Control of emulsion drop production in flow focusing microfluidics

Generating droplets using flow-focusing microfluidics in multiphase flows has reached its limit that it cannot generate submicrometer droplets in size. Flow focusing geometry together with an electric field has been used to make smaller droplets in microchannels. The droplet size was controllable by the flow rate ratio as well as the electric field. The droplets size decreased as the voltage increased. A Taylor cone was formed to generate very fine droplets which were less than 1m? in diameter. The tip made smaller droplets due to the tangential force by the electric field. A small inner flow rate and high electric field were required to form a stable Taylor cone in a DC electric field. The droplet size, however, was not stable at a small water flow rate because the flow rate was not as accuate as required. When I used a modified syringe pump with more accurate flow rate control, I was able to obtain a stable set of data. A small change in droplet size occurred at low voltage. The drop size changed dramatically, when the voltage was high enough. I also observed how an AC electric field affects the droplet size. The droplet size was not solely determined by the voltage. This is because of the imbalance of the supplied flow rate and the emitted flow rate. I also found that the droplet size is related to the tip position of the dispersed phase. The droplet size decreased as the tip stretched more. Typically, the microfluidic device generated monodispese droplets in narrow size distribution. It also generated a bigger droplet followed by a smaller one consecutively at low flow rate ratio of inner and outer fluid flow ()265.0/09.0??oiQQ. To understand this instability of drop formation, a numerical calculation was conducted. The simulation results showed inside of the tip still pointed downstream after it generated a big droplet. Then, the tip generated another smaller droplet while the tip was stretched. Finally, the tip moved back and began a new cycle.