Superfluid helium-4 is the
superfluid form of
helium-4, an
isotope of the element
helium. A superfluid is a
state of matter in which the matter behaves like a
fluid with zero
viscosity. The substance, which looks like a normal liquid, flows without friction past any surface, which allows it to continue to circulate over obstructions and through pores in containers which hold it, subject only to its own
inertia.