Superfluids created by bacteria

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.

https://www.quantamagazine.org/swarming-bacteria-create-an-impossible-superfluid-20180726/