This paper reports flow visualization experiments of droplet emulsions in the microfluidics that is composed of a flow-focusing (cross-junction) structure with a converging-diverging nozzle shaped section. The channel structure was made of polydimethylsiloxane (PDMS) with a channel of 73 μm in depth and 100 μm in width. Oil as the continuous phase and water as the dispersed phase were injected into the cross-junction inlets to form water-in-oil droplets. It is found that adding the converging-diverging section to the cross-junction significantly reduces the droplet size without largely increases the overall flow resistance and still achieve excellent uniformity (with coefficient of variation less than 3.5%). Throat of the converging-diverging section determines the breakup location of droplet emulsion. The size of droplets is controllable in a wide range (6-116μm) through the variation of throat width between 23 and 100 μm in conjunction with the flow rates of continuous and dispersed phases. This emulsion technique demonstrates that droplets with diameter as small as 10 μm or less can be produced in microchannels having dimensions on the order of 100 μm without use of surfactants.
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