Chemical interactions in active droplets

Abstract

Active droplets serve as simple artificial model systems to elucidate the chemical and hydrodynamics-driven interactions between biological organisms. We investigate the pairwise interactions between such micellar solubilization based active droplets, capable of self-generating chemical and hydrodynamic fields. We experimentally demonstrate that the solute Péclet number (Pe), characterizing the relative strength of its convective to diffusive transport, plays a crucial role in determining how the chemical and hydrodynamic fields impact their interactions. At low Pe, strong chemo-repulsive interactions dominate, resulting in consistent scattering behavior regardless of the approach symmetry. In contrast, at high Pe, hydrodynamic interactions become dominant, enabling transient contact, synchronized motion, or reduced repulsion. Furthermore, by analyzing droplet–wake interactions across both high and low Pe regimes, we demonstrate that the scattering behavior is invariant to the relative approach orientation and is fundamentally governed by the droplet’s intrinsic chemical polarity, which in turn is set by its Pe number. Our results not only offer robust experimental validation of recent theoretical predictions on the scattering behavior of active droplets but also present a clear rationale for the observed angular deflection and its invariance to the approach angle of droplets. Our findings establish a systematic framework correlating the Pe-dependent intrinsic chemical polarity of droplets to the scattering outcomes in both droplet–droplet and droplet–wake interactions. These mechanistic insights advance our understanding of active droplet interactions and can serve as a basis for modeling and engineering collective behaviors in chemically active systems within the framework of multi-agent interactions, where simple local interaction rules result in emergent collective dynamics.