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Experimental and Computational Analysis of Bubble Generation Combining Oscillating Electric Fields and Microfluidics

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Experimental and Computational Analysis of Bubble Generation Combining Oscillating Electric Fields and Microfluidics Experimental and computational analysis of bubble generation combining oscillating electric fields and microfluidics A thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy By Anjana Kothandaraman Supervised by: Professor Mohan Edirisinghe And Professor Yiannis Ventikos Department of Mechanical Engineering University College London Torrington Place, London WC1E 7JE United Kingdom December, 2017 1 | P a g e Declaration I, Anjana Kothandaraman, confirm that the work presented in this thesis is my own. Where information has been derived from other sources, I confirm that this has been indicated in the thesis. -------------------------------------- Anjana Kothandaraman 2 | P a g e Abstract Microbubbles generated by microfluidic techniques have gained substantial interest in various fields such as food engineering, biosensors and the biomedical field. Recently, T-Junction geometries have been utilised for this purpose due to the exquisite control they offer over the processing parameters. However, this only relies on pressure driven flows; therefore bubble size reduction is limited, especially for very viscous solutions. The idea of combining microfluidics with electrohydrodynamics has recently been investigated using DC fields, however corona discharge was recorded at very high voltages with detrimental effects on the bubble size and stability. In order to overcome the aforementioned limitation, a novel set-up to superimpose an AC oscillation on a DC field is presented in this work with the aim of introducing additional parameters such as frequency, AC voltage and waveform type to further control bubble size, capitalising on well documented bubble resonance phenomena and properties. Firstly, the effect of applied AC voltage magnitude and the applied frequency were investigated. This was followed by investigating the effect of the mixing region and electric field strength on the microbubble diameter. A capillary embedded T-junction microfluidic device fitted with a stainless steel capillary was utilised for microbubble formation. A numerical model of the T-Junction was developed using a computational fluid dynamics-based multiphysics technique, combining the solution of transport equations for mass and momentum (Navier-Stokes Equations), a Volume of Fluid algorithm for tracking the gas-liquid interfaces, and a Maxwell Equations solver, all in a coupled manner. Simulation results were attained for the formation of the microbubbles with particular focus on the flow fields along the detachment of the emerging bubble. Experimental results indicated that frequencies between 2 − 10 푘퐻푧 have a pronounced effect on the bubble size, whereas elevated AC voltages of 3 − 4 푘푉푃−푃 promoted bubble elongation and growth. It was observed that 3 | P a g e reducing the mixing region gap to 100 μ푚 facilitated the formation of smaller bubbles due to the reduction of surface area, which increases the shear stresses experienced at the junction. Reducing the tip-to-collector distance causes a further reduction in the bubble size due to an increase in the electric field strength. Computational simulations suggest that there is a uniform velocity field distribution along the bubble upon application of a superimposed field. Microbubble detachment is facilitated by the recirculation of the dispersed phase. A decrease in velocity was observed upstream as the gas column occupies the junction suggesting the build-up in pressure, which corresponds to the widely reported ‘squeezing regime’ before the emerging bubble breaks off from the main stream. The novel set-up described in this work provides a viable processing methodology for preparing microbubbles that offers superior control and precision. In conjunction with optimised processing parameters, microbubbles of specific sizes can be generated to suit specific industrial applications. 4 | P a g e Impact Statement Microbubbles are perceived to introduce interesting textural elements in perishable items such as bread, cakes, chocolates, champagne and sparkling drinks. From a consumer’s point of view, products containing bubbles have a sophisticated mouth feel, this is achieved using a gas which represents a zero calorie ingredient thus replacing fat. With nutraceuticals taking an expanding share of the market, it is believed that finer and uniform bubbles will enhance the palatability of functional foods (Ahmad et al., 2012, Shen et al., 2008a). On the other hand, bacterial biosensing is a rapidly developing field, with particular reference to industrial applications and in public health as a result of the advancements taking place