Transport phenomena induced by micrometer size drops and bubbles are aesthetic, scientifically challenging, and play an important role in applications with relevance to the 21st century such as healthcare and energy efficiency. For example, microbubbles injected in a microchannel can enhance heat and mass transfer by inducing vortices; micro drops can transport reagents and encapsulate cells to perform chemical reactions and biological experiments in a microfluidic chip, with high sensitivities and low sample consumptions. This thesis first describes an invention of a novel technique for generating drops and bubbles on demand in a microfluidic chip. The technique involves a PDMS chip with one or several microliter-size chambers driven by piezoelectric actuations. Individual aqueous microdrops are dispensed from the chamber to a main transport channel filled with an immiscible fluid, in a process analogous to atmospheric drop on demand dispensing. The drop formation process is characterized with respect to critical dispense parameters such as the shape and duration of the driving pulse. Several features of this drop on demand technique with direct relevance to lab on a chip applications are presented and discussed, such as the ability to merge drops of different reagents and the ability to encapsulate single cells. The next part of this thesis shows the study on the dynamics of gas-liquid interfaces, bounded by micro-geometric features such as the channel walls of a microfluidic chip. The efficiency of three different micro-geometries at anchoring the interface is compared. The effects of ultrasound on the interface are also investigated. The sonicated interface exhibits harmonic traveling waves or standing waves, the latter corresponding to a higher ultrasound level. Standing capillary waves with subharmonic and superharmonic frequencies are also observed, and are explained in the framework of parametric resonance theory, using the Mathieu equation. In the end, a very efficient gas-liquid separation method using an integrated hydrophobic porous membrane is demonstrated and explained. Bubbles generated at a T-junction are transported towards the gas removal section, where they slide along a hydrophobic membrane until complete removal. This efficient gas removal process occurs provided four criteria are simultaneously respected. The first criterion is that the bubble length needs to be larger than the channel diameter. The second criterion is that the gas plug should stay on the membrane for a time sufficient to transport all the gas through the membrane. The third criterion is that the gas plug travel speed should be lower than a critical value: otherwise a stable liquid film between the bubble and the membrane prevents mass transfer. The fourth criterion is that the pressure difference across the membrane should not be larger than the Laplace pressure to prevent water from leaking through the membrane.
|School Location:||United States -- New York|
|Source:||DAI-B 72/04, Dissertation Abstracts International|
|Keywords:||Bubbles, Drops on demand, Fluid mechanics, Lab on a chip, Microfluidics, Multiphase flow|
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