Stefan Clara

Resonant pressure wave setup for simultaneous sensing of longitudinal viscosity and sound velocity of liquids

Increasing demands for online monitoring of liquids have not only resuted in many new devices relying on well-established sensing parameters like shear viscosity but also initiated research on alternative parameters. Recently, the longitudinal viscosity has been evaluated as a promising candidate because the devices arising enable the bulk of the liquid to be probed rather than a thin surface layer. We report on a multi-purpose sensor which allows simultaneous measurement of the sound velocity and longitudinal viscosity of liquids. The device embodiment features a cube-shaped chamber containing the sample liquid, where one boundary surface carries a flush-mounted PZT transducer. In operation, the transducer induces standing, resonant pressure waves in the liquid under test. We studied the influences of sound velocity and longitudinal viscosity on the generated pressure waves by means of the Navier–Stokes equation for adiabatic compressible liquids and exploited both parameters as the basic sensing mechanism. Furthermore, a three-port network model describing the interaction of the transducer and sample liquid was developed in order to be applied for extracting the parameters of interest from the raw measurement data. Finally, we demonstrate the device and method by carrying out and discussing test measurements on glycerol–water solutions.

A Viscosity and Density Sensor Based on Diamagnetically Stabilized Levitation

We investigate the feasibility of viscosity and density measurements using diamagnetically stabilized levitation of a floater magnet on pyrolytic graphite. This principle avoids any clamping structures in the measurement chamber and is, therefore, not suffering under unknown mounting conditions and is furthermore easy to integrate into microfluidic systems. The only part that has to be in contact with the liquid is the floater magnet. Immersing it in a liquid, buoyancy forces will come into play. Keeping the levitation height of the floater magnet constant in different liquid surroundings by accordingly adjusting the lifter magnet, the buoyancy force and, therefore, the density of the fluid can be determined from these adjustments. For more accurate results, a magnetic field modeling was used to determine the levitation height of the floater magnet out of the superposed magnetic fields of both magnets. For viscosity measurements, we add an additional ac-driven coil to the setup, which yields a superposed alternating force on the floater magnet causing periodic vibrations of the floater magnet. The vibrations are damped according to the viscosity of the surrounding fluid. By performing a frequency sweep, the frequency response of the damped spring mass resonator can be obtained where the resonance frequency for our setup is around 6 Hz. Furthermore, the influence of the levitation height on the resonance characteristics was examined by studying the resonance frequency and quality factor for different lifter magnet positions.

Viscosity and density sensor principle based on diamagnetic levitation using pyrolytic graphite

We investigate the feasibility of viscosity and density measurements using diamagnetically stabilized levitation of a floater magnet on pyrolytic graphite. Immersing the floater magnet in a liquid, buoyancy forces will come into play. Keeping the levitation height of the floater magnet constant in different liquid surroundings by accordingly adjusting the lifter magnet, the buoyancy force and therefore the density of the fluid can be determined from these adjustments. For viscosity measurements, we add an additional AC-driven coil to the setup, which yields a superposed alternating force on the floater magnet causing periodic vibrations of the floater magnet. The vibrations are damped according to the viscosity of the surrounding fluid. By performing a frequency sweep, the frequency response of the damped spring mass resonator can be obtained where the resonance frequency for our setup is around 6 Hz.

Utilizing the transient response of an acoustic transmission setup utilizing pressure waves to determine physical liquid parameters

Ultrasonic sensors are suitable for the determination of physical fluid parameters like e.g., viscosity, mass density and sound velocity. In this contribution we present the concept of a recently devised sensor setup utilizing transient pressure waves to determine the longitudinal viscosity of fluids. Besides the basic sensor setup an according 1D-model, an implemented PSPICE model of the whole sensor system and a comparison of simulation results to measurement results obtained with a prototype device are presented.

