Outside of the imaginations of physics teachers, frictionless devices are hard to come by. But putting a bunch of swimming bacteria into a drop of water achieves just that: a fluid with zero resistance to motion. Incredibly, that resistance (or viscosity, as it’s properly known) can even go negative, creating a self-propelling liquid that might, say, turn a motor in a way that seems to defy the laws of thermodynamics. Recent work explains how bacteria conspire to pull off the improbable.
“For a normal fluid it’s impossible because the whole thing would be unstable,” said Aurore Loisy, a physicist at the University of Bristol in the United Kingdom and a coauthor of one of the new studies, “but for bacteria somehow it works.”
Physicists have long dreamt of getting something for nothing, even if only in outlandish thought experiments. In the 1860s James Maxwell conjured up an all-knowing demon who could shunt fast air molecules to one side of a room and slow molecules to the other, creating a temperature difference that could power an engine. With marginally more practicality, in 1962 Richard Feynman lectured about a microscopic gear that, when jostled by air molecules, would turn in only one direction, driving a motor. But such ideas are dashed by the Second Law of Thermodynamics, which insists that the sorting or the turning must generate heat that dooms both schemes. As the poet Allen Ginsberg put it, you can’t win, and you can’t break even.
Recently, evidence has been mounting that while a free lunch is off the table, a cheap snack might be feasible with a system built around a living fluid. Experimental oddities began to surface in 2015 when a French team confirmed that solutions of E. coli and water could get unnaturally slick. Sandwiching a drop between two small plates, they recorded the force needed to make one plate slide at a certain speed. Liquids usually get harder to stir, or more viscous, when they contain additional suspended particles (think water vs. mud), but the opposite turns out to be true when the particles can swim. When the solution was around half a percent E. coli by volume, keeping the plate moving required no force at all, indicating zero viscosity. Some trials even registered negative viscosity, when the researchers had to apply a bit of force against the plates’ motion to keep them from speeding up. The liquid was doing work, which for any inert fluid would have meant a violation of the Second Law.
The straightforward conclusion was that the organisms were swimming in a way that neutralized the solution’s internal friction to produce something like a superfluid, a liquid with zero resistance. The apparent thermodynamics violation was an illusion because the bacteria were doing the work to offset or overcome the viscosity. [Continue reading…]