Swimming
bacteria at work: from reduction of viscosity to rotation of gears.
Department of Ecology and Evolutionary Biology
Princeton University
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 7 compared to the
viscosity of the same liquid without bacteria or with nonmotile
bacteria. The reduction in viscosity is observed in two complementary
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. In addition, we describe a
class of systems in which aerobic swimming bacteria moving randomly in
a fluid film power submillimeter gears and primitive systems of gears
decorated with asymmetric teeth. The directional rotation is observed
only in the regime of collective bacterial swimming and the
gears’ angular velocities depend on and can be controlled by the
amount of oxygen available to the bacteria.