(45c) Reduction of Viscosity in Suspension of Swimming Bacteria

Measurements of the shear viscosity in suspensions of swimming Bacillus subtilis in free standing liquid films have revealed that the viscosity can decrease by up to a factor of five compared to the viscosity of the same liquid without bacteria or with non-motile bacteria. The reduction in viscosity is observed in two complimentary experiments: one studying the decay of a large vortex induced by a moving probe and another measuring the viscous torque on a rotating magnetic particle immersed in the film. The measured viscosity depends on the concentration and swimming speed of the bacteria. We derive the effective viscosity for dilute suspensions of swimming bacteria. Due to the bacterium's asymmetric shape, interactions with a prescribed generic (such as pure shear or planar shear) background flow cause the bacteria to preferentially align in directions in which self-propulsion produces a reduction in the effective viscosity.