Generation and applications of bubbles and droplets in microfluidic systems

This thesis described development and characterization of the process that generates bubbles, droplets, and polymer particles using microfluidic flow-focusing geometries. The process that forms bubbles and droplets in a flow-focusing geometries exhibits rich and complex dynamics, and such multiphase flow by bubbles and droplets are useful in applications in a variety of different fields in chemistry, biology and material synthesis.Chapter 1 introduced the concept of microfluidics, methods to generate bubbles and droplets in microfluidic systems, and their importance in a range of applications. Chapter 2 and 3 discusses the generation of bubbles and droplets in a flow-focusing geometry: Chapter 2 discusses a formation of bubbles in a parallel, coupled flow-focusing generator. The timing of formation of bubbles in multiple (two, and four) synchronized in that class of flow-focusing generators, and the mechanisms of the generation of bubbles that lead to synchronization of the timing were studied. Chapter 3 described the process that formed multi-disperse sets of bubbles in a flow-focusing junction comprising multiple rectangular sections. The studies of the detailed mechanisms of the formation of bubbles elucidated influences of the flow parameters and geometry of the flow-focusing junctions on the size and dispersity of bubbles that generated in those junctions. Chapter 4 and 5 explored dynamic generation of patterns by bubbles and droplets flowing through a microfluidic channels: Chapter 4 discussed interfacial instabilities on aqueous droplets suspended in hexadecane. Different types of formation of patterns by droplets, and dynamic shear-driven effects were observed when interfacial tension was lowered sufficiently by adding surfactants in both aqueous and organic phases. Chapter 5 described formation of composite structure of stable lattices of bubbles and droplets by simultaneous use of multiple flow-focusing generators. Chapter 6, 7 and 8 discussed the development of emulsion-based applications in optofluidics, material synthesis, and chemical communications: Chapter 6 discussed the development of the diffraction gratings using self-assembled lattice structures of bubbles. Changes in flow parameters demonstrated dynamic switching of the periodicity of the gratings. Chapter 7 discussed generation of monodisperse, drug-loaded poly (lactic-co-glycolic acid) (PLGA) particles using microfluidic flow-focusing generators. The size-dependent kinetics of drug release was demonstrated. In addition, kinetics of drug release from particles fabricated using a microfluidic method and a conventional method were compared. Chapter 8 describes a new method to generate a series of optical pulses by aligning an optical mask below a microfluidic channel in which transparent droplets flows through an opaque continuous phase. This study is intended to explore a new field in chemical communications---that is, generation, transmission and detection of information using chemistry and chemical properties of matter.