Swimmer-tracer scattering at low Reynolds number

TL;DR

This paper investigates how tracer particles move in active fluids influenced by different microswimmers, revealing that force-free swimmers typically create loop-shaped trajectories that are robust against Brownian motion effects.

Contribution

The study provides analytical and numerical insights into tracer scattering by various microswimmers, highlighting the topological nature of the induced trajectories.

Findings

Force-free swimmers induce loop-shaped tracer trajectories.

Loop structures depend on microswimmer hydrodynamics.

Loops persist despite Brownian motion effects.

Abstract

Understanding the stochastic dynamics of tracer particles in active fluids is important for identifying the physical properties of flow generating objects such as colloids, bacteria or algae. Here, we study both analytically and numerically the scattering of a tracer particle in different types of time-dependent, hydrodynamic flow fields. Specifically, we compare the tracer motion induced by an externally driven colloid with the one generated by various self-motile, multi-sphere swimmers. Our results suggest that force-free swimmers generically induce loop-shaped tracer trajectories. The specific topological structure of these loops is determined by the hydrodynamic properties of the microswimmer. Quantitative estimates for typical experimental conditions imply that the loops survive on average even if Brownian motion effects are taken into account.