The robust design of ultrasonic devices for use in oceanographic environments

Abstract

The Earth’s oceans host an enormous range of natural resources, over 90 billion kg of fish and shell fish are caught each year (WorldOceanReview, 2011). The oceans play an overriding role in climate regulation, removing vast quantities of carbon from the atmosphere. It is believed phytoplankton could account for more than half the earth’s oxygen production (ConsciousAlliance, 2011). Monitoring these biological and chemical characteristics offers an invaluable insight into the way our oceans work, which can then be used to generate and verify reliable models of the global ecosystem. In-situ sensors have been identified as the way forward, due to the ever changing properties of the world’s oceans in terms of chemistry and biology. The sensors will have to overcome harsh working conditions and process large quantities of data, whilst using as little power as possible. The work undertaken for this thesis aims to develop ultrasonic standing wave particle manipulation techniques for use in an oceanographic environment. Ultrasonic particle manipulation techniques are generally confined to ceramic devices, which are incompatible with oceanographic sensing on a large scale deployment. This work has bridged that gap and developed fresh approaches to ultrasonic techniques in polymer devices.

In addition to this, novel manufacturing methods have been developed to improve the robustness of the devices or to make the technology more compatible with cheaper, quicker and easier manufacturing techniques. A specific problem in oceanographic sensing is biofouling – the build up of microorganisms in and around the sensor. This project has investigated the feasibility of using ultrasonic techniques to reduce this build up, in particular, the formation of biofilms within sensors. The use of ultrasonics to reduce biofouling has been investigated by others, but it generally focuses on acoustic streaming techniques which induces mixing and has high power requirements incompatible with remote sensing (Sankaranarayanan et al., 2008). This work was carried out in conjunction with the Centre for Marine Microsystems in Southampton. The centre is developing robust high performance metrology systems for use in oceanographic science. Four distinct class of sensors will all be looked at, Chemical, Physical, Nucleic acid sensors and a ?-flow cytometer (UniversityOfSouthampton, 2011).

To assist in the development of oceanographic ultrasonic sensing platforms, one dimensional and two dimensional modelling was carried out in Matlab (R2014b) and ANSYS (12.0). The models were used to design new devices as well verify experimental results. In particular, ANSYS was used to investigate the mechanism behind standing wave particle manipulation devices. The modelling investigated the robustness of such a device and their suitability for scaling up and integration into an oceanographic sensor. Once the computational modelling had been carried out, devices were built using a variety of manufacturing techniques. As required, the techniques were adapted and optimised for the production of ultrasonic oceanographic sensors. The work went on to qualitatively and quantitatively analyse the affects of ultrasonic techniques in an oceanographic sensor. In particular, the formation of biofilm within a polymer sensor was analysed. Image processing software was optimised for the experiments then used to identify the effects of ultrasonic standing wave techniques.

The work shows that it is possible to reduce the build up of biofilms within polymer devices over substantial time periods using ultrasonic standing wave techniques. The devices used differ from conventional ceramic devices in that less than an exact half wave was set up across the fluid channel. This means there are non zero forces acting at the polymer/fluid boundary which is beneficial for the reduction of biofilm formation. The work also identifies a mechanism for the alignment of particles within a microfluidic device through the use of surface acoustic waves, though it was not possible to verify the computational results experimentally. A novel manufacturing technique using spin coating was developed that would allow easier construction of surface wave devices. In addition to this, a new type of device was developed utilising the transparent properties of lithium niobate in a bulk acoustic wave configuration. In the process of carrying out this work an experimental method has been developed allowing the depth of particles within devices to be ascertained through long exposure images. The length of the particle streak allows the position of the particle relative to a solid/fluid boundary to be inferred, and the data can be presented in such a way to build up a picture of the device characteristics.

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Identifiers

ORCID for Martyn Hill: orcid.org/0000-0001-6448-9448

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