# Magnetotactic bacteria in a droplet self-assemble into a rotary motor

## TL;DR

This study demonstrates that magnetotactic bacteria confined in droplets can self-assemble into a rotary motor under a magnetic field, producing reversible vortex flows and measurable mechanical energy, revealing new insights into active matter self-organization.

## Contribution

It introduces a novel biological self-assembled rotary motor driven by magnetic fields, combining microbiology and physics to explore active matter dynamics.

## Key findings

- Bacteria form a reversible vortex under magnetic field.
- The vortex exerts torque on the surrounding oil phase.
- Mechanical energy from the motor is quantitatively characterized.

## Abstract

From intracellular protein trafficking to large scale motion of animal groups, the physical concepts driving the self-organization of living systems are still largely unraveled. Selforganization of active entities, leading to novel phases and emergent macroscopic properties, recently shed new lights on these complex dynamical processes. Here we show that, under the application of a constant magnetic field, motile magnetotactic bacteria confined in water-in-oil droplets self-assemble into a rotary motor exerting a torque on the external oil phase. A collective motion in the form of a large-scale vortex, reversable by inverting the field direction, builds-up in the droplet with a vorticity perpendicular to the magnetic field. We study this collective organization at different concentrations, magnetic fields and droplets radii and reveal the formation of two torque-generating areas close to the droplet interface. We characterize quantitatively the mechanical energy extractable from this new biological and self-assembled motor.