Bubbles: sensors for the micro world

Thesis

It has been proposed that coated gas microbubbles, currently used as ultrasound contrast agents could also be used as microscale sensors due to the sensitivity of their acoustic response to changes in their environment. However, their behaviour is not fully understood and there remains considerable scope for improving their characterisation. The aim of this thesis is to improve the theoretical description of microbubble dynamics under ultrasound excitation with the ultimate aim of assessing the regimes in which they could be exploited most effectively as sensors.

Previous theoretical and experimental work relating to the confinement and acoustic excitation of microbubbles is reviewed. Specifically, optical trapping as a method for the isolation and manipulation of individual bubbles is studied for use in developing a sensor. An assessment of the existing models’ validity is undertaken. This is followed by the development of models for optical trapping of single microbubbles, and the coupled radial and translational motion of a microbubble under ultrasound excitation, which includes time dependent phenomena. The latter model is used to perform a sensitivity analysis to determine the uncertainty associated with using microbubbles as sensors. The potential for uniquely characterising the shell of the microbubble from experimental data is also assessed.

Subsequent chapters present the results from a combination of computer simulations and experimental data, used to develop and assess the validity of the new models for describing microbubble behaviour. Particularly, the model is used to simulate the response of a dilute suspension of microbubbles undergoing large amplitude oscillations and single microbubbles undergoing lipid shedding. The optimal regimes in which microbubbles may be utilised as sensors for liquid physical properties and local pressure variations are then assessed. Finally, a summary of the conclusions and areas for further work is presented.

Authors: Harfield, C

• Publication date: 2014
• DOI: 10.5287/ora-d562wmnke
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: Oxford University, UK
• Language: English
• Keywords: ultrasound; low-index optical trapping; microbubble
• Subjects: Biomedical engineering; Engineering & allied sciences