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 behavior in multibody systems. However, developing synthetic, inanimate embodiments of a chemomechanical framework for generating non-reciprocal and collective interactions of tunable strength and directionality has been challenging. Emulsions, which are mixtures of immiscible liquids characterized by chemical inhomogeneity and non-equilibrium states, are unique materials in which to study how spatially controlled gradients affect chemical “communication” leading to organized assembly and motion. We present a source-sink framework for rationalizing and predicting micelle-mediated interactions between droplets of different chemistries and explore the ramifications for chemically programmable active fluids. We show how chemotactic signaling between microscale oil droplets of different chemistries in micellar surfactant solutions can result in predator-prey-like non-reciprocal chasing interactions. Our findings demonstrate how chemically-minimal emulsion systems can be designed with controllable chemotactic interactions to generate emergent self-organization and collective behaviors reminiscent of biological systems.