SUPERFLUID CAN CLIMBSWALLS
SCIENCE FACTS
You don't have to worry about a soft drink spontaneously overflowing its rim or shooting up and out of the straw from which you're trying to drink. That's because soft drinks are nothing like the superfluid helium.
Researchers have known for decades that if you cool liquid helium just a few degrees below its boiling point of –452 degrees Fahrenheit (–269 degrees Celsius) it will suddenly be able to do things that other fluids can't, like dribble through molecule-thin cracks, climb up and over the sides of a dish, and remain motionless when its container is spun.
No longer a mere liquid, the helium has become a superfluid—a liquid that flows without friction. "If you set [down] a cup with a liquid circulating around and you come back 10 minutes later, of course it's stopped moving," says John Beamish, an experimental physicist at the University of Alberta in Edmonton. Atoms in the liquid will collide with one another and slow down. "But if you did that with helium at low temperature and came back a million years later," he says, "it would still be moving."
Like plenty of other physics experiments that make you go—"Huh?"—superfluidity flows from the counterintuitive rules of quantum mechanics. But unlike other quantum stuff, superfluid helium's weird behavior is visible to the naked eye.
An early sign of helium's odd behavior was observed back in 1911 by the Dutch physicist and 1913 Nobel physics laureate Heike Kamerlingh Onnes, a master of refrigeration who was the first to liquefy helium. Onnes found that helium (technically, the helium 4 isotope) began to readily conduct heat below –455.67 degrees F (–270.92 degrees C), also known as the lambda point.
It wasn't until 1938 that the Russian physicist Pyotr Kapitsa and, independently, the British duo of John Allen and Don Misener measured the flow rate of helium below that temperature through a pair of glass disks attached to a plunger and a long, thin glass tube, respectively. The viscosity was so low that Kapitsa, who won his own Nobel Prize for the work, coined the term "superfluid" to describe it—after "superconductor," the term for a material that conducts very high electric currents without resistance.
Key to the effect is helium's unique ability to remain liquid down to absolute zero (–459.67 degrees F, or –273.15 degrees C), the temperature at which atoms theoretically stop moving. When most liquids are cooled, the slight attraction between atoms in the fluid finally begins to overcome heat vibrations, and the particles settle into a regular order, namely a solid. But helium atoms are so light and weakly drawn to one another that even when ordinary atomic motions have quieted, the atoms jiggle with zero-point motion, a slight momentum imparted by the quantum uncertainty principle. Hence, they never settle into the solid state.
Helium's liquidity at low temperatures allows it to carry out a transformation called Bose–Einstein condensation, in which individual particles overlap until they behave like one big particle. Atoms acting in unison don't behave like individual atoms. "If you march in unison, you don't collide with each other," says Moses Chan, who studies superfluidity at Pennsylvania State University in University Park.
Researchers like to think of superfluid helium as a mixture of two fluids, one normal and one superfluid. Different experiments bring out the contrasting characters of the two fractions. The simplest "experiment" is to watch as a container full of liquid helium suddenly springs a leak as it is cooled below the lambda point and the frictionless superfluid fraction begins to pour through microscopic cracks that the normal liquid fraction cannot enter. ("Super-leaks" have been the bane of scientists working with liquid helium since the early days, Beamish says.) But stir the same helium like coffee and the normal liquid fraction will resist the motion, imparting viscosity to the superfluid mixture, after all.
As the temperature falls, the superfluid fraction takes up a greater share of the mixture. In the field's gold-standard experiment, researchers measure the ratio of the two fractions by placing a sample in a cylindrical metal container suspended by a wire. When they impart a twist to the wire, the cylinder will rotate one way and then the other. But only the normal fraction will rotate with the cylinder, because of friction between it and the cylinder walls; the superfluid portion cuts right through the normal fluid and remains still. As the superfluid fraction increases, the cylinder rotates faster, as if the cylinder were losing weight (technically, inertia).
Superfluid helium's dual nature is at work again when it climbs the walls of a containerAny liquid will coat the sides of a dish in which it sits—thanks again to the slight attraction between atoms—but the liquid's internal friction limits how far the coating may spread. In superfluid helium, the frictionless film slithers over the whole container, creating a sort of arena through which the superfluid can flow. If the liquid has somewhere to fall after it climbs out of the dish, it will drip from the bottom of the container until it siphons out all the superfluid pooled above it.
The same principle underlies another famous demonstration in which superfluid rapidly shoots out of an open, heated glass tube packed with fine powder at the bottom. Called the superfluid fountain, it occurs because the superfluid outside of the tube rushes in to cool down the superfluid that has been warmed by the inside of the tube. (Allen, the co-discoverer of superfluidity, is said to have discovered the effect after he shined a pocket flashlight onto a glass tube of liquid helium.)
Work on superfluid helium has already netted three Nobel Prizes and may yet garner more. In 2004 Penn State's Chan and Eun-Seong Kim rotated a ring full of solid helium at 26 atmospheres of pressure and found that as they cooled the helium below the critical temperature, the rotational frequency increased, just as it does with liquid helium. Half a dozen laboratories, including Beamish's, are studying the "supersolid" effect, but researchers still aren't sure which elements of the solid would condense into a single Bose–Einstein state.
The trick now is to see if the supersolid can produce the equivalent of super-leaks or other well-known super-effects. "If other unique properties can be convincingly shown," Beamish says, "everyone would agree it's a new phase of matter."
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