in biotechnology and microelectronics. Microbubbles have been employed in acoustic biosensing platforms as the shell properties influence the electroacoustic response of the bubble. Fine bubbles provide a large surface throughput to bacteria, ranging from 35 − 250 μm (Mann and Krull, 2004, Mahalingam et al., 2015). The methodology described in this thesis generates bubbles to the required size range for the aforementioned application. Alternative procedures to obtain purified water have been investigated due to the steady decline in fresh water sources. Microbubbles have also been introduced into bioreactors to extract biogases such as methane in waste water (Al-mashhadani et al., 2016). Large quantities of microbubbles of < 150 μ푚 are desired in waste water treatment processes due to their high surface area to volume ratio which is essential to separate particulates from potable water. The apparatus presented in this work can be parallelised in order to generate a large quantity of bubbles this application necessitates. In order to target biomedical applications such as ultrasound contrast agents, the acceptable size range is approximately 2 − 10 휇푚, without compromising the production rate or stability of the microbubbles. One of the main findings of this work was that frequency is an important contributor to the reduction in microbubble diameter. Higher frequencies 5 | P a g e displayed favourable results in terms of size reduction. This lays the framework for targeting the biomedical field by incorporating an amplifier which has higher frequency capabilities to achieve microbubble sizes within the desired range. The methodology presented in this research has tremendous potential to prepare microbubbles for various industries. The microbubbles produced are monodisperse and provide the user with a high degree of control over the processing parameters. The experimental apparatus also benefits from low power consumption (1.2푊) with prospects to parallelise the T- junctions to increase the scalability. Furthermore, it paves way to construct a portable machine to target each of these applications in economical and scalable way. 6 | P a g e Publications Journal Papers Kothandaraman A, Harker A, Ventikos Y, and Edirisinghe, M. Novel Preparation of Microbubbles by Integrating Oscillating Electric Fields with Microfluidics (under review). Kothandaraman A, Qureshi M, Ventikos Y, Alfadhl Y and Edirisinghe M. Study of the Effect of the Mixing Region Geometry and Collector Distance on Microbubble Formation using a Capillary Embedded Microfluidic Device coupled with AC-DC Electric Fields (in preparation). Patent Kothandaraman A, Edirisinghe M, Ventikos Y. International Patent Application: “Microbubbles and their generation”, PCT/GB2016/053497, November 2016. Conference presentations Kothandaraman A, Edirisinghe M, Ventikos Y. The Microfluidic Congress, London, October 2016 (Poster Presentation) Kothandaraman A, Qureshi M, Ventikos Y, Alfadhl Y and Edirisinghe M. International Conference on Nanochannels, Microchannels and Minichannels, Washington DC, July 2016 (Technical presentation). 7 | P a g e Kothandaraman A, Edirisinghe M, Ventikos Y UK Society for Biomaterials Conference, London, UK, June 2016 (Technical presentation). Kothandaraman A, Edirisinghe M, Ventikos Y. Departmental PhD conference, Department of Mechanical Engineering, UCL, London, UK, June 2015 (Poster and Technical Presentation). Kothandaraman A, Edirisinghe M, Ventikos Y. Departmental PhD conference, Department of Mechanical Engineering, UCL, London, UK, July 2014 (Poster and Technical Presentation). 8 | P a g e Acknowledgments I would like to express my sincere thanks to my primary supervisor Professor Mohan Edirisnghe for believing I could achieve great heights. His continual support, valuable guidance and patience have been a great source of motivation. My thanks are equally due to my secondary supervisor Professor Yiannis Ventikos, who has not only imparted a wealth of knowledge to me, but has always been very approachable whenever I needed any advice. It has been an honour to be under the supervision of Prof. Edirisinghe and Prof. Ventikos. When times have been tough, support from near and dear ones have always pushed me to persevere and achieve the end goal. I would like to extend my thanks to Dr. John Vardakis for always guiding me in the right direction and Mr. Adrian Matei for his exquisite sense of humour and deep discussions. I would like to give my sincerest gratitude to my father Mr. R Kothandaraman, who has been my role model, inspiration and my source of strength, and my mother, Mrs. Sheela Kothandaraman, whose affection and encouragement are invaluable and has always been a strong pillar of support. I credit all my accomplishments to them. To my family, thank you. Last but not least, I would like to thank God for giving me the determination and endurance
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