In a superfluid gas, the atoms can flow without resistance. One of the clearest ways to distinguish such a superfluid from a normal gas is to rotate it. A sample of normal gas rotates like an ordinary object. But a sample of superfluid can rotate only when it forms vortices similar to mini-tornadoes. This gives a rotating superfluid the appearance of Swiss cheese, where the holes are the cores of the mini-tornadoes.
This image shows the vortices observed in three rotating superfluids: samples of sodium atoms (green cartoon), tightly bound lithium molecules (red-blue cartoon) and loosely bound pairs of lithium atoms. The larger-scale vortex in the background is the eye of hurricane Isabel in the summer of 2003.
Credit: Andre Schirotzek, Massachusetts Institute of Technology