Braided mixing in confined chiral active matter

TL;DR

This paper investigates how chiral active microswimmers in confined environments can be optimized to enhance fluid mixing by analyzing the entanglement of their trajectories through braid theory.

Contribution

It introduces a modified chiral Vicsek model and demonstrates how specific flow patterns maximize mixing efficiency using braid analysis.

Findings

Optimal mixing occurs with a stable vortex droplet and oscillating phase.

Braid entanglement correlates with mixing efficiency.

Parameter tuning enhances chaotic mixing in microswimmer systems.

Abstract

Efficient mixing of fluids is essential in many practical applications to achieve homogeneity. For microscopic systems, however, both diffusion and turbulence are ineffective methods to achieve chaotic mixing due to the low Reynolds number, hence either active stirring or inducing turbulence through geometric boundary effects are generally implemented. Here, we study a modified chiral Vicsek model, where active microswimmers act as moving rods, stirring the surrounding substrate. We study the degree of mixing in the patterns formed by interplay between confinement, chiral motion and alignment interactions. This mixing is computed by considering the entanglement of spacetime trajectories of the particles, which forms a braid. Optimising the finite-time braiding exponent of this braid then yields a set of constituent parameters of the system, showing that a pattern consisting of a local stable vortex droplet and an ordered oscillating phase achieves the highest degree of mixing.