Wetting-induced interfacial instability: A mechanism for droplet emission at air-liquid interfaces.

High-throughput production of monodisperse microdroplets has revolutionized many fields, typically relying on shear-induced emulsification in intricate microfluidic channels to induce the Rayleigh-Plateau instability. This mechanism exhibits low robustness due to its high dependence on the physical properties and flow conditions of fluids. Here, we report a robust emulsification mechanism-wetting-induced interfacial instability-for droplet emission. We find that, when pendant microdroplets in the air contact with an immiscible wetting bulk phase, it triggers interfacial instability in the hanging droplets and then their rapid breakup into the bulk phase. This simplifies the monodisperse microdroplet production using a nozzle positioned above an air-liquid interface, requiring no complex microchannels. We demonstrate that this method exhibits highly scalable production and exceptional robustness against variations in physical properties and flow conditions of fluids, including highly viscous non-Newtonian fluid (56,600 millipascal-seconds). This mechanism provides a simpler alternative to the traditional Rayleigh-Plateau instability for emulsification, offering opportunities for industrial applications and insights into microscale interfacial science.