Generation of emulsion droplets and micro-bubbles in microfluidic devices

Abstract

Droplet-based microfluidic devices have become a preferred versatile platform for various fields in physics, chemistry and biology to manipulate small amounts of liquid samples. In addition to microdroplets, microbubbles are also needed for various pro- cesses in the food, healthcare and cosmetic industries. Polydimethylsiloxane (PDMS) soft lithography, the mainstay for fabricating microfluidic devices, usually requires the usage of expensive apparatus and a complex manufacturing procedure. In ad- dition, current methods have the limited capabilities for fabrication of microfluidic devices within three dimensional (3D) structures. Novel methods for fabrication of droplet-based microfluidic devices for the generation microdroplets and microbubbles are therefore of great interest in current research. In this thesis, we have developed several simple, rapid and low-cost methods for fabrication of microfluidic devices, especially for generation of microdroplets and mi- crobubbles. We first report an inexpensive full-glass microfluidic devices with as- sembly of glass capillaries, for generating monodisperse multiple emulsions. Different types of devices have been designed and tested and the experimental results demon- strated the robust capability of preparing monodisperse single, double, triple and multi-component emulsions.

Second, we propose a similar full-glass device for generation of microbubbles, but with assembly of a much smaller nozzle of a glass capillary. Highly monodisperse microbubbles with diameter range from 3.5 to 60 microns have been successfully produced, at rates up to 40 kHz. A simple scaling law based on the capillary number and liquid-to-gas flow rate ratio, successfully predicts the bubble size. Recently, the emergent 3D printing technology provides an attractive fabrication technique, due to its simplicity and low cost. A handful of studies have already demonstrated droplet production through 3D-printed microfluidic devices. However, two-dimensional (2D) flow structures are still used and the advantage of 3D-printing technique has not been fully exploited. Therefore, we apply 3D printing technology to fabricate 3D-miniaturized fluidic device for droplet generation (single emulsion) and droplet-in-droplet (double emulsion) without the need for surface wettability treat- ment of the channel walls, by utilizing 3D geometry design and fabrication. A scaling law is formulated to predict the drop size generated in the device. Furthermore, magnetically responsive microspheres are also produced with our emulsion templates, demonstrating the potential applications of this 3D emulsion generator in chemical and material engineering. Finally, we design and 3D-print a hybrid ?plug-and-play? microfluidic droplet generator, which involves a 3D-printed channel chamber and commercial tubings and fittings. By combination of 3D-printed part and market-available parts, this device can be easily assembled and disassembled, which provides a great flexibility for different demands. A scaling law has been proposed for prediction of drop size generated in the device. Furthermore, a 3D-printed concentration gradient generator and a droplet merging device based on the droplet generator have been developed to demonstrate the great scalability of 3D-printing technology.