with different resonator geometries including higher modes will be used to develop the theoretical description further. Up to now some new unexpected effects (e.g. nonlinear duffing behavior, peak splitting) were found, which will be investigated in the follow-up project. A possible application of this effect for broadband pressure measurement into the near high vacuum range is emerging. For studying the interaction with paramagnetic fluids (mainly oxygen) we plan to include a magnetic field into the resonator setup. From answering these scientific questions we expect deeper insight for finding an optimal layout of a resonator system for a particular technical application.

• DFG Programme: Research Grants
• International Connection: Jordan
• Cooperation Partner: Dr. Abdallah Ababneh