# Geometry-driven collective ordering of bacterial vortices

## TL;DR

This study demonstrates how geometric boundary conditions can control collective vortex patterns in bacterial suspensions, revealing a design principle for active matter systems through experiments and modeling.

## Contribution

It introduces a geometric control method for bacterial vortices and links pattern transitions to a specific geometric quantity using a Vicsek-like model.

## Key findings

- Circular microwells isolate and control vortex formation.
- Transition from parallel to anti-parallel vortices depends on geometry.
- Pattern formation includes chiral configurations in quadruplet microwell arrays.

## Abstract

Controlling the phases of matter is a challenge that spans from condensed materials to biological systems. Here, by imposing a geometric boundary condition, we study controlled collective motion of Escherichia coli bacteria. A circular microwell isolates a rectified vortex from disordered vortices masked in bulk. For a doublet of microwells, two vortices emerge but their spinning directions show transition from parallel to anti-parallel. A Vicsek-like model for confined self-propelled particles gives the point where two spinning patterns occur in equal probability and one geometric quantity governs the transition as seen in experiments. This mechanism shapes rich patterns including chiral configurations in a quadruplet of microwells, thus revealing a design principle of active vortices.