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

Credit: © GETTY IMAGES

Add sufficient non-adsorbing polymers to a colloidal suspension, and the colloids will precipitate. This is caused by the attractive (depletion) forces that arise between colloids when they are too close for the polymers to fit in the space between. Such an entropic-driven phase separation has also been observed in suspensions of non-motile bacteria. Now, Jana Schwarz-Linek and colleagues find that a suspension of self-propelled Escherichia coli needs roughly three times more polymer to precipitate. The researchers explain that bacterial self-propulsion competes with depletion forces, and that therefore live E. coli experience an attraction that is weaker than that between passive colloids. They also observed that when the strength of the attraction is too feeble to cause phase separation (but not aggregation), the competition between self-propulsion and entropic attraction also makes clusters of bacteria rotate unidirectionally, and that the net torque results from the random forces exerted by a cluster's outermost bacteria. The researchers' computer simulations of self-propelled spheres with a short-range intersphere attraction show that the observed phenomena should be generic to active matter.