Swimming bacteria work together to go with the flow

Swimming bacteria can reduce the viscosity of ordinary liquids like water and make them flow more easily, sometimes down to the point where the viscosity becomes zero: the flow is then frictionless.

A suspension of bacteria confined between two plates behaves as a negative viscosity fluid: pulling the top solid plate to the left make it move to the right. A. Loisy, J. Eggers, and T. Liverpool, University of Bristol
A suspension of bacteria confined between two plates behaves as a negative viscosity fluid: pulling the top solid plate to the left make it move to the right. A. Loisy, J. Eggers, and T. Liverpool, University of Bristol

Viscosity is a fluid’s resistance from the flow. For instance, oil does not flow as quick as water when spilled out on the grounds that its viscosity is larger. This restriction to motion is caused at the microscopic level by the contact between the liquid particles.

Up to this point, zero viscosity (likewise alluded to as superfluidity) had just been found in quantum systems at low temperatures near absolute in suspensions of microbes. But a couple of years ago, zero viscosity was measured in suspensions of bacteria.

This is possible because bacteria are not like water molecules, they are alive or ‘active’. By swimming together, they can generate forces in the fluid that are large enough to counterbalance friction, such that the bacterial suspension behaves as a superfluid.

Now researchers from the University of Bristol report that bacterial suspensions can have a negative viscosity, a property is strictly forbidden by the law of physics for ordinary fluids, but which becomes allowed when living organisms are present.

Dr Aurore Loisy, Research Associate in Applied Mathematics & Theoretical Physics at the University of Bristol said, “While this phenomenon has been observed experimentally, a detailed theory about how it works has been missing.”

“Furthermore, a fundamental question was still open: can this phenomenon be pushed even further, so that the apparent viscosity becomes negative, meaning that the sheared bacterial fluid could actually do work rather than require work to be done on it to keep flowing, something which is impossible in passive or ‘conventional’ fluids?”

In order to disclose the concept of negative viscosity, scientists studied how the self-organization and collective motion of microorganisms suspended in a fluid modify its hydrodynamic properties.

They showed that it is theoretically possible to make a suspension of bacteria behave as a negative viscosity fluid and that this behavior can be triggered and controlled by playing with the size of the container.

Dr. Loisy said, “This shows us that microscopic bacterial activity can be converted into macroscopic useful mechanical power. In other words, it is possible to make bacteria work together and use that to power larger devices.”

“It is important to note that we haven’t got something for nothing, as in we’ve not broken the laws of thermodynamics. The bacteria are converting the nutrients they have ingested into mechanical work by swimming and the ‘food for the bacteria’ would be the fuel (source of the energy) of the putative machine.”

“The next step is to observe this phenomenon experimentally, for example by turning a rheometer into a bacteria-powered rotary motor.”

The study is published in the Physical Review Letters.