| This thesis describes the formation and applications of bubbles and droplets in microfluidic systems. Microfluidic devices, with channels that are tens to hundreds of microns wide, are useful for fundamental studies of fluidic systems and in biological, medical, chemical and optical applications. The addition of bubbles and droplets to the microfluidic toolkit has expanded the reach of microfluidics further into these fields.; Chapter 1 explains the mechanism by which liquid squeezes off bubbles and droplets in a t-junction generator. A model, based on the mechanism and supported by experimental data, predicts dependence of the sizes of bubbles and droplets on the ratio of the rates of flow of the fluids.; Chapter 2 reports the contribution of bubbles to the pressure drop across a microchannel. In the absence, or at low and high concentrations, of surfactant, there is little flow of liquid past the bubbles. Under these conditions, the number of bubbles in the channel predominantly contributes to the overall pressure drop. At intermediate concentrations, the liquid flows rapidly past the bubbles; here the total length of the bubbles is the dominant contributor to the pressure drop.; Chapter 3 describes a microfluidic device that exhibits reversible, nonlinear behavior. Droplets of ink in hexadecane travel through a section of a microfluidic network in which the channel splits into two branches that reconnect downstream to form a loop. They enter the loop separated by uniform time-intervals, but exit separated by periodic or aperiodic sequences of intervals. This nonlinear behavior reverses when the droplets move in the opposite direction through the device. This reversible, nonlinear behavior allows for the coding and decoding of information.; Chapter 4 describes the application of bubbles to the problem of mixing liquids efficiently---a process that is typically limited by diffusion in microfluidic systems. Liquids flow through the device, and bubbles form at t-junction generators, due to the vacuum created using a syringe connected to the outlet of the device. Since only a syringe is required for the device to function, it is fully portable. Chapter 4 demonstrates the use of the device to perform a common enzymatic assay. |
