Microfluidic synthesis of uniform hollow spheres with internal nanostructures induced by diffusive phase separation

Abstract

Droplet-based microfluidic systems provide uniform single or double emulsion using co-flowing, T-junction or flow-focusing channel geometries. However, previous microfluidic approaches generally limit the minimum achievable size of single and double emulsions to a few micrometers and tens of micrometers, respectively. Here, I present new strategies for the design and development of microfluidic devices for preparation of ultrafine emulsion droplets which have not been achieved with previous microfluidic approaches. This thesis also describes the synthesis of functional nano/microparticles or capsules using emulsions as templates. In Chapter 2, submicron single emulsions were produced via the tip-streaming process using a 3-D flow-focusing device. The 3-D flow-focusing channel featured a square waveform geometry, where the energy and pressure required for droplet breakup were significantly reduced. In addition, the hydraulic resistance was adjusted by modulating the channel heights. The pressure range for tip-streaming mode was enlarged significantly compared with conventional two-dimensional flow-focusing devices. Therefore, monodisperse emulsions were produced under this tip-streaming mode for as long as 48 hours. Using the uniform submicron emulsion droplets as confining templates, biocompatible polyethylene glycol (PEG) particles were produced with a narrow size distribution at the sub-micrometer scale. In Chapter 3, I demonstrate a microfluidic technology that generates ultrafine double emulsions less than a few micrometers in a single emulsification process. This technology utilized the diffusive phase separation of a single emulsion involving tetrahydrofuran (THF) as a co-solvent. To perform this technique, we developed PDMS-glass hybrid microfluidic devices with a three-dimensional topography that could tolerate the exposure to strong organic solvents. A variety of hydrophilic or hydrophobic nano/microcapsules could be fabricated via this method withou...