Simulations and theory indicate that the “synchronized swimming” of bacteria occurs in much sparser suspensions of the microorganisms than expected.
While being one of the most important unsolved questions in physics, the phenomenon of turbulence is not commonly associated with microorganisms.
Yet experiments on suspensions of swimming bacteria have revealed that they exhibit chaotic collective motion reminiscent of the turbulence seen around an airplane wing or in a waterfall: when the concentration of bacteria is high enough, they start to swirl around in vortex-like patterns that constantly change size and direction. The mechanism of this phenomenon is thought to be related to the mutual reorientation of bacteria in the fluid flow they create while swimming, but the precise mechanism has proven to be elusive.
In a recent paper, researchers from Lund, Cambridge and Edinburgh add another piece to this puzzle. Using theory and large-scale computer simulations, Joakim Stenhammar, Cesare Nardini, Rupert Nash, Davide Marenduzzo, and Alexander Morozov demonstrate that even at very low densities, individual microorganisms "feel" the presence of the others and their motion is thus never fully independent. As the concentration of bacteria is increased, the strength of these correlations build up to eventually form a chaotic state strongly similar to that seen in experiments.
Dr Alexander Morozov commented: "Up to now it was thought that low-density suspensions of swimming microorganisms are random and featureless, and that the correlations appear suddenly at the onset of collective motion. Surprisingly, we see measurable signs of these correlations at very low densities of microswimmers that significantly affect physical properties of the suspension."