Ulrike Hempel

Quartz crystal resonator sensors under lateral field excitation—a theoretical and experimental analysis

AT-cut quartz crystals have been driven with the so-called lateral field excitation. When applying a voltage to the two electrodes placed on one crystal surface and separated by a small gap an electric field is generated, which is mainly confined in the lateral direction. Extraordinary changes can be observed in the conductance spectrum when applying a liquid to the other (bare) surface of the crystal. In contrast to traditional quartz crystal sensors, these changes must be attributed to electrical properties of the adjacent medium. It is suggested that a redistribution of the exciting electric field from the lateral toward the thickness direction occurs. The assumption is supported by simulations which allow insights into the piezoelectric excitation and transduction mechanism of the acoustic device under varied electrical boundary conditions at the bare surface as well as the shear displacement patterns involved. Results from 10 MHz plano–plano and 6 MHz plano–convex sensors which have been exposed to liquids of varying permittivity show the strong dependence of the sensor response on liquid permittivity which overlies the known dependence on density–viscosity.

Lateral field excited quartz crystal resonator sensors for determination of acoustic and electrical properties of liquids

Specifically designed lateral field excited (LFE) quartz crystal resonator sensors have been applied to liquids with systematically varied acoustic and electrical properties. Dramatic changes in the conductance spectrum including an extraordinary shift of the resonance frequency have been found when the bare sensing surface of the crystal is exposed to water [1]. It will be shown that the sensitivity to electrical properties of the liquid becomes effective via changes in the acoustic properties of the crystal and therefore appears in the complete admittance spectrum. Finite element simulations supporting this conclusion are presented in [2]. Here we report about the contributions of liquid density/viscosity and liquid permittivity to the sensor signal. Since the contributions superimpose, an empirical scheme could be developed to separate the mechanical from the electrical properties of the liquid.

Lateral Field Excited Acoustic Wave Devices: A new Approach to Bio-Interface Sensing

10 MHz AT-cut quartz crystals have been excited by placing two electrodes on one crystal surface. An electric field is generated, which is not only laterally confined in the plane of the quartz crystal disk, but also penetrates into the medium adjacent to the opposite bare sensor surface. Due to the mechanical and electrical nature of medium-sensor-interaction, access to additional physical material parameters is anticipated. Up to now the influence of an adjacent mediums electrical and mechanical property changes on the sensor response has not been examined in detail for Lateral Field Excited (LFE) sensors. In order to determine these influences, the sensor impedance spectrum has been investigated while the sensor was exposed to liquid acoustic loads exhibiting varying permittivity and conductivity. The associated results will be discussed in terms of reorganization of the electric field distribution within the quartz.

Particle characterisation in highly concentrated dispersions using ultrasonic backscattering method

Determining particle size and concentration in highly concentrated suspensions and emulsions is challenging, especially under process conditions. In general, ultrasound therefore can be used for particle characterisation due to the ability of sound waves to pass opaque dispersions, whereas optical detection principles mostly are limited to low particulate contents. An established acoustic method, the ultrasonic attenuation spectroscopy, uses a transmission setup for measuring the attenuation of a dispersion. A major drawback of this measurement method is caused by the fact, that the measuring gap tends to plug, which again limits the inline capability. To overcome this limitation, an ultrasonic reflection setup is used for gathering the sound waves, which are reflected, respectively backscattered by the dispersion. Statistically analysing the corresponding backscattering signal yields the sound attenuation as well as a scattering intensity equivalent. Both measurement parameters can be shown to be sensitive against particle size and concentration. Based on a single scattering theory, a semi-empirical approach is presented for interpretation of measurement results with respect to particle size and concentration. Measurements, performed on a glass beads in water dispersion, show good agreement with theory for dimensionless wave number 0.1

Phase boundary characterization in liquids by acoustic waves

Precise inline process analysis, as an instrument for securing product qualities and enhanced process efficiency, nowadays often requires not only a temporal but also a spatial investigation of relevant physical process parameters. From the variety of available methods that allow for a tempo-spatial medium analysis, the ultrasonic-based tomographic approach and its direct application for inline monitoring of liquid systems is discussed in the present contribution. As an application example, the localization and characterization of an a priori unknown multiphase layer system has been performed by means of a simple ultrasonic transmitter–receiver arrangement. The data acquisition is based on the ultrasonic waves that are transmitted through the medium under test. Those signals represent the input of the tomographic process imaging. Using a linear model describing the homogeneous wave propagation and a fast estimation algorithm running on a normal PC, the extraction of information regarding layer thicknesses, materials and boundary properties has been performed in real time. The corresponding results are verified by simulations and semi-numeric studies and are additionally confirmed by first experimental tests.

