Transport Powered by Bacterial Turbulence

We demonstrate that collective turbulentlike motion in a bacterial bath can power and steer the directed transport of mesoscopic carriers through the suspension. In our experiments and simulations, a microwedgelike “bulldozer” draws energy from a bacterial bath of varied density. We obtain that an optimal transport speed is achieved in the turbulent state of the bacterial suspension. This apparent rectification of random motion of bacteria is caused by polar ordered bacteria inside the cusp region of the carrier, which is shielded from the outside turbulent fluctuations.

Figure 1

(a) Experimental carrier speed $v/v_0$ as a function of the bacterial three-dimensional volume fraction $c$. The insets show the temporal progress of the carrier positions (right) and a snapshot of the carrier (left) indicating the characteristic spatial dimensions and the schematic representation (blue line). See also movie 1 in the Supplemental Material [32]. (b) Numerically obtained transport speed for varying swimmer concentration $\varphi$ and carriers of two different contour lengths: $L=\text{262\hspace{0.17em}\hspace{0.17em}}\mu \mathrm{m}$ (squares) corresponding to the experimental conditions and half the length $L=131\mu \mathrm{m}$ (circles). Magnitude of vorticity $\mathrm{\Omega }$ and averaged bacterial swimming speed $v_p/v_0$ for various concentrations are shown in the inset. See also movies 2 and 3 in the Supplemental Material [32].

Figure 2

Intensity plots for different bacteria concentrations, $\varphi =0.2$ (top row) and $\varphi =0.7$ (bottom row): local bacterial concentration around the carrier, normalized by the total concentration (left) as well as averaged bacteria orientations $⟨\cos \phi ⟩$ (middle) and $⟨\sin \phi ⟩$ (right).

Figure 3

Concentration difference between the wake of the carrier and its front $\mathrm{\Delta }c=c_w-c_f$ obtained from (a) experiments and (b) simulations.

Figure 4

Normalized local magnitude of vorticity obtained from simulations (left column) and experiment (top right). Bottom right: Illustration of swirl shielding in the carrier cusp—bacteria in the shielded area (light colored) are indicated by arrows and the unshielded area is marked by dark color. Typical swirl radius $R$ for different bacteria concentrations is obtained as the first minimum of the equal-time spatial velocity autocorrelation function [39].

Figure 5

(a) Probability distributions of the carrier displacement direction $\vartheta$: Experimental results (left) measured after a time of $0.03\mathrm{s}$ and numerical results (right) after a time of ${10}^{-3}\tau$ for various bacterial concentrations. The definition of the displacement angle $\vartheta$ is shown in the inset. (b) Orientational ordering of the bacteria within the swirl shielded area. Characteristic configurations are illustrated by sketches.