Certain types of swimming bacteria can lower the viscosity of an ordinary liquid, sometimes even turning it into a superfluid, according to work done by researchers in France. The team studied how the collective swimming motion of bacteria can substantially alter a fluid’s hydrodynamic properties. In some cases, the change is so great that highly active bacteria create a “negative-viscosity” liquid and are then pushed along by the fluid itself. The researchers suggest that the energy from such bacterial suspensions could be used to drive tiny mechanical motors in microfluidic systems.

The viscosity of a liquid is a measure of its resistance to being forced to flow. For example, honey and oil have much higher viscosities than water, and therefore do no flow as easily. Viscosity arises because of collisions between neighbouring particles in a fluid that move at different velocities. On the other hand, a superfluid – such as liquid helium – is a type of ideal fluid that has zero viscosity and flows as if it is not affected by surface tension or gravity.

All shook up

In the past, scientists had suspected that the presence of certain bacteria in a fluid could cause a change in its viscosity because of bacterial motion. Some models even sought to explain how this might happen: for example, the swimming movement of the bacteria being thought to make local changes in the liquid’s flow as the bacteria align themselves to reduce the velocity gradient of the liquid.

Now, Harold Auradou of the University Paris-Sud and colleagues have studied the well-known Escherichia coli, or E. coli, bacteria. These are a type of “pusher swimmer” that force fluid to flow outwards away from their flagella as they propel themselves forward. Auradou’s team studied E. coli suspensions made up of varying amounts of bacteria in solutions of water and just enough nutrients to keep the cells alive, but not adequate for them to reproduce. The flow of the solutions was studied as they were spun at different speeds in a rheometer – a device that applies shear stress through a rotating outer wall and is used to measure the viscosity of a liquid.

Bacterial brews

With this set-up, the researchers were able to determine that, for low to moderate stress values, the bacteria do indeed lower the viscosity of the liquid, as predicted. When the team then increased the number of bacteria and “doped” them with extra nutrients, the higher activity meant that the viscosity plummeted to zero – and even below zero.

The team still cannot say for sure what causes the viscosity drop, although the pusher-swimming motion may play a key role. Auradou and colleagues are confident, however, that the viscosity drop was indeed caused by the motion of the bacteria, rather than their mere presence, because adding dead bacteria to the solution made no difference to the viscosity. The team says that it may, in theory, be possible to somehow harness the viscosity-lowering ability of such bacterial cocktails. This could involve placing tiny rotors in the fluid that would be dragged around and could power a small device such as a microfluidic pump.

The research is published in Physical Review Letters.