Liquid helium has fascinated scientists ever since it was first produced by the Dutch physicist Heike Kamerlingh Onnes in 1908. One of the biggest surprises was that liquid helium-4 forms a substance known as helium-II when it is cooled below 2.2 K. Helium-II is a mixture of two interpenetrating fluids: superfluid helium-4, which can flow without resistance, and normal helium-4.
Now Demosthenes Kivotides, Carlo Barenghi and David Samuels at the University of Newcastle in the UK have discovered another surprising feature of liquid helium. They have predicted that the coupled motion of the normal and superfluid components can give rise to an intriguing structure that contains three vortex rings (Science 2000 290 777).
In the February issue of Physics World, Charles Adams of the Department of Physics, University of Durham, UK, investigates this surprising phenomenon.
