Collective generation of milliemulsions by step-emulsification

Emulsification is a key step in many processes for the production and functionalization of dispersed liquid systems. Here, we report a versatile and robust device that generates monodisperse milliemulsions by stepemulsification. In contrast to the conventional design in which each channel is physically separated, we use a shallow plateau sandwiched between two parallel glass strips to connect all channels in a microcapillary film (MCF) before emerging in a deep reservoir. Because of the open plateau that connects different channels, the flow tips from neighboring channels may get immediately in contact with each other; this interaction may lead to the relative movement and deformation of the flow tips, to repulsion or even coalescence, enabling droplet generations from different channels to synchronize. By simply tuning the interaction, we achieve Janus droplets, drops of fluids mixed at different ratios and mixed drops of different compositions. The in situ generation of droplets with excellent control is essential for various applications.