Quantum Rotation Observed in Helium Nanodroplets
Take a material to an extreme, and you can expect to see some weird things happen. One example is superconductivity, and another is superfluidity, in which a liquid will flow with zero viscosity. Helium, when cooled enough, can become a superfluid, and researchers at Berkeley Lab and the SLAC National Accelerator Laboratory have recently answered a question about how nanodroplets of helium behave.
Superfluidity is the result of the atoms in the liquid coupling together to act as though they are a single particle. This removal of viscosity can lead to some interest behaviors, such as the liquid climbing up walls. For superfluid helium, we have known for decades that rotating it will cause quantum vortices to form, regularly spaced throughout the liquid. Whether this behavior also occurs in an isolated nanoscale droplet has been an open question for some time now, and if they are present, that would mean the entire droplet acts like a single quantum object, instead of a mixture of particles.
To create the nanodroplets, helium is passed through a nozzle that has been cooled to below 10 K, and shot into a vacuum chamber at almost 200 m/s. To image the droplets before they had a chance to vanish, the researchers had to use SLAC's Linac Coherent Light Source to create very short pulses of high energy X-rays. Xenon atoms were also added to the helium, to make the vortices visible, and indeed they were. In fact the vortices behaved like those in larger samples of superfluid helium, though the nanodroplets were also clocked spinning 100,000 times faster than any previous rotating superfluid helium sample. Despite that high speed though, the droplets did not deform, like normal liquid drops would. Potentially, studying these drops could lead to a better understanding of superfluidity, thanks to how isolated the property is.
Source: Berkeley Lab