Predator–prey interactions between droplets driven by non-reciprocal oil exchange

Chemotactic interactions are ubiquitous in nature and can lead to non-reciprocal and complex emergent behaviour in multibody systems. However, developing synthetic, inanimate embodiments of a chemomechanical framework to generate non-reciprocal interactions of tunable strength and directionality has been challenging. Here we show how chemotactic signalling between microscale oil droplets of different chemistries in micellar surfactant solutions can result in predator–prey-like non-reciprocal chasing interactions. The interactions and dynamic self-organization result from the net directional, micelle-mediated transport of oil between emulsion droplets of differing composition and are powered by the free energy of mixing. We systematically elucidated chemical design rules to tune the interactions between droplets by varying the oil and surfactant chemical structure and concentration. Through the integration of experiment and simulation, we also investigated the active behaviour and dynamic reorganization of multidroplet clusters. Our findings demonstrate how chemically minimal systems can be designed with controllable, non-reciprocal chemotactic interactions to generate emergent self-organization and collective behaviours reminiscent of biological systems. Interactions that generate directed movement in response to a chemical stimulus occur in nature but have been difficult to realize in synthetic systems. Now, it has been shown that asymmetric micelle-mediated exchange of haloalkanes can be used to create tunable chasing interactions between chemically distinct microdroplets. Collective interactions lead to the formation of droplet assemblies with emergent self-organization and collective behaviours.