Geometry-driven collective ordering of bacterial vortices

Area of Science:

• Active matter physics
• Microbiology
• Soft condensed matter

Background:

• Controlling collective motion in active matter is crucial across disciplines, from materials science to biology.
• Disordered vortex motion is common in bacterial populations, posing challenges for predictable control.

Purpose of the Study:

• To investigate the controlled collective motion of *Escherichia coli* bacteria using geometric confinement.
• To understand the transition in spinning directions of emergent vortices in confined bacterial systems.

Main Methods:

• Utilized circular microwells to impose geometric boundary conditions on *Escherichia coli*.
• Observed vortex formation and spinning dynamics in single and multiple microwell configurations.
• Employed a Vicsek-like model for confined self-propelled particles to analyze experimental observations.

Main Results:

• A single circular microwell isolated a rectified vortex, contrasting with disordered bulk motion.
• Doublets of microwells induced two vortices with a transition from parallel to anti-parallel spinning.
• A geometric quantity was identified as the governing factor for the observed spinning pattern transitions.

Conclusions:

• Geometric boundary conditions effectively control collective bacterial motion and active vortex formation.
• The study reveals a fundamental design principle for creating ordered patterns in active matter systems.
• Identified mechanisms can shape diverse patterns, including chiral configurations, in confined bacterial populations.