Bacteria can thin liquids to make them flow more easily. Astonishingly, they can do this to the point where the liquid they are swimming in becomes a superfluid, something with zero viscosity – a phenomenon previously only witnessed under extreme conditions, such as temperatures close to absolute zero.
For the study, Dr. Harold Auradou of the University of Paris-Sud added E. coli to liquids in different concentrations. To make sure the bacteria focused on swimming, he put in enough nutrients to keep them alive, but not enough to tempt them to reproduce. Each solution was then spun in a rheometer to measure how the concentration of bacteria affected the ability of the liquid to flow.
The effect was something like what physicists see as they test electrical resistance when cooling metals and certain ceramics. At first, rising concentrations of bacteria caused a steady reduction in viscosity, allowing the liquid to flow more freely. This is in keeping with modelling done in 2009 on how elongated lifeforms, propelling themselves using tiny flagella, would influence the liquids in which they swim.
However, just as falling electrical resistance can suddenly drop to zero, Auradou reports in Physical Review Letters that in some circumstances all traces of shear resistance vanishes. In other words, the liquid acts in a manner similar to liquid helium below -271° C (-456° F), whose party trick behavior dazzles the mind.
“If you use dead bacteria, nothing changes,” Auradou told Nature, which demonstrates the effect is a function of the swimming behavior, rather than the bacteria’s shape. Indeed, while all strains of E. coli produced very low viscosity at concentrations between 0.6% and 2.4% by volume, it is only the most actively swimming strains that turn the solution in which they move into a superfluid.
Nevertheless, Auradou does not claim to know how zero viscosity is achieved. “We believe that there is a kind of collective motion of the bacteria that we don't understand yet,” he said. Almost certainly, the tail-like flagella have something to do with it. Viscous conditions make swimming hard for the tiny bacteria, obstructing them from reaching their food, so the flagella have evolved to interrupt the forces between molecules that are one of the root causes of viscosity.
Auradou’s discovery required the use of equipment generally considered obsolete. Modern rheometers “are made to measure higher viscosity and higher shear rate,” Auradou says, and couldn’t track resistance this low. Combining an old rheometer with modern computer controls created a device capable of distinguishing between low and zero viscosity.
At the moment, this is basic research without direct practical applications, but Auradou suggests it could prove useful in the production of tiny motors.