Dynamics of Microfluidic Aqueous Two-Phase Compound Droplets

Abstract

An aqueous two-phase system (ATPS) composed of dextran and polyethylene glycol provides a reliable and biocompatible platform for purification of biomedical products and cellular macromolecules. The incorporation of such an ATPS into microfluidics would make automated on-chip purification of desirable proteins and other products possible. In addition, microfluidic aqueous two-phase droplets could potentially be used to better mimic intracellular uid environment. In order to control the physical and topological behaviors, it is therefore crucial to understand both the hydrodynamics and the thermodynamics of the aqueous two-phase microfluidic droplets. This thesis aims to address some of the relevant issues on such complex microfluidic droplets from hydrodynamic and thermodynamic point of view, which will help in better control and monitoring of on-chip ATPS. The first part of the project focuses on the creation and characterization of microfluidic aqueous two-phase droplets. Continuous and uniform droplets can be successfully produced at a Y-shaped junction. The microfluidic droplets exhibit a continuum of morphologies for different ow speeds and compositions and they have been well classified to facilitate subsequent discussions. Some other interesting experimental phenomena have been observed, including the very fine reticulate heterogeneous fluid structures, fractal emulsions and the creation of micron-sized satellite ATPS droplets. They can potentially have great industrial applications. In the second part, the underlying physics has been investigated. The rationales for transitions between different droplet morphologies have been investigated in analogy with the existing theory on droplet dynamics in an unconfined linear Stokes flow. Then, the characteristic size of the fluid filaments under strong shear stress has been determined by the technique of Fast Fourier Transform of the images. The interface thickness for compositions near the binodal line is calculated based on the Cahn-Hilliard theory. Surprisingly, we find that the characteristic size of the fine fluid filaments approaches the order of the interfacial thickness for compositions in close vicinity of the binodal line. In addition, the possibility of homogenization by applying very strong shear is also discussed based on existing scientific literature. The effect of chaotic mixing on the droplet morphology induced by the presence of a meandering section has also been studied.