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Bacterial teamwork decreases resistance to flow

The physics of active fluids is a topic that has recently emerged at the interfaces between fluid mechanics, statistical physics and biology. It covers an area of applications extremely broad ranging from collective properties of flocks of birds, schools of fish but also at a much smaller scale, it deals with the very singular constitutive properties of fluids loaded with bacteria. These microorganisms propelled autonomously in a viscous environment and transmit their momentum to the fluid at a microscopic scale. Recently, a collaboration between teams from the Université Paris-Sud (LPS - UMR 8502, FAST - UMR 7608) and the ESPCI (PMMH - UMR 7636)) showed a surprising property of these bacterial fluids. By shearing an Escherichia coli suspension, these researchers demonstrated that the viscosity of the solution decreases with the concentration of bacteria, contrary to what is commonly accepted since Einstein for suspensions of solid grains. In addition, for very concentrated solutions, the macroscopic viscosity (i.e. the resistance to shear) may cancel, like a superfluid ! This result means that the engine power of bacteria oriented in the flow fully compensates the viscous dissipation. Apart from the fundamental interest of this result, which confirms some theoretical conjectures on active matter, the ability to reduce viscous dissipation may have important implications for understanding basic mechanisms governing the transport of bacteria in confined environments such as biological networks or soils (contamination or biological transfer).

Suspending spherical particles in a fluid increases its resistance to flow as was observed and demonstrated by Albert Einstein. But what happens if these particles are self-propelled ? One way to address this question is to measure the viscosity of active suspensions (liquids consisting of self-propelled particles). Bacteria such as Bacillus subtilis and Escherichia coli (E. coli) have rod shape bodies equipped with helical flagella that are rotated by motors embedded in their membrane. These unicellular organisms are thus able to swim by exerting a force on the surrounding liquid. Based on the hydrodynamic theory of active fluids, it was predicted that these swimming organisms could change substantially the viscosity of the fluid. Models suggest that pusher-swimmers — organisms that force fluid to flow out, away from their tails — will lower the viscosity by aligning themselves such that their pushing contributes to the velocity gradient (see Fig. A). However, the experimental confirmation of such a prediction requires the ability to measure the fluid viscosity at very low shear rates.

To this end, we used a simple Couette rheometer : the suspension is placed in the annulus between two concentric cylinders. To shear the fluid, the outer cylinder is rotated at a set rate. The liquid drags the inner cylinder, exerting a torque on it. From measurements of this torque, one can infer the shear stress and thus the fluid viscosity, defined as the ratio of stress to the applied shear rate. We have modified the rheometer by controlling the inner cylinder rotation using a computerized feedback loop in order to measure ultralow shear stresses in a very sensitive way. We used this device to measure the viscosity of a suspension of motile, but not dividing bacteria. We studied the viscous response of samples at different bacteria concentrations and different shear rates.

The results of such measurements were surprising : as the number of bacteria was increased, the viscosity dropped. For a sufficiently high concentration of active E. coli, the solution’s viscosity dropped all the way to zero, meaning that we were able to measure no torque on the probe. In particular environmental conditions, the cell motility could be enhanced and in these conditions a negative viscosity was measured (Fig. B).

This means that bacteria can completely compensate for fluid friction, turning it into a frictionless liquid, akin to a superfluid. To explain such an observation, we propose that elongated bacteria align their bodies along the stretching axis of the external flow and generate additional flow that further stretches the fluid in the same direction. At high enough concentrations, we propose that bacteria act collectively to push the fluid along, effectively thinning it. However, at this stage a microscopic understanding of how bacteria coordinate their response to shear to achieve a state of frictionless flow is still missing.

Although we are not able yet to harness bacterial power for energy generation at a macroscopic scale, one can reasonably imagine that bacteria could be used as mixers to thin and stir the flow in complex environments like soils or biological networks.

Reference :

Turning Bacteria Suspensions into Superfluids
H. Matías López et al.
Phys. Rev. Lett. 115, 028301 (2015).
See also the Synopsis.

Contact :

Carine Douarche