Pushing the boundaries of microfluidics; an experimental study on high-speed droplet formation in confined microchannels using air as the continuous phase*

Microfluidic systems for generation of liquid emulsions provide versatile tools for many applications in natural sciences. Droplet formation in these systems are performed in confined microchannels and as a result of the flow of two immiscible liquids (e.g. water in oil). Here we characterize droplet generation in an alternative format where continuous oil phase is replaced by gas in the microchannel. Droplet breakup is experimentally studied in a flow-focusing type channel in which a middle aqueous phase is pinched off by a high-speed air flow. The microchannels are fabricated in PDMS and feature a non-planar architecture enabling the liquid to be fully surrounded by the air flow. We investigate the Dripping and Jetting regimes for this system and identify the effect of geometry as well as flow conditions. We report droplet generation in the order of 10 kHz with diameters of 50µm and 150µm in the Dripping regime while for the Jetting regime we obtain droplets of 10µm 50µm at frequencies higher than 100 kHz which is an order of magnitude higher than the state-of-the-art for oil-based systems. The outcomes of this work are useful in many areas, namely pharmaceutical industries where uniform droplets can be generated purely in air.