Bacteria swimming in a fluid can reduce its viscosity to zero.

Unlike most liquids, superfluids have zero viscosity and so can behave in an unusual way, sometimes even defying gravity. Harold Auradou at the University of Paris-Sud in France and his colleagues found that they could alter the viscosity of a normal liquid solution by varying the concentration of Escherichia coli swimming in it. As the researchers spun the solution around in a rheometer (which measures viscosity), the motion of the bacteria's propulsive tails compensated for the forces between the molecules in the liquid, decreasing its viscosity. Once there were around 6 billion strong swimmers in suspension, the viscosity fell to zero.

Phys. Rev. Lett. 115, 028301 (2015)