Probing the Phase Behavior of Complex Fluids Using Microliter Droplet Reactors
Microfluidic droplet-based techniques allow for the formation of extremely uniform emulsions with precise control over the contents of each phase. In this thesis, we exploit these small scale fluid manipulation techniques to study several related phenomena in the area of complex fluids engineering: phase separation and dewetting in three-phase liquid systems, partitioning of high value nanomaterials, and the coalescence and creaming of dilute oil-in-water emulsions. Phase separation occurs, for example, in aqueous systems composed of two or more dissimilar polymers. Using a model system composed of polyethylene glycol, dextran, and water, we study dehydration-driven wetting transitions in a microfluidic array-type device, and then use this technique to estimate the very low interfacial tension between the aqueous phases as a function of polymer concentration. Next, we scale up the droplet-based fluid handling techniques to develop a millifluidic platform for screening the effect of water clarifiers on the stability of dilute crude-oil-in-water emulsions. Finally, the tools and techniques previously developed are adapted and extended to study aqueous two phase extraction, which has recently emerged as a promising technique for sorting synthetic nanomaterials such as single walled carbon nanotubes. The partition coefficient of a nanotube depends on many system parameters, including the presence and concentration of surfactants and salts, and the properties of the phase forming polymers. Knowledge of these relationships provides insight into the fundamental processes governing partitioning and are required for rational design of large scale separation processes. We demonstrate that a microliter scale droplet-based fluid handling approach coupled with inline absorption spectroscopy can facilitate the identification and optimization of the critical parameters for aqueous two phase extraction of carbon nanotubes. The primary contribution of this thesis is a set of robust tools and techniques to quickly, and with high compositional resolution, quantify phenomena such as phase change and partitioning in complex fluid systems.