Date of Award

Winter 2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Micro and Nanoscale Systems

First Advisor

Hisham Hegab

Abstract

The primary goal of the work presented in this dissertation is to generate microbubbles of a diameter of less than 15μm inside micro channels. In order to achieve this detailed understanding, the facts and limitations behind the generation of microbubbles inside a micro channel are determined from existing literature. The major limitations of the current bubble/droplet generators are found to be the bubble confinement effect, the merging of bubbles, the difficulty in determining bubble diameter and the need for smaller channels to generate smaller bubbles. A device eliminating these drawbacks is conceptualized, and its feasibility is studied using COMSOL® and found to be successful. Based on the findings of the initial studies, prototypes of a new generation of microbubble/droplet generators are developed in this work. This generator utilizes a fused silica tube as the channel carrying the secondary fluid. The micro channel into which the bubbles/droplets are formed is made on a silicon wafer and is sealed using a glass plate. Both cross-flow and co-flow bubble generation devices are analyzed as part of this work. The new generation devices have the advantage of generating unconfined spherical bubbles inside a micro channel while keeping the pressure drop across the channel as low as possible. The new generation cross-flow bubble generator is able to produce bubbles smaller than that possible with existing devices. The cross-flow devices were found to be more efficient in producing microbubbles of smaller diameter in comparison to a similar co-flow device operating under identical conditions. In order to produce smaller microbubbles inside micro channels, a flow focusing technique is introduced in both cross-flow and co-flow devices. Using flow focusing techniques has made it possible to produce microbubbles smaller than that possible without flow focusing. The major parameters that affect bubble formation inside micro channels are determined using a parametric study of both the fluid properties and the geometry of micro channels. A mathematical model is developed to predict the bubble diameter at its detachment from the orifice in a cross-flow device and is validated using experimental data. Surface tension and drag force are found to be the major factors in determining the bubble diameter at detachment. Major achievements of this work are summarized below: 1) A new generation of a microbubble/droplet generator capable of producing unconfined microbubble/droplet is developed in this study. 2) The merging problem of bubbles inside the micro channel immediately after the bubble detaches from orifice is observed for the first time during this study. 3) A microbubble of diameter 11μm is generated in the micro channel of hydraulic diameter 162µm. 4) The bubbly region of bubble formation inside a micro channel is further divided into three in this work: confined region, active region and saturation region. 5) The flow focusing technique inside the cross-flow devices is introduced and studied for the first time during this work. 6) Bubbles of diameter 6µm were produced using the flow focusing technique inside the micro channel of hydraulic diameter 162μm. 7) Circular micro channels were found to be more efficient than straight channels when used in a cross-flow device.

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