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

Figure 1

Experimental setup 1: a thin liquid film with bacteria spans between four movable fibers. A micromanipulator with a magnetic deflecting system is used to initiate a large vortex through movement of the probe.

Figure 2

Velocity of $0.53\mu \mathrm{m}$ fluorescent markers vs swimming speed $V_b$. Dots depict experimental data; solid line is the fit $V_m=0.46V_b+7$. Inset: Swimming speed $V_b$ vs time in the course of filling of the experimental chamber with nitrogen. Only part of the velocity decay is shown.

Figure 3

Viscosity for 6 different concentrations of bacteria. ${\nu }_0$ is the viscosity of the liquid without bacteria. Inset: instant viscosity vs time during decay of the vortex for density $n=2.9\times {10}^{10}$. The dashed line is the average value of the viscosity during the slow phase of decay. See movies 1 and 2 in 19.

Figure 4

Left: a thin liquid film containing a bacterial suspension and submersed Ni particle spanning between four movable fibers. Two pairs of magnetic coils create a rotating magnetic field (four green arrows). Right: Field of view of the microscope. The particle’s magnetic moment is shown by a short yellow arrow and the external magnetic field by a long green arrow. See movie 3 in 19.

Figure 5

Viscosity vs speed of the bacteria for two concentrations: (a) $n\approx 1.8\times {10}^{10}{\mathrm{cm}}^{-3}$ (▪) and (b) $n\approx {10}^{10}{\mathrm{cm}}^{-3}$ (●). Corresponding concentrations are indicated in Fig. 3 [areas (a) and (b)]. ${\nu }_0$ is the viscosity of the solution of immobilized bacteria. Inset: Magnetic torque $T_m$ (▪) calculated as $\sin (\varphi )$ and typical velocity of tracers $V_m=2⟨|V_r|⟩$ (●) vs time. The dashed line shows the nonphysical values of $T_m$ and $\nu$ calculated for a magnetic particle that is stuck.