Self-Sustained Density Oscillations of Swimming Bacteria Confined in   Microchambers
  We numerically study the dynamics of run-and-tumble particles confined in two chambers connected by thin channels. Two dominant dynamical behaviors emerge: (i) an oscillatory pumping state, in which particles periodically fill the two vessels and (ii) a circulating flow state, dynamically maintaining a near constant population level in the containers when connected by two channels. We demonstrate that the oscillatory behaviour arises from the combination of a narrow channel, preventing bacteria reorientation, and a density dependent motility inside the chambers. 
M. Paoluzzi, R. Di Leonardo, L. Angelani
cond-mat.soft cond-mat.stat-mech