Water flows best when it’s chock-full of synchronized-swimming bacteria, a new experiment finds.
It may appear that water flows easily. After all, a stream of it flows a lot faster than, say, a stream of honey. But water doesn’t flow nearly as fast as liquid helium. Such a frigid liquid flows with almost no resistance. Indeed, it is said to have zero viscosity. (Viscosity is a measure of a fluid’s resistance to stress. It corresponds to the idea of how “thick” a liquid is.)
But now, by coaxing billions of cells to work together, researchers have made a small sample of a bacteria-laden water solution show no resistance to flow.
“The results are pretty compelling,” says Raymond Goldstein. He is a physicist at the University of Cambridge in England. The new study, he says, demonstrates that the motion of microbes can drive the large-scale behavior of liquids.
The new finding appears in the July 10 Physical Review Letters. Physicists Héctor Matías López and Harold Auradou at Paris-Sud University and their colleagues authored the new paper.
These researchers started with a small cup filled with water, nutrients and E. coli bacteria. There were enough nutrients to fuel the swimming of bacteria, but not enough energy to allow the microbes to divide. Then the physicists dipped a cylindrical probe into the cup. They slowly rotated the cup and measured the force of the twist, or torque, exerted by the solution on the probe.
A viscous fluid like honey would tug on and spin the probe. Water also would tug on the probe, just not as much. When infused with a strain of very active E. coli, however, the water solution exerted no torque on the probe. That indicates zero viscosity. In some trials, the viscosity actually became negative: The cup rotated counterclockwise. But the solution exerted a clockwise torque on the probe.
Before the cup spun, the bacteria had been swimming about randomly, Auradou says. But theoretical studies suggest that once the liquid starts to flow, the E. coli coordinate their motion. As the rod-shaped bacteria swim, they push water in front and behind themselves. Liquid fills in from the sides. That nudges neighboring bacteria closer together and causes them to line up and swim in a similar direction. The bacteria’s collective pushing increases the speed at which adjacent layers of water can rush past each other. That gives the solution a more efficient — and less viscous — flow.
The new finding may be especially useful in the lab. Tiny amounts of fluid can be difficult to analyze because samples can get stuck in micro-size passageways. Bacteria may help by ensuring that scientists can measure every last drop.