Macroscopic bioinspired magnetic active matter and the physical limits of magnetotaxis

TL;DR

This paper investigates the physical limits of magnetotactic bacteria's magnetic sensing, showing how increased magnetic interactions lead to clustering that impairs their swimming ability, using bio-inspired experiments and models.

Contribution

It combines experiments, simulations, and analytic estimates to identify an upper bound on magnetosome size affecting magnetotactic bacteria's performance.

Findings

Increasing dipolar strength causes clustering in magnetic active matter.

Beyond a certain magnetic moment, clustering hinders effective swimming.

Long-range magnetic interactions influence phase behavior of anisotropic active matter.

Abstract

Magnetotactic bacteria (MTB) are endowed with an exquisite orientation mechanism allowing them to swim along the geomagnetic field lines. This mechanism consists of a chain of bio-synthesized magnetic nano-crystals that endow MTB with a permanent magnetic moment. Although the physics behind the minimum size of this biological compass is well understood, it is yet unclear what sets its maximum size. Here we combine macroscopic bioinspired experiments, calibrated simulations, and analytic estimates to show that increasing dipolar strength can drive magnetic active matter from a freely swimming regime into clustered states. Using a physical model with parameters relevant to MTB, we infer a plausible physical upper bound on useful magnetosome-chain magnetic moments: beyond a threshold, clustering and the formation of compound bodies are expected to hinder effective swimming and reduce magnetotactic performance. Our macroscopic bio-inspired experiment and physical model show how long-range magnetic interactions reshape the phase behavior of anisotropic active matter and provide a programmable platform for studying magnetic active matter across scales.