He co-discovered superfluidity in helium-3 with Robert C. Richardson and Douglas Osheroff. It is a characteristic property of a fluid with zero viscosity which therefore flows without loss of kinetic energy. When stirred, a superfluid forms cellular vortices that continue to rotate indefinitely. It occurs in two isotopes of helium (helium-3 and helium-4), when they are liquified by cooling to cryogenic temperatures.