An Electromagnetically Actuated Oscillating Sphere Used as a Viscosity Sensor

We present a novel viscosity sensor principle which utilizes an oscillating metal sphere attached to a wire acting as mechanical spring. The oscillation is electromagnetically excited using four actuation coils. The arrangement of the coils allows a linear oscillation in one plane but also a circular motion if the actuation coils are driven accordingly. The latter causes a constant speed of the sphere and thus, a stationary flow around the sphere, which allows probing a different rheological regime compared with the oscillation mode featuring continuous acceleration and deceleration. For viscosity measurements, the drag force acting on the sphere is the crucial parameter in the sensing approach. The calculation of this force requires modeling of the magnetic field generated by the actuation currents in the coils. To verify the model for the magnetic field and the forces, a COMSOL simulation was performed and the results were compared with those obtained with the model. Finally, the approach was verified by means of measurements using different viscous liquids (viscosity standard oils and water glycerol mixtures).

A viscosity sensor utilizing an electromagnetically actuated oscillating sphere

We present a novel viscosity sensor principle which uses an oscillating metal sphere attached to a wire acting as mechanical spring. The oscillation is electromagnetically excited using four actuation coils. The arrangement of the coils allows a linear oscillation in one plane but also a circular motion if the actuation coils are driven accordingly. The latter causes a constant speed of the sphere and thus a stationary flow around the sphere, which allows probing a different rheological regime (more comparable to conventional falling ball viscometers) compared to the oscillation mode featuring continuous acceleration and deceleration.

A liquid properties sensor utilizing pressure waves

Miniaturized sensors for fluid viscosity often utilize shear vibrations and thus measure a thin film of fluid. To probe the bulk of a sample, pressure waves can be utilized instead. Then, however, the so-called longitudinal viscosity is determined, which can be equally useful for condition monitoring applications. Moreover, this parameter has not yet been investigated in detail such that material data are scarce. In this paper, we report on a prototype setup utilizing standing acoustic pressure waves in a small sample chamber. The impact of these resonances on the impedance of a PZT transducer is modeled and investigated experimentally. It is demonstrated that with this setup sound velocity and the longitudinal viscosity of liquid samples can be investigated.

Investigation and Modeling of an Acoustoelectric Sensor Setup for the Determination of the Longitudinal Viscosity

We present a two transducer setup suited for the determination of the second coefficient of viscosity, sometimes also termed acoustic viscosity. We present the basic sensor setup and according models in frequency and time domain allowing to extract the acoustic viscosity from the measurement data. We illustrate the approach using experimental data obtained with a demonstrator device. The setup, which has potential for further miniaturization, is operated in the time domain. Unwanted spurious effects and imperfections, such as diffraction, acoustic matching losses, and transducer losses, are discussed and according calibration and correction strategies are presented.

A Differential Pressure Wave-Based Sensor Setup for the Acoustic Viscosity of Liquids

We investigate a differential sensor setup utilizing acoustic pressure waves, which aims at the determination of the acoustic viscosity (or the second coefficient of viscosity) of highly viscous liquids with shear viscosities in the range of several 100 mPa · s and above. The whole setup is modeled in PSPICE and investigated experimentally. The presented approach is suitable for further miniaturization and is based on a differential configuration, such that spurious effects associated with the parameter spread of the used lead-zirconate-titanate transducers ideally cancel out. We demonstrate the operation in time domain (avoiding spurious interference effects), present an according calibration procedure with a known reference liquid, and discuss the effects of diffraction. Finally, the experimental results demonstrate the applicability of the setup.

Resonator sensor array for synovial fluid characterization

We introduce a resonating viscosity-density sensor array measurement setup for the characterization of synovial fluid, the joint-lubricant in humans and animals, in an isolated and protective environment to prevent degradation of the fluid sample due to exposure to air. Our measurement technique requires very low sample volumes of 28 μl or less, and offers a more reliable alternative to capillary breakup tests already in use. The described method can be extended to characterize other biological and non-Newtonian fluids.