superfluidity Helium is a unique element because it remains liquid at atmospheric pressure, even at temperature near the absolute zero (-273.15^o). The inter atomic force between two helium atoms is very small. The random thermal motions even at such low temperatures overcome this force. Below 2.2K, the viscosity of liquid He⁴ vanishes, as a result of which He⁴ can flow through very narrow slit without friction. This phenomenon is known as superfluidity. Kapitza discovered superfluidity in liquid helium.

At very low temperature the velocities of helium atoms are small. This makes the de Broglie wavelength (l =h/p) large compared to the atomic dimensions. The de Broglie wavelengths of adjoining atoms overlap making the liquid to acquire quantum mechanical properties.

Superfluidity has also been observed in He³, a fermion system, although at a much lower temperature.