# Swimming bacteria in Poiseuille flow: the quest for active Bretherton-Jeffery trajectories

## TL;DR

This study experimentally tracks swimming E. coli bacteria in Poiseuille flow, confirming a kinematic model of their trajectories and revealing new features like quasi-planar paths and drift angles, despite noise effects.

## Contribution

The paper experimentally validates an active Bretherton-Jeffery model for bacteria trajectories and derives new analytical features related to bacterial shape and flow interactions.

## Key findings

- Confirmed swinging and shear tumbling trajectories
- Derived quasi-planar piece-wise trajectories
- Identified drift angles causing cyclic paths

## Abstract

Using a 3D Lagrangian tracking technique, we determine experimentally the trajectories of non-tumbling E. coli mutants swimming in a Poiseuille flow. We identify a typology of trajectories in agreement with a kinematic "active Bretherton-Jeffery" model, featuring an axi-symmetric self-propelled ellipsoid. In particular, we recover the "swinging" and "shear tumbling" kinematics predicted theoretically by Z\"ottl et al. Moreover using this model, we derive analytically new features such as quasi-planar piece-wise trajectories, associated with the high aspect ratio of the bacteria, as well as the existence of a drift angle around which bacteria perform closed cyclic trajectories. However, the agreement between the model predictions and the experimental results remains local in time, due to the presence of Brownian rotational noise.