Drahne, Ulrike; (2021) Investigation of bubble fragmentation using acoustical measurements. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Bubbles in the ocean mediate the gas exchange between ocean and atmosphere. This gas exchange is vital for the Earth's climate, yet bubble mediated gas exchange is poorly understood, and therefore parametrization in climate models is limited. Enhanced understanding of bubble mediated gas exchange will therefore improve climate models. In medicine, bubbles are used to improve the quality of ultrasound images by enhancing the image contrast. Micro-bubbles are injected into the blood stream and are excited into oscillation by external ultrasound. Bubble oscillation increases the signal-to-noise ratio in the images making it easier to recognize anomalies. In the biomedical industry, bio-reactors are used to cultivate cells. Stirring or shaking of the reactors generates bubbles, and cells adhere to the sticky bubble surface. When the bubble burst the cell is damaged resulting in loss of cells, time and increased costs. Here, research is required to reduce bubble fragmentation. Acoustic measurements were used to study bubble sizes of parent and daughter bubbles. Using bubbles with an initial radius of approximately 3mm, bubble pinch-off and bubble fragmentation was investigated experimentally for water temperatures ranging from 3-30°C and for a surface tension value between 0.032N/m and 0.070N/m. The measured acoustic signals were fitted with a successfully validated numerical fit algorithm. For varying water temperature and surface tension all the volume split ratios exhibited similar U - shapes with a minimum for similar sized daughter bubbles. The experimental data do not agree with existing theoretical fragmentation models, that predict a maximum for similar sized daughter bubbles. Current theoretical fragmentation models are presumably based on inaccurate assumptions and require revision, e.g. by accounting for the fragmentation mechanism resulting in one large and one small bubble, rather than relying on general energetics arguments. Particle Image Velocimetry (PIV) was used to visualise the 2D flow conditions caused by two colliding water jets leading to bubble fragmentation. Three different pump rotation speeds of 40, 50 and 60rpm and four different heights of the vertical offset (0, 1, 2 and 3cm) between both flow tubes were investigated. The location of bubble fragmentation in the 2D flow field was investigated for the same pump speeds and tube offsets. Additionally, location of bubble fragmentation at 30rpm, 1cm offset and a surface tension of 0.031N/m was analysed. This allowed for comparison between fragmentation processes and the local conditions that were responsible for bubble fragmentation. The location of bubble fragmentation was independent of the flow velocity through the glass tubes, the vertical offset between the tubes and water surface tension. Fragmentation mostly occurred in a preferred range of maximum shear rate of 15-25 1/s. Fragmentation process and location of bubble fragmentation were independent of water temperature (which chiefly affects water viscosity) and surface tension. For a water temperature range of 3-30°C and a surface tension range of 0.031-0.070N/m, a universal volume split ratio was observed showing that parent bubbles mostly fragmented into one large and one small bubble, and that similar sized daughter bubbles were rarely observed. These findings are significant since firstly, a universal volume split ratio means that the fragmentation mechanism is independent of water conditions, and secondly, a universal volume split ratio means that daughter bubble sizes in the open ocean may be similar anywhere in the global ocean, i.e. in polar as well as in tropical regions.

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