Abstract
This thesis presents an electric-field triggered droplet formation scheme that is capable of producing sub-femtoliter droplets on demand. Pressure-driven flow focusing scheme is enhanced by a time-varying electric field, which acts against interfacial tension and causes the breakup of droplets. More specifically, Maxwell stress is induced on the interface, where droplets are produced via tip streaming process. PDMS microfluidic devices with embedded solid electrodes are utilized to realize the proposed on-demand droplet formation scheme. In the prototype demonstration, low melting-temperature solder mixed with magnetic nanoparticles is inductively heated and injected into microchannel to form solid electrodes. Meanwhile, a diaphragm is mounted on top of the junction¸ which can be deformed pneumatically to adjust the channel geometries and therefore to control the droplet formation process. It is demonstrated that the emulsification process is controlled by the applied pressures and electric field, and extremely-small droplets with volumes less than 1 femtoliter can be readily produced and collected. Using a voltage ramp (0 to 400 V in 0.05 seconds)applied across a distance of 6 mm, droplets of diameters less than 1 μm are produced. As such, the demonstrated microfluidic scheme could potentially realize the controllability on the formation of sub-femtoliter droplets, which are desired for a variety of chemical and biological applications.