Microfluidic preparation of water-in-oil-in-water emulsions with an ultra-thin oil phase layer.

We developed a novel microfluidic device to prepare monodisperse water-in-oil-in-water (W/O/W) emulsions with an ultra-thin (<1 microm) oil phase layer. This microfluidic device was composed of two microchannel junctions, one of which had a step structure, and a uniformly hydrophobic surface for effective oil removal from W/O/W droplets. At the first junction, an internal aqueous phase was transformed into slug-shaped water-in-oil (W/O) droplets by a flow-focusing mechanism. At the second junction equipped with the step structure, the preformed slug-shaped W/O droplets were introduced into an external aqueous phase and were transformed into spherical W/O droplets. In the downstream area of the second junction, the W/O droplets were released from the hydrophobic surface of the microchannel into the external aqueous phase by inertial lift force and were transformed into W/O/W droplets. During this process, most of the oil phase was effectively removed from the W/O droplets: the bulk of the oil phase flowed along the hydrophobic surface of the microchannel. The thickness of the oil phase layer of the resulting W/O/W droplets was ultra-thin, less than 1 microm. The volume of the internal aqueous phase of the W/O/W droplets reflected that of the W/O droplets and was controlled by the flow rates of the internal aqueous phase and oil phase during W/O droplet formation. We successfully demonstrated encapsulation of water-soluble molecules and polymer particles into the prepared W/O/W emulsion.