superfluidity, tendency of liquid helium below a temperature of 2.19°K; to flow freely, even upward, with little apparent friction. Helium becomes a liquid when it is cooled to 4.2°K;. Special methods are needed to cool a substance below this temperature, which is very near absolute zero (see Kelvin temperature scale Kelvin temperature scale, a temperature scale having an absolute zero below which temperatures do not exist. Absolute zero , or 0°K;, is the temperature at which molecular energy is a minimum, and it corresponds to a temperature of −273.
..... Click the link for more information. ; low-temperature physics low-temperature physics, science concerned with the production and maintenance of temperatures much below normal, down to almost absolute zero, and with various phenomena that occur only at such temperatures.
..... Click the link for more information. ). When the temperature reaches 2.19°K;, the properties of liquid helium change abruptly, so much so that ordinary helium is known as helium I and helium below this temperature is known as helium II. The transition temperature between helium I and helium II is known as the lambda point because a graph of certain properties of helium takes a sharp turn at this temperature and resembles the Greek letter lambda (&Lgr;). Liquid helium II flows easily through capillary tubes that resist the flow of ordinary fluids (see capillarity capillarity or capillary action, phenomenon in which the surface of a liquid is observed to be elevated or depressed where it comes into contact with a solid.
..... Click the link for more information. ) and a Dewar flask Dewar flask [for Sir James Dewar ], container after which the common thermos bottle is patterned. It consists of two flasks, one placed inside the other, with a vacuum between. The vacuum prevents the conduction of heat from one flask to the other.
..... Click the link for more information. filled with helium II from a larger container will empty itself back into the original container because the liquid helium flows spontaneously in an invisible film over the surface of the flask. The behavior of helium II can be partially understood in terms of certain quantum effects (see quantum theory quantum theory, modern physical theory concerned with the emission and absorption of energy by matter and with the motion of material particles; the quantum theory and the theory of relativity together form the theoretical basis of modern physics.
..... Click the link for more information. ). Helium stays a liquid down to absolute zero because its zero-point energy is such that it cannot become a solid without giving up an amount of energy that is less than that allowed by the quantum theory. Similarly, quantum restrictions keep helium II from behaving like a normal fluid because the energy interactions associated with friction and viscosity in normal fluid flow involve amounts not possible for helium II.

superfluidity

Unusual property of liquid helium cooled below −455.75 °F (−270.97 °C). At such low temperatures, helium exhibits an enormous rise in heat conductivity and rapid flow through capillaries or over the rim of its container. To explain such behaviour, the substance is described in terms of a “two-fluid” mixture model consisting of normal helium and superfluid helium. In normal helium the atoms are in excited states (see excitation), whereas in superfluid helium they are in their ground state. As the temperature is lowered below −455.75 °F, more of the helium becomes superfluid. It is assumed that the superfluid component can move through the container without friction, thereby explaining the unusual behaviour.

superfluidity [¦sü·pər·flü′id·əd·ē]

(cryogenics)

The frictionless flow of liquid helium at temperatures very close to absolute zero through holes as small as 10^-7centimeter in diameter, and for particle velocities below a few centimeters per second.

Superfluidity

The frictionless flow of liquid helium at low temperature; also, the flow of electric current without resistance in certain solids at low temperature (superconductivity).

Both helium isotopes have a superfluid transition, but the detailed properties of their superfluid states differ considerably because they obey different statistics. ⁴He, with an intrinsic spin of 0, is subject to Bose-Einstein statistics, and ³He, with a spin of ½, to Fermi-Dirac statistics. There are two distinct superfluid states in ³He called A and B.

The term “superfluidity” usually implies He II or the A and B phases of ³He, but the basic similarity between these and the “fluid” consisting of pairs of electrons in superconductors is sufficiently strong to designate the latter as a charged superfluid. Besides flow without resistance, superfluid helium and superconducting electrons display quantized circulating flow patterns in the form of microscopic vortices. See Bose-