AbstractsEngineering

This dissertation investigates the emulsification of aqueous liquid in immiscible organic liquid in various microfluidic environments, and addresses both experimental characterization and theoretical interpretation of the dynamics and design guidelines, as well as an application of microfluidic emulsification in fabrication of disk-like colloidal particle suspensions for studying its sedimentation behavior. In an attempt to understand the dynamics of drop formation in flow-focusing microfluidic channels, especially for an explanation of a transition from unique drop size to bimodal oscillating drop sizes as observed in the experiments, numerical simulation is developed to use the volume-of-fraction method to model the drop formation, and the simulation results help to interpret the transition in the theory of saddle-node transition in drop formation, as well as show the importance of selecting proper orifice length in flow-focusing microfluidic channel design. The electric technique for controlling of microfluidic emulsification is explored by a detailed study on low-frequency alternating-current electro-flow-focusing (EFF) emulsification in microfluidic channels. It is found that the droplet size variation is not a monotonic function of the electric field as in the case of direct-current EFF emulsification, which originates from the relaxation oscillation of the flow rate through the Taylor cone, and a power-law droplet size distribution was obtained at the voltage ramping-up stage. This emulsification process was modeled in analog to the charge accumulation and release in a resistor-capacitor electric circuit with an adjustable resistor, and the simulated data exhibit good agreement with the experiments. As an application of the microfluidic emulsification, a method of fabricating disk-like wax colloidal particle suspensions using electrospray is reported. Based on this technique, the first measurement of the hindrance function for sedimentation and creaming of disk-shaped colloids via the analytical centrifugation is reported. Disks align with the external flow right above the volume fraction of a few percent and this effect is extremely sensitive to the aspect ratio of disks. Due to this alignment effect, disk sedimentation/creaming demonstrate distinct trends in dilute and semi-dilute region.