Layer guided shear acoustic wave sensors

Martin, F., 2003. Layer guided shear acoustic wave sensors. PhD, Nottingham Trent University.

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Acoustic wave micro-sensors with pure or dominant shear horizontal polarisation, employed for liquid operations with Love mode devices, are amongst the most promising because they have the highest sensitivity among all known acoustic sensors. In this thesis a model is presented for the mass sensitivity of Love waves on a finite thickness substrate. It is shown that the model includes shear horizontal acoustic plate modes (SH-APM). A relationship is derived between the slope of the dispersion curve and the mass sensitivity. A new type of sensor, a layer guided SH-APM, is proposed and it is shown that significant enhancement of mass sensitivity, compared to a SH-APM, occurs. In addition, the peak mass sensitivity for Love waves on a semi-infinite substrate is shown to scale directly with frequency provided no mode change occurs. Optimisation of guiding layer thickness corresponds to strong dispersion so phase and group velocities can be quite different. A formula relating the difference in group and phase velocities to the mass sensitivity has been derived and can be extended to a wide range of other acoustic wave sensors.

In the work of this thesis Love wave devices have been optimised in terms of mass sensitivity by increasing the wave-guide layer. A polymer waveguide was created with increasing thickness and higher order modes have been identified. Elastic, visco-elastic and liquid sensing operation at the first, second and third Love wave modes were achieved. Higher order modes were shown to exhibit substantial sensitivity in insertion loss to mass loading. A novel interrogation system based on a pulsed signal technique has been developed to investigate solid-liquid interactions or mass deposition at the interface of a surface acoustic wave sensor. The technique provided essential sensor parameters such as attenuation, phase change and time delay with the advantage of reflection peaks and acoustic modes discrimination, fast response and high sensitivity. The system was successfully operated for the detection of biological agents in liquid and directly compared to a continuous wave system employing a network analyser. Insertion loss sensitivity to the deposition of evaporated metals, spin-coated polymer and liquid sensing has been directly investigated for the first, second and third Love modes. In analogy to the phase velocity mass sensitivity, the sensitivity in insertion loss to mass deposition has been related to the slope of the insertion loss curve with normalised guiding layer thickness. Phase velocity liquid loading sensitivity by phase measurements was also investigated for the first, second and third Love mode. It has been shown that higher order Love wave modes exhibit substantial sensitivity to mass loading by insertion loss and phase measurements. The effect of dispersion on Love wave sensors has been considered experimentally. Measurements of phase and group velocities, and insertion loss were obtained by systematically increasing the guiding layer thickness and by liquid phase sensing experiments using solutions of poly(ethylene glycol) of varying molecular weight and concentrations. The increase in sensitivity with guiding layer thickness is significant and the data confirms the theoretical prediction that group velocity is a more sensitive parameter than the phase velocity to visco-elastic and liquid loading.

Item Type: Thesis
Creators: Martin, F.
Date: 2003
ISBN: 9781369314656
Divisions: Schools > School of Science and Technology
Record created by: Linda Sullivan
Date Added: 21 Sep 2020 14:42
Last Modified: 28 Jul 2023 13:57

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