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We present a new technique for 3-D position sensing and active magnetic levitation of a steel ball for use in a levitating ball viscometer. In order to achieve a stable levitation, a very sensitive positioning measurement system is mandatory. For this task the differential transformer principle was chosen to realize a 3-D position measurement. This leads to a purely magnetic sensor and actuator system without the need for other transducer types such as optical readout. The actuation utilizes power efficient switch-mode electronic circuitry which opens the possibility of upscaling the device, if demanded, for future applications. It is shown that this switch-mode actuation can be combined directly with the position measurement when special switching patterns are applied. A position resolution of ∼ 100 µm in all three axial directions at a sample rate of 476.19 Hz is achieved. For viscosity sensing, the steel ball is magnetically driven to orbital movements of variable revolution frequency of up to 2.5 Hz within a fluid chamber. The frequency response is analyzed and related to the shear viscosity of the fluid under test. As a proof of concept, measurements in various viscous liquids were performed with the prototype, showing promising results in the range of 1–10 mPa s. The principle may also be of interest for applications beyond viscosity sensing, such as fluid mixers, or as actuators in microfluidic devices.

We present a robust viscosity measurement system based on a torsionally oscillating pipe. The sensitive surface of the sensor performs periodic movements in the fluid to be sensed, generating a shear wave that penetrates the fluid. Due to this interaction, the resonance characteristic of the structure is affected, in particular the quality factor decreases with increasing viscosity. The pipe is mounted at its center where it features a nodal point of the preferred resonant mode, reducing temperature issues while simultaneously enabling high quality factors. A mathematical model is presented illustrating how different parameters influence the sensitivity of the sensor. Long-term measurements were performed to demonstrate the time stability of the sensor setup. [ABSTRACT FROM AUTHOR]

A recently introduced method for robust determination of the parameters of strongly damped resonances is evaluated in terms of achievable accuracy. The method extracts and analyzes the locus of the resonant subsystem of noisy recorded complex spectra, such that the interfering influences of many environmental factors are eliminated. Estimator performance is compared to the absolute lower limit determining the Cramér–Rao lower bound (CRLB) for the variance of the estimated parameters. A generic model that is suitable for representation of a large class of sensors is used and analyzed. It is shown that the proposed robust method converges to the CRLB for low measurement noise. [ABSTRACT FROM AUTHOR]

When solute molecules in a liquid evaporate at the surface, concentration gradients can lead to surface tension gradients and provoke fluid convection at the interface, a phenomenon commonly known as the Marangoni effect. Here, we demonstrate that minute quantities of ethanol in concentrated sodium hydroxide solution can induce pronounced and long-lasting Marangoni flow upon evaporation at room temperature. By employing particle image velocimetry and gravimetric analysis, we show that the mean interfacial speed of the evaporating solution sensitively increases with the evaporation rate for ethanol concentrations lower than 0.5 mol %. Placing impermeable objects near the liquid-gas interface enforces steady concentration gradients, thereby promoting the formation of stationary flows. This allows for contact-free control of the flow pattern as well as its modification by altering the objects shape. Analysis of bulk flows reveals that the energy of evaporation in the case of stationary flows is converted to kinetic fluid energy with high efficiency, but reducing the sodium hydroxide concentration drastically suppresses the observed effect to the point where flows become entirely absent. Investigating the properties of concentrated sodium hydroxide solution suggests that ethanol dissolution in the bulk is strongly limited. At the surface, however, the co-solvent is efficiently stored, enabling rapid adsorption or desorption of the alcohol depending on its concentration in the adjacent gas phase. This facilitates the generation of large surface tension gradients and, in combination with the perpetual replenishment of the surface ethanol concentration by bulk convection, to the generation of long-lasting, self-sustaining flows.

Advanced fluid models relating viscosity and density to resonance frequency and quality factor of vibrating structures immersed in fluids are presented. The numerous established models which are ultimately all based on the same approximation are refined, such that the measurement range for viscosity can be extended. Based on the simple case of a vibrating cylinder and dimensional analysis, general models for arbitrary order of approximation are derived. Furthermore, methods for model parameter calibration and the inversion of the models to determine viscosity and/or density from measured resonance parameters are shown. One of the two presented fluid models is a viscosity-only model, where the parameters of it can be calibrated without knowledge of the fluid density. The models are demonstrated for a tuning fork-based commercial instrument, where maximum deviations between measured and reference viscosities of approximately ±0.5% in the viscosity range from 1.3 to 243 mPas could be achieved. It is demonstrated that these results show a clear improvement over the existing models.

We provide an overview of recent achievements using quartz tuning forks for sensing liquid viscosity and density. The benefits of using quartz crystal tuning forks (QTFs) over other sensors are discussed on the basis of physical arguments and issues arising in real world applications. The path to highly accurate and robust measurement systems is described and a recently devised system considering these findings is presented. The performance of the system is analyzed for applications such as the mixing ratio measurement of fuels, diesel-soot contamination for engine oil condition monitoring, and particle size characterization in suspensions. It is concluded that using properly designed systems enables a variety of applications in industry and research.

This paper reviews our recent work on vibrating sensors for the physical properties of fluids, particularly viscosity and density. Several device designs and the associated properties, specifically with respect to the sensed rheological domain and the onset of non-Newtonian behavior, are discussed.