Confinement Stabilizes a Bacterial Suspension into a Spiral Vortex

Francis G. Woodhouse,Raymond E. Goldstein,John O. Kessler,Jörn Dunkel,Hugo Wioland

doi:10.1103/physrevlett.110.268102

Abstract

Confining surfaces play crucial roles in dynamics, transport, and order in many physical systems, but their effects on active matter, a broad class of dynamically self-organizing systems, are poorly understood. We investigate here the influence of global confinement and surface curvature on collective motion by studying the flow and orientational order within small droplets of a dense bacterial suspension. The competition between radial confinement, self-propulsion, steric interactions, and hydrodynamics robustly induces an intriguing steady single-vortex state, in which cells align in inward spiraling patterns accompanied by a thin counterrotating boundary layer. A minimal continuum model is shown to be in good agreement with these observations.

Highlights

Geometric boundaries and surface interactions are known to have profound effects on transport and order in condensed matter systems, with examples ranging from nanoscale edge currents in quantum Hall devices [1,2] to topological frustration in liquid crystals (LCs) tuned by manipulating molecular alignment at confining surfaces [3]
In spite of considerable recent interest [4,5,6,7,8], the effects of external geometric constraints and confining interfaces on collective dynamics of active biological matter [9,10], such as polar gels [11,12] and bacterial [13,14,15,16,17,18] or algal suspensions [19], are not yet well understood, not least owing to a lack of well-controlled experimental systems
While some progress has been made in understanding the dynamics of dense bacterial suspensions in bulk [16,18,23,24,25,26], microorganisms often live in porous habitats like soil, where encounters with interfaces or three-phase contact lines are common [13,14,27]

Summary

Introduction

Geometric boundaries and surface interactions are known to have profound effects on transport and order in condensed matter systems, with examples ranging from nanoscale edge currents in quantum Hall devices [1,2] to topological frustration in liquid crystals (LCs) tuned by manipulating molecular alignment at confining surfaces [3]. The vortex flow described here is purely azimuthal and accompanied by a thin counterrotating boundary layer, consisting of cells swimming opposite to the bulk. We observe that the cells arrange in spirals with a maximum pitch angle of up to 35 relative to the azimuthal bulk flow direction [Fig. 1(b)].

Results

Conclusion