Photoreversible fragmentation of a liquid interface for micro-droplet generation by light actuation

We describe a method to induce by light a reversible switch from a continuous two-phase laminar flow to a droplet generating regime, in microfluidic devices with a usual water-in-oil flow focusing geometry. It consists in adding a photosensitive surfactant to the aqueous phase to modulate using light the interfacial energy between flowing liquids and the microfluidic substrate. We show that UV irradiation induces liquid fragmentation into monodisperse water microdroplets and that many cycles of reversible and rapid switches (<2 s) between continuous laminar flows and stable droplet regimes can be realized. By spatially controlling the application of the light stimulus, we also demonstrate the first spatially resolved remote induction of droplet generation.