An advanced viscosity and density sensor based on diamagnetically stabilized levitation

We present a viscosity and density measurement principle based on diamagnetically stabilized levitation of a test object (a floating magnet) in the fluid to be characterized. Miniaturized resonant viscosity sensors are usually operated at relatively high frequencies, ranging from the lower kHz to tens of MHz, which utilize shear waves penetrating the liquid in the close vicinity of the vibrating surface thus only enabling the sensing of a small liquid film on the surface. With our approach, we reduce the resonance frequency which increases the penetration depth of the shear wave. Due to the freely levitated measurement body and the magnetic readout no mechanical or electrical connections into the measurement chamber are necessary, making the setup particularly useful for sterile, toxic or poisonous fluids. The design of the setup allows different modes of operation for the floater magnet, e.g., linear oscillations along the (vertical) z-axis and rotational oscillations around the (horizontal) x- or y-axis. In this contribution we analyze the rotational oscillation mode and present a theoretical model. Different additional features such as frequency tunability or the influence of the levitation height are examined. Measurements are discussed that prove the theoretical model and demonstrate the functionality of the principle. The use of rotatory oscillations instead of linear movements leads to a reduced measurement time and less influence of the boundaries.