Dynamics of a free bacterial vortex

Micro-particles around a collapsing bubble are focused into a cluster of high concentration. Inspired by this phenomenon, we develop a new microfluidic device to study the phase behavior of micro-swimmers of high densities, without using any confinement or external field. After being radially aligned by a contracting gas bubble, bandings and a vortex are observed in a bacterial suspension. We study the periodic structure of the bandings during their formation and how they finally become bacterial turbulence. The result might help to enrich our understanding on active matter collective motion, which might enhance dispersion of microbes in a natural environment where bubble collapse regularly happens.