Finite Element Analysis of Lateral Field excited thickness shear sensors

Lateral field electroded (LFE) sensors have been recently introduced that can study the permittivity and conductivity (electrical properties) of liquids in contact with the surface opposite the electroded side. The unique feature of these sensors is that the response depends in part on changes in the electrical field distribution in the quartz blank due to the electrical properties in the liquid. This work uses finite element analysis (FEA) to model the past plano-plano devices and a new plano-convex design as the electrical boundary conditions on the side opposite the electrodes change from free of surface charge to constant potential, either grounded or floating. Results are presented for the mode shapes, frequencies, and motional capacitance (Cm) of several modes present in the blanks. The Cm of the different modes changes dramatically between these electrical boundary condition extremes. The plano-convex design has better-defined mode shapes at the expense of sensing dynamic range.

Compact RF Impedance-Spectrum-Analyzer For Lateral Field Excited Liquid Acoustic Wave Sensors

A compact RF impedance-spectrum-analyzer (RF-ISA) has been developed and applied to a Lateral Field Excited (LFE) liquid acoustic wave sensor, which is sensitive to both, the mechanical and the electrical liquid property changes. The developed RF-ISA was used to determine the impedance spectrum of a LFE sensor exposed to various liquids with varying electrical and mechanical properties and to calculate the series resonance frequency and resistance at the sensors conductance maximum. The combination of the LFE sensor and the RF-ISA electronics results in a compact and portable liquid sensing unit, which not only has many sensing applications but also can provide insight into the piezoelectric excitation and transduction mechanisms.

Determining liquid properties by extraordinary acoustic transmission through phononic crystals

This contribution shows how a resonance-induced extraordinary acoustic transmission through a phononic crystal structure can be used as sensor for liquid properties. The phononic crystal consists of a metal plate with a periodic array of holes. Ultrasound propagates in a way that the incidence direction of sound is perpendicular to the plate. A characteristic transmission peak has been found to strongly depend on liquid sound velocity. The respective peak maximum frequency serves as measure for liquid composition. Numerical calculations based on FDTD and FEA reveal more insides to the propagation characteristics, in particular the presence of specific plate modes. Experimental investigations using a laser vibrometer and the Schlieren method support the theoretical findings.

Application of a Lateral Field Excited Acoustic Wave Device as a New Sensor Principle in Biointerface Analysis

AT-cut quartz crystals have been excited by applying a lateral electric field in the plane of the disk. Deposition of self-assembled monolayers proves the expected sensitivity of the sensor to small mass changes. When subjected to a liquid acoustic load the sensor exhibits an extremely high sensitivity to permittivity and conductivity of the liquid. In combination with an additional artificial interface layer, the influence on the resonance will be discussed in terms of reorganization of the electric field distribution within the quartz. Due to the mechanical and electrical nature of medium-sensor-interaction, access to interfacial properties is anticipated.

Sensor-Actuator-Based Network for an Early-Warning System in Extreme Weather Conditions

Abstract Damages caused by extreme weather have increased dramatically. Although extreme weather cannot be prevented, in many cases damages could be avoided or at least contained if affected citizens and institutions were specifically warned in sufficient time. In this context an early warning system is presented that registers approaching thunderstorms by means of specifically developed sensors which are located in close proximity to each other. The meteorological data obtained from this close-meshed sensor network in addition to the conventional techniques of radar and satellite provide the basis for short-term weather and rainwater run-off prognoses that are locally confined to small urban areas. In order to provide an efficient, situation-dependent and reliable distribution of warnings, an information logistic engine has been developed which combines spatial information of affected citizens or technical actuators, its information demand and the necessary processes for the automatic prevention of hazards. Using such information as input of hydrodynamic simulations of the sewer network, allows alerting citizens and emergency services of flooded streets. This is but one practical application of such an early warning system. Due to the combination of those heterogeneous actuators in an actuator network the process of mitigation of danger in extreme weather conditions can optimally be supported.