Self-Starting Micromotors in a Bacterial Bath

Micromotors pushed by biological entities, such as motile bacteria, constitute a fascinating way to convert chemical energy into mechanical work at the micrometer scale. Here we show, by using numerical simulations, that a properly designed asymmetric object can be spontaneously set into the desired motion when immersed in a chaotic bacterial bath. Our findings open the way to conceive new hybrid microdevices exploiting the mechanical power production of bacterial organisms. Moreover, the system provides an example of how, in contrast with equilibrium thermal baths, the irreversible chaotic motion of active particles can be rectified by asymmetric environments.

Figure 1

Rotary micromotor in a bacterial bath. Snapshots are taken at three different simulation times, $t=10$, 12, and 14 s. Each bacterium is represented by a spherocylinder (with aspect ratio $1:2$) with a white head pointing in the direction of the self-propelling force. The arrow at the center of the gear evidences the counterclockwise rotation at an average angular velocity ${\omega }_0\simeq 0.21\mathrm{rad}/\mathrm{s}$.

Figure 2

Angular velocity $\omega$ (in $\mathrm{rad}/\mathrm{s}$) of the micromotor as a function of time (in seconds). The black line refers to a single run. The red (lighter) line is the average over 100 independent runs. After a short transient regime (due to initial configuration of bacteria), a fluctuating velocity around a mean value ${\omega }_0\simeq 0.21\mathrm{rad}/\mathrm{s}$ is observed. Inset: same as main plot for a symmetrically shaped micromotor.

Figure 3

Sketch of the collision of a single bacterium with the rotor boundary. Arrows are the forces exerted by the bacterium onto the rotor. (a) Bacteria coming from the left area with respect to the normal leave the gear. (b) Bacteria from the right get stuck at the corner exerting a torque on the rotor.