Abstract

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 viscosity depends on the concentration and swimming speed of the bacteria.

The effective viscosity of dilute suspensions of swimming bacteria from the microscopic details of the interaction of an elongated body with the background flow is derived. An individual bacterium propels itself forward by rotating its flagella and reorients itself randomly by tumbling. Due to the bacteriums asymmetric shape, interactions with a background flow cause the bacteria to preferentially align in directions in which self-propulsion produces a significant reduction in the effective viscosity.

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