Coexistence of different droplet generating instabilities: new breakup regimes of a liquid filament

The coexistence of multiple droplet breakup instabilities in a Step-emulsification geometry is studied. A liquid filament, which is confined in one dimension by channel walls and surrounded by a co-flowing immiscible continuous phase, decays into droplets when subject to a sudden release of confinement. Depending on the filament aspect ratio and liquid flow rates, an unexpectedly rich variety of droplet breakup regimes is found. All of these breakup regimes are composed of two basic instabilities, i.e. a step- and a jet-instability, that coexist in various combinations on the same filament. Surprisingly, even an asymmetric breakup regime is found, producing droplet families of significantly different diameters, while the filament is subject to a fully symmetric flow field. We suggest key physical principles explaining the spontaneous symmetry breaking and the transitions between individual droplet breakup regimes. The particular ability to produce distinct droplet families from a single filament is demonstrated to allow for simultaneous concentration and encapsulation of particles into one droplet family while excess bulk liquid is released into another family of droplets.