Christian Riesch

Miniaturized sensors for the viscosity and density of liquids-performance and issues

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.

Characterizing Vibrating Cantilevers for Liquid Viscosity and Density Sensing

Miniaturized liquid sensors are essential devices in online process or condition monitoring. In case of viscosity and density sensing, microacoustic sensors such as quartz crystal resonators or SAW devices have proved particularly useful. However, these devices basically measure a thin-film viscosity, which is often not comparable to the macroscopic parameters probed by conventional viscometers. Miniaturized cantilever-based devices are interesting alternatives for such applications, but here the interaction between the liquid and the oscillating beam is more involved. In our contribution, we describe a measurement setup, which allows the investigation of this interaction for different beam cross-sections. We present an analytical model based on an approximation of the immersed cantilever as an oscillating sphere comprising the effective mass and the intrinsic damping of the cantilever and additional mass and damping due to the liquid loading. The model parameters are obtained from measurements with well-known sample liquids by a curve fitting procedure. Finally, we present the measurement of viscosity and density of an unknown sample liquid, demonstrating the feasibility of the model.

A suspended plate viscosity sensor featuring in-plane vibration and piezoresistive readout

Miniaturized viscosity sensors are often characterized by high-resonance frequencies and low-vibration amplitudes. The viscosity parameter obtained by such devices is therefore not always comparable to those probed by conventional laboratory equipment. We present a novel micromachined viscosity sensor with relatively low operating frequencies in the kHz range. The sensor utilizes Lorentz force excitation and piezoresistive readout. The resonating part consists of a rectangular plate suspended by four beam springs. The first mode of vibration is an in-plane mode. Thus, the contribution of the moving plate to the device damping is low, whereas the overall mass is high. This principle improves the quality factor and gives additional freedom to the device designer. This paper presents the device concept, the fabrication process and a prototype of the viscosity sensor. Measurement results demonstrate the feasibility of the device and show that the damping of the device is an appropriate measure for the viscosity.

A micromachined doubly-clamped beam rheometer for the measurement of viscosity and concentration of silicon-dioxide-in-water suspensions

In this contribution we demonstrate the feasibility of a sensor system for viscosity and concentration measurement of complex liquids, in particular suspensions of silicon dioxide particles in water. The sensor system is based on a doubly clamped micromachined beam vibrating in the sample liquid, and an optical readout utilizing a DVD player pickup head. The vibrating beam features resonance frequencies in the range of several 10 kHz, and higher mechanical amplitudes than microacoustic sensors, e.g., quartz thickness shear mode (TSM) resonators or surface acoustic wave (SAW) devices. We show that the damping of the beam is dominated by the viscosity of the liquid, and that this relation also holds for the considered complex liquids, whereas a TSM resonator sensor fails to detect the steady state shear viscosity of the suspensions.

A Novel Sensor System for Liquid Properties Based on a Micromachined Beam and a Low-Cost Optical Readout

For the measurement of liquid parameters like viscosity and density vibrating micromachined structures offer useful alternatives to bulky conventional measurement equipment. Evaluating the shift of the resonance frequency and the change of the damping factor of such devices allows the simultaneous determination of viscosity and density. Furthermore, these sensors can even be used for complex liquids like emulsions where other microacoustic sensors like TSM quartz resonators, in comparison to conventional laboratory viscometers, measure in a different rheological domain. In our contribution we present a novel implementation of a viscosity and density sensor system utilizing a doubly-clamped vibrating micromachined beam. The beam deflection is determined by means of a laser pickup head as it is used, e.g., in DVD drives, yielding high sensitivity and a low-cost setup at the same time. The measured damping factor correlates well with the viscosity of the respective liquid, whereas the resonance frequency is also influenced by the liquids density.

Novel Readout Electronics for Thickness Shear-Mode Liquid Sensors Compensating for Spurious Conductivity and Capacitances

In this paper, a readout circuit for a thickness shear-mode (TSM) resonator is presented which can be used for sensing applications in liquids such as viscosity sensing. The system features compensation of both spurious capacitances and conductances in parallel to the resonator. This allows measurements even in conductive liquids without the need for an elaborate sealing of the sensor. The influence of the spurious elements is determined by means of two orthogonal synchronous detectors and eliminated by active compensation using voltage-controlled amplifiers (VCAs). Furthermore, the circuit is robust against possible phase errors. The basic concept is discussed in detail, and a readout circuit is developed. A prototype is presented, and sample results are given, demonstrating the feasibility of the approach

A Novel Combined Rheometer and Density Meter Suitable for Integration in Microfluidic Systems

In this contribution we present a combined rheometer and density meter based on two vibrating membranes carrying electrically conductive paths for excitation and readout. The liquid is contained in a 100 mul volume between the rectangularly clamped membranes. The vibration is excited by Lorentz forces arising from a static magnetic field provided by a permanent magnet and the current through the excitation path going back and forth on the vibrating part of the membrane. The sensor element is designed in such a way that the viscous liquid is subjected to shear stress. Additional conductive loops on the membrane perform the sensor readout by means of the induced voltage due to motion in a static magnetic field. The measured frequency response in a range from 500 Hz to 15 kHz allows the determination of the fluids mass density and viscosity. This novel sensor design is well suited for miniaturization and the integration in microfluidic platforms.

Characterizing Resonating Cantilevers for Liquid Property Sensing

Miniaturized liquid sensors are essential devices in online process or condition monitoring. In case of viscosity and density sensing, microacoustic sensors such as quartz crystal resonators or SAW devices have proved particularly useful. However, these devices basically measure a thin-film viscosity, which is often not comparable to macroscopic measurements. Miniaturized cantilever-based devices are interesting alternatives for such applications, but here the interaction between the liquid and the oscillating beam is more involved. In our contribution we describe a measurement setup, which allows the investigation of this interaction for different beam cross-sections. We present an analytical model based on an approximation of the immersed cantilever as an oscillating sphere comprising the effective mass and the intrinsic damping of the cantilever and additional mass and damping due to the liquid loading. The model parameters are obtained by a curve fitting procedure.

Remote electromagnetic excitation of miniaturized in-plane plate resonators for sensing applications

For the purpose of measuring the viscosity of complex structured liquids, a novel type of resonator is employed. A metallic frame with a spring-mounted plate in its center is placed in a magnetic field exhibiting a constant and a superposed sinusoidal component. This way, a dominant in-plane motion is excited. The numerical analysis of the frequency response shows a useable viscosity range up to approx. 1 Paldrs in a frequency range below 1500 Hz. Experiments demonstrate the feasibility of the concept, which is suited for further miniaturization.

A micromachined suspended plate viscosity sensor featuring in-plane vibrations and integrated piezoresistive readout

We present a novel resonant miniaturized sensor for the viscosity of liquids. The sensing element is a rectangular plate suspended by four beam springs. This plate vibrates in-plane. Thus, the plates contribution to the device damping is low, but the plate increases the moving mass of the resonator. This principle aims at improving the quality factor, and gives additional freedom to the device designer. The sensor was fabricated by micromachining, and features Lorentz force actuation and piezoresistive readout. We present a device prototype and show by experiment, that the quality factor of the sensor is an appropriate measure for the viscosity.