title: Confinement stabilizes a bacterial suspension into a spiral vortex.
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.
authors.affiliation: Department of Applied Mathematics and Theoretical Physics, Centre for Mathematical Sciences, University of Cambridge, Wilberforce Road, Cambridge CB3 0WA, United Kingdom.
authors.forename: Hugo
authors.initials: H
authors.lastname: Wioland
authors.forename: Francis G
authors.initials: FG
authors.lastname: Woodhouse
authors.forename: Jörn
authors.initials: J
authors.lastname: Dunkel
authors.forename: John O
authors.initials: JO
authors.lastname: Kessler
authors.forename: Raymond E
authors.initials: RE
authors.lastname: Goldstein
mesh_terms: D000465:Algorithms; D001412:Bacillus subtilis / Q000737:chemistry / Q000502:physiology*; D001431:Bacteriological Techniques; D008954:Models, Biological; D013499:Surface Properties; D013535:Suspensions / Q000737:chemistry; D013550:Swimming
chemical_list: D013535:Suspensions