| Summary: | This project uses experimental techniques to explore the effects of surface active agent (surfactant) adsorption on a droplet of oil suspended in a flow. The fluorescent surfactant Rhodamine-6G was used to enable the use of optical techniques to visualise the build-up of surfactant at the rear of the droplet and its effects on the internal circulation within the droplet. This was done to enable an exploration of how surfactant accumulates behind a droplet with the aim of predicting the behaviour based on the internal circulation for non- fluorescentt surfactants. An experiment was designed along with calibration procedures in order to utilise the non-intrusive measurement techniques laser-induced fluorescence and particle image velocimetry to measure the volume of surfactant held behind the droplet, the angle of the visible cap caused by the build-up of Rhodamine-6G, and the cap angle of the stagnant region obtained by measuring the velocity of the circulation within the droplet. Laser-induced fluorescence (LIF) was used to visualise the development of a surfactant-rich cap that formed at the rear of the oil droplet as it rose through an aqueous solution, with different bulk concentrations of surfactant. The aqueous solution soluble surfactant, adsorbed to the forwards facing part of the droplet, was transported around to the rear where it accumulated in a surfactant cap before being swept back into the ow behind the droplet. The fluorescent properties of Rhodamine-6G were utilised to measure the size of the visible surfactant cap angle and the volume of surfactant stored within the cap. The results showed that increased concentrations of surfactant caused a larger volume of surfactant to be held behind the droplet with larger droplets resulting in smaller surfactant cap angles. Particle image velocimetry (PIV) was used to explore the effects of surfactant at the interface on the internal circulation within the droplet. As surfactant accumulated an area of very low velocity at the rear of the droplet appeared. This was quantified by measuring the tangential velocity around the droplet with the area of low velocity signifying the presence of a stagnant cap. Measuring the internal velocity around the droplet close to the interface showed that the stagnant cap angle had large growth over the initial region of the tank, with larger droplets resulting in smaller surfactant cap angles. The visible cap angle measured by LIF was related to the stagnant cap angle measured by PIV to compare how the results for each technique developed over the height of the tank. The angles for both techniques exhibited the same trends as the droplet height increased, although they showed different magnitudes revealing that the experimental procedure could be improved in the future to obtain better agreement.
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