Over the last few years considerable research attention has been directed towards droplet-based microfluidic devices because of their numerous applications in chemical and biological fields, to name a few. Specifically, gas-liquid droplet systems are of great importance for applications in which a gaseous phase is required instead of a second liquid phase. In this paper we experimentally investigate the manipulation of water droplets in flow-focusing configurations using a high inertial air stream. Compared to a T-junction geometry, the flow-focusing geometry provides the injection of highly inertial air on both sides of the droplet generation region, producing a more consistent droplet generation process in this type of gas-liquid microfluidic system. For this study, we changed the width of the liquid channel, the air flow rate, and the liquid flow rate in order to experimentally investigate their effects on the flow regime and generation frequency. The interactions of different geometrical and physical parameters produce three distinct flow regimes in the gas-liquid flow rate space (co-flow, jetting, and dripping). The controlled size and generation rate of droplets in this scheme provide the capability for precise and oil-free delivery of discrete microliter volumes of fluid.

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