Superfluid helium as a vacuum

What is the minimum number of atoms needed for superfluidity? According to the results of a remarkable molecular spectroscopy experiment, there is definite proof that a system of only 60 atoms can be superfluid. Slava Grebencv and colleagues at the Max Planck Institute for Fluid Dynamics in Göttingen, Germany, found that small molecules dissolved in droplets of liquid helium can rotate freely, just like they do in a vacuum (S Grebencv, J P Tocnnies and A Vilesov 1998 Science 279 2083). The molecules rotate freely because the surrounding helium is a "superfluid" – a liquid without viscosity. In effect, the liquid helium acts as a vacuum.