Spatiotemporal control of cargo delivery performed by programmable self-propelled Janus droplets

Self-propelled droplets capable of transporting cargo to specific target locations are desired tools for many future applications. Here we propose a class of active droplets with programmable delivery time that are attracted or repelled by certain obstacle geometries. These droplets consist of a water/ethanol mixture and are dispersed in an oil/surfactant solution. Owing to a mass exchange between fluid phases during self-propulsion, the initially homogeneous droplets spontaneously de-mix and evolve into characteristic Janus droplets. Cargo molecules, like DNA, can be separated into the trailing ethanol-rich droplet and are carried to their target location “like in a backpack”. The delayed onset of phase separation provides a handle to control the time frame of delivery, while long-ranged hydrodynamic interactions and short-ranged wetting forces are exploited to achieve the desired spatial specificity with respect to obstacle geometry and surface chemistry. Self-propelled droplets capable of transporting cargo to specific targets have major interest in medical applications. In this article, the authors report on a one step cargo delivery method using swimming droplets.