Effects of flow control on the production of emulsion drops

It is well-known that in a coflow setting with immiscible fluids, the size of a generated emulsion drop is controlled by the capillary number associated with the outer fluid (Ca_o), while the frequency of formation is by the Weber number of the inner fluid (We_i). Additionally, the dripping regime is bound by We_i <1 and Ca_o < 1. These results were obtained in a glass-based microfluidic device, using syringe pumps to inject the fluids. It has also been shown using 2D-PDMS devices with a flow focusing setting that the injection method (syringe pump – flow control and pressurized canisters – pressure control) affects the size and frequency of formation of the drops. In this work, we show that using coflow in glass devices (3D flow), the drop size is also affected by the different injection methods, and the region where dripping exits is extended for We_i > 1. The fact these changes are not present when the experiments are performed in air points to the surface tension as the key factor in this process. Using different surfactants, we monitored the surface stresses in the fluid-fluid interface to understand how the flow profile is affected by the injection method.