Self-propulsion of liquid droplet assemblies controlled by the functionalities of their components

The self-propulsion of droplet assemblies consisting of droplets of 1-decanol and either an ethyl salicylate (ES) or a composite droplet of ES and liquid polydimethylsiloxane (PDMS) is reported. The ES-PDMS composite droplets have an ES core covered by a PDMS layer that stabilizes the assembly significantly. Their self-propulsion exhibits characteristic predator-prey behavior, with a decanol droplet closely chasing the ES or ES-PDMS composite droplet, forming a bound droplet pair. Furthermore, the stability that PDMS gives the system enables us to construct more complex assemblies, such as two, three, and four decanol droplets closely chasing an ES-PDMS droplet, whose motion patterns depend strongly on the symmetry in the structure of the assemblies. Our findings demonstrate that long-lived assemblies composed of droplets with distinct functionalities can serve as a versatile platform for developing self-organizing and adaptive droplet systems, functioning as "droplet robots".