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 inwardly-spiralling patterns accompanied by a thin counterrotating boundary layer. A minimal continuum model is shown to be in good agreement with these observations.
@article{arxiv.1304.2875,
title = {Confinement Stabilizes a Bacterial Suspension into a Spiral Vortex},
author = {Hugo Wioland and Francis G. Woodhouse and Jörn Dunkel and John O. Kessler and Raymond E. Goldstein},
journal= {arXiv preprint arXiv:1304.2875},
year = {2013}
}