How is helium turned into a liquid and a superfluid?
Helium makes your voice go squeaky and keeps your balloons floaty, but how can it be made even cooler?
Asked by: Toby Carter, by email
At -269°C, helium gas condenses to become a liquid. Cool it even further and it becomes a state of matter called a superfluid. In this state it has no measurable viscosity and so does some odd things, such as climbing up the walls of a dish, leaking through apparently solid materials and staying motionless while its container is spun.
To create the liquid and superfluid states, you cool down helium gas to a few degrees above absolute zero. This is achieved by compressing the gas, and then expelling it through a small nozzle. As the gas expands, it rapidly cools (you’ll have noticed this effect if you’ve ever used an aerosol deodorant). The process is repeated until the gas that rushes out of the nozzle is cold enough to condense to a liquid, then if you repeat the cycle a few more times the helium will become cold enough to turn to a superfluid.