  Chemotactic interactions are ubiquitous in nature and can lead to
nonreciprocal and complex emergent behavior in multibody systems. Here we show
how chemotactic signaling between microscale oil droplets of different
chemistries in micellar surfactant solutions can result in predator-prey-like
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.
The nonreciprocal behavior occurs in a wide variety of oil and surfactant
conditions, and we systematically elucidate chemical design rules for tuning
the interactions between droplets by varying oil and surfactant chemical
structure and concentration. Through integration of experiment and simulation,
we also investigate the active behavior and dynamic reorganization of
multi-droplet clusters. Our findings demonstrate how chemically-minimal systems
can be designed with controllable, non-reciprocal chemotactic interactions to
generate emergent self-organization and collective behaviors reminiscent of
biological systems.
