Confinement stabilizes a bacterial suspension into a spiral vortex.

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

Department of Applied Mathematics and Theoretical Physics, Centre for Mathematical Sciences, University of Cambridge, Wilberforce Road, Cambridge CB3 0WA, United Kingdom.

Physical review letters 2013 Jun 28

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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.

Citation

Hugo Wioland, Francis G Woodhouse, Jörn Dunkel, John O Kessler, Raymond E Goldstein. Confinement stabilizes a bacterial suspension into a spiral vortex. Physical review letters. 2013 Jun 28;110(26):268102