Proc. Natl Acad. Sci USA 109, 4052–4057 (2012)

Throwing non-adsorbing polymers into a system of colloidal particles induces an osmotic pressure that drives the particles together. When the colloids are passive, this depletion mechanism can lead to phase separation — a process tuned by the polymer concentration. But what happens when the colloids are capable of moving themselves around? Jana Schwarz-Linek and colleagues have shown that bacteria can act as motile colloids that exhibit intriguing self-assembly behaviours.

A quick calculation reveals that the bacterial propulsion forces should be of the same order as those associated with the depletion mechanism. Schwarz-Linek et al. found that the ensuing competition between activity and interparticle attraction suppressed phase separation, meaning that higher polymer concentrations were needed to push the system towards coexistence.

When the interparticle attraction was too weak to overcome the motile force — but strong enough to cause phase separation in passive systems — the group noticed that the bacteria formed tiny unidirectionally rotating clusters. The chiral symmetry was broken by the torque associated with the bacterial flagella, but simulation results suggested that formation of these microrotors may be a generic property of attractive active colloids.