In this project, we are investigating a new method to selectively control the bubble movement within a dynamic flow condition, using what is known as "acoustic radiation force (ARF)". The acoustic radiation force results from the average dynamic interaction between the oscillating bubble and the incident acoustic (pressure) waves in the surrounding liquid. Though this phenomenon has been well observed and studied over many decades, the full potential in its application is far from complete. In particular, we are investigating the feasibility of ARF in developing a new air trap to filter microbubbles in medical procedures such as cardiopulmonary bypass and hemodialysis (Air bubble in blood stream, known as air embolism, is harmful and requires a system that safely prevents the passage of bubbles/microbubbles into the patient's bloodstream). 

However, in order to experimentally test the new concept, a new bubble generation technique is needed as currently available techniques only produce either continuous streams of microbubbles, or relatively large single bubbles. As such, the first part of the project was to design a new single bubble generator which produces single microbubbles so that the interaction between multiple bubbles (e.g. coalescence & secondary acoustic force) can be controlled.

The size of a microbubble, which dictates its surface to volume ratio, plays an important role in how it interacts with fluids, surfaces, and (electric or acoustic) fields. Therefore, many biomedical and scientific applications require accurate bubble size control. To address this need, a new method of controlled generation of single microbubbles was developed in this study, which uses timed gas injection and liquid co-flow. In this device, a pressure pulse is used to inject a small, controlled burst of air through a micropipette to generate individual microbubbles. As an added degree of freedom, an external co-flow of water was used to control the drag force which can influence when the bubble detaches from the micropipette. By limiting the air flow through a micropipette with an automatically actuated pressure pulse and by adjusting the co-flow rate, the present method was able to generate single microbubbles in the range of 62.6 - 150.8 µm in diameter (without the use of surfactants). Compared to other reported gas injection techniques which generate sub-millimetric single bubbles, the 62.6 µm bubbles generated in this study represent a 37% reduction in measured diameter.

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