In this week’s PANDA meeting (that’s Pattern Formation, Nonlinear Dynamics and Applications) at Leeds University’s Department of Applied Mathematics, Dr Suzanne Fielding, a physicist from Durham University, presented the results of her theoretical research on "active fluids". The strange texture of folds and swirls, pictured above, is predicted by her mathematical model.
The symmetry-based theories of condensed matter physics are usually used to model inanimate materials like semiconductors, liquid crystals and magnets. But Fielding has applied them to understand the swarming behaviour of microscopic swimmers such as amoebae. These organisms are so numerous and tiny that, en masse, they form a fluid with liquid-crystalline properties, making shades of light and dark in polarized light (see picture), like a liquid crystal display (LCD).
This active fluid of living organisms flows in peculiar ways. A fluid’s viscosity is a measure of how “thick” it is – how hard you have to push to make it flow at a given rate. So water has a low viscosity, but treacle’s viscosity is high. Fielding has used her model to calculate the viscosity of a dense swarm of microbes.
“Even at zero stress, it can spontaneously flow with a finite shear rate,” she says. “So it has exactly zero viscosity. It’s a superfluid!”
This is a very strange result. Superfluids – liquids that flow without friction – are rare, and have previously only been encountered at extremely low temperatures, close to absolute zero. If Fielding’s predictions are correct, this would be the first example of superfluidity at room temperature. Whereas the “traditional” superfluid, liquid helium, relies on the weird quantum physics of ultra-low temperatures to achieve perfect frictionlessness, Fielding’s active fluid simply relies on the hard work of billions of microbes swimming furiously.
So what’s it going to be used for? The applications of a room-temperature superfluid have yet to be invented, since no-one ever anticipated such a discovery, but they will surely be impressive. Ideas anyone?
“Nonlinear dynamics and rheology of active fluids: simulations in two dimensions”, S. M. Fielding, D. Marenduzzo and M. E. Cates, Physical Review E 83 (2011) 041910