Collective motion in bacterial suspensions is scale-free

Area of Science:

• Microbiology
• Fluid Dynamics
• Soft Matter Physics

Background:

• Swimming microorganisms like E. coli exhibit collective behaviors driven by hydrodynamic interactions.
• Bacterial suspensions can self-organize into complex patterns, leading to large-scale chaotic flows known as bacterial turbulence.
• The existence of an intrinsic length scale governing these patterns remains an open question in active matter research.

Purpose of the Study:

• To investigate the emergence of large-scale flows in E. coli suspensions.
• To determine if bacterial collective motion possesses an intrinsic length scale.
• To elucidate the mechanisms driving active turbulence under confinement.

Main Methods:

• Experiments were conducted using E. coli suspensions confined in flat cylindrical chambers.
• The confinement height was systematically varied over more than two orders of magnitude.
• Critical density for collective motion and emergent length/time scales were measured.

Main Results:

• The critical density for collective motion onset scales inversely with confinement height.
• Observed length and time scales increase sharply near onset, limited by confinement.
• Both scales show power-law dependence on confinement height, indicating scale-free behavior.
• Transient coherent vortices, thousands of times larger than individual bacteria, were observed.

Conclusions:

• Bacterial collective motion, or bacterial turbulence, is scale-free, lacking an intrinsic length scale.
• Confinement height is a critical factor determining the scales of active turbulence.
• Results provide crucial data for theoretical models of active matter dynamics.