Superfluid storm erupts

One of the major discoveries of the last century of physics has been flipped on its head. Scientists have discovered that a stirred teacup of supercooled liquid helium will not swirl forever, as previously thought.

Neither does it flow completely without resistance through a pipe.

Instead, a microscopic ‘storm’ layer at the helium’s surface puts the brakes on. The discovery, described in the journal Physical Review Letters, reveals a new complexity to the supremely odd ‘superfluid’ state of matter. And it could lead to improved superfluid cooling systems, such as the colossal arrangement used to deep-freeze the 27 kilometre ring of the Large Hadron Collider.

While water can gush easily from a faucet, honey slowly drips from a spoon. That’s because the two liquids have wildly different viscosity — a resistance to flow coming from how the liquid molecules bump into and tangle with one another.

One of the great shocks of twentieth century physics was the discovery of a liquid with exactly zero viscosity — supercooled liquid helium.

At about minus-271C, or just two degrees above absolute zero, the behaviour of liquid helium turns surreal. No open container can hold it, because the liquid flows up the walls and spills out. In 1937 Soviet and American physicists independently realised it was a new form of matter: a superfluid.

To help describe the bizarre ‘zero-viscosity’ behaviour of superfluid helium, physicists often use the analogy of a tea (or coffee) cup. Stir a teacup of superfluid helium and it would keep swirling forever.

“Or,” says George Stagg, a mathematician at Newcastle University, “at least this is what has always been believed.”

Stagg and his colleagues have found that this ‘eternal flow’ is, and always has been, a mistaken concept. Instead, the superfluid does have an ‘emergent’ viscosity, arising from its interaction with the walls of its container.

The problem is that at small scales, every surface is somewhat rough, a landscape of peaks and valleys a few molecules tall. As the superfluid flows over these peaks and valleys, “mini tornadoes” are created. The interaction of these tornadoes with one another slows down the flow of the superfluid—at least at its edges.

“These swirling vortices tangle together like spaghetti and just like when you drain your spaghetti and leave it for too long in a pan they stick together, creating a slow-moving boundary layer between the free-moving fluid and the surface,” says Stagg.

The behaviour had not been detected before because theorists usually only consider ideally smooth surfaces. Stagg and his team made the discovery when they set out to explain experiments performed at Lancaster University, which monitored the flow of superfluid helium past a metal rod.

The team’s computer models revealed the tangling tornadoes, and slowed flow. Often physicists use idealised models, from perfectly flat surfaces to spherical chickens in a vacuum, to describe how something behaves at the simplest possible level. But it’s a recurring lesson that, in the real world, the behaviour of materials is often dominated by their defects, and imperfections. Just like we are.