Minimal physicochemical heterogeneity shapes the collective dynamics of active droplets

Active droplets are an ideal platform to investigate how collective behavior, akin to that of living systems, emerges from the interplay of simple self-propelling units. Non-trivial ordered structures and dynamics can arise from the interaction of identical agents, but apparently minor differences among them can dramatically reverberate on a larger scale. How and to what extent a minimal asymmetry in the droplet composition and, consequently, in their individual behavior affects the properties of the collective is an open question. Here, we report the study of 50 self-propelled oil droplets floating on the surface of an aqueous solution. 25 droplets are stained with a red dye, and the other 25 are stained blue: this seemingly minor chemical difference profoundly affects the droplets’ interfacial tension properties, giving rise to an emergent behavior that deviates qualitatively from that of a homogeneous counterpart. A tracking pipeline developed ad hoc is applied to three independent realizations of the system: the analysis of the droplet trajectories reveals a quantitatively reproducible evolution unfolding in a sequence of qualitatively distinct stages, and showcases a complex interplay between the relative mobility of the constituents and the emergent organization of the collective. The results reported in this work support that the population dynamics of active droplets can be remarkably sensitive to minimal physicochemical heterogeneity while remaining reproducible, opening the way to the rational design of active systems with controllable emergent behavior.