Chemical interactions in active droplets

TL;DR

This study uses active droplets to model biological interactions, revealing how chemical and hydrodynamic fields influence their pairwise behaviors depending on the solute Péclet number, with implications for understanding collective dynamics.

Contribution

It demonstrates how the Péclet number governs the dominance of chemo-repulsive or hydrodynamic interactions in active droplets, providing a new framework for tuning their pairwise interactions.

Findings

At low Pe, chemo-repulsion dominates, preventing contact.

At high Pe, hydrodynamics lead to physical engagement.

Chemical trail interactions are always chemo-repulsive.

Abstract

Interactions among biologically active agents is facilitated by their self-generated chemical and hydrodynamic fields. In order to elucidate the pair-wise interactions between such micro-organisms, we employ active droplets as a model system, capable of self-generating chemical and hydrodynamic fields. We demonstrate that the solute P\'eclet 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. Our findings reveal that at low $Pe$, the interaction is predominantly governed by chemo-repulsive effects, leading to droplets avoiding physical contact. Conversely, at elevated $Pe$, hydrodynamic interactions become more influential, leading to physical engagement. However, irrespective of $Pe$, the interaction of a droplet with the chemical trail of another droplet is always governed by chemo-repulsive effects. Furthermore, our results establish that the chemo-repulsive deflection/rebounding of droplets is influenced by the droplets' inherent chemical polarity, as determined by its $Pe$, independent of their approach orientation. Our findings offer a methodology for tuning the outcomes of binary interactions among chemically active droplets, laying the groundwork for potential studies on their collective dynamics.