Alignment and scattering of colliding active droplets

TL;DR

This paper investigates how active droplets collide and interact, revealing that symmetric collisions tend to align their trajectories, while oblique collisions lead to diverse scattering behaviors, influencing collective dynamics.

Contribution

It extends previous head-on collision analysis to include oblique collisions, proposing a reduced model to characterize alignment and scattering in active droplet interactions.

Findings

Symmetric collisions lead to trajectory alignment.

Oblique collisions result in complex scattering behaviors.

Chemical wake dynamics influence collision outcomes.

Abstract

Active droplets emit a chemical solute at their surface that modifies their local interfacial tension. They exploit the nonlinear coupling of the convective transport of solute to the resulting Marangoni flows to self-propel. Such swimming droplets are by nature anti-chemotactic and are repelled by their own chemical wake or their neighbours. The rebound dynamics resulting from pairwise droplet interactions was recently analysed in detail for purely head-on collisions using a specific bispherical approach. Here, we extend this analysis and propose a reduced model of a generic collision to characterise the alignment and scattering properties of oblique droplet collisions and their potential impact on collective droplet dynamics. A systematic alignment of the droplets' trajectories is observed for symmetric collisions, when the droplets interact directly, and arises from the finite-time rearrangement of the droplets' chemical wake during the collision. For more generic collisions, complex and diverse dynamical regimes are observed, whether the droplets interact directly or through their chemical wake, resulting in a significant scattering.