Parhizkar, M; (2014) Preparation of monodisperse microbubbles in a capillary embedded T-Junction device and the influence of process control parameters on bubble size and stability. Doctoral thesis , UCL (University College London).

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Abstract

The main goal for this work was to produce microbubbles for a wide range of applications with sizes ranging between 10 to 300 μm in a capillary embedded Tjunction device. Initially the bubble formation process was characterized and the factors that affected the bubble size; in particular the parameters that reduce it were determined. In this work, a polydimethylsiloxane (PDMS) block (100 x 100 x 10 mm3) was used, in which the T-shaped junction was created by embedded capillaries of fixed outer diameter. The effect of the inner diameter was investigated by varying all the inlet and outlet capillaries’ inner diameter at different stages. In addition, the effect of changes in the continuous phase viscosity and flow rate (Ql) as well as the gas pressure (Pg) on the resulting bubble size was studied. Aqueous glycerol solutions were chosen for the liquid phase, as they are widely used in experimental studies of flow phenomena and provide a simple method of varying properties through dilution. In addition, the viscosity could be varied without significantly changing the surface tension and density of the solutions. The experimental data were then compared with empirical data derived from scaling models proposed in literature, which is widely used and accepted as a basis of comparison among investigators. While the role of liquid viscosity was investigated by these authors, it was not directly incorporated in the scaling models proposed and therefore the effect of viscosity was also studied experimentally. It was found that bubble formation was influenced by both the ratio of liquid to gas flow rate and the capillary number. Furthermore, the effect of various surfactant types and concentrations on the bubble formation and stability were investigated. Preliminary studies with the current T-junction set-up indicated that producing microbubbles with size ranging from 50-300 μm was achievable. Subsequently, the study progressed to optimise the junction to produce smaller bubbles (~ 20 μm) by directly introducing an electric field to the T-junction set-up and assisting the bubble breakup with the combination of microfluidic and electrohydrodynamic focusing techniques. Finally, in this thesis, a novel method that combines microfluidics with electrohydrodynamic (EHD) processing to produce porous BSA scaffolds from microbubble templates with functional particles and/or fibres incorporated into the scaffolds’ structure is presented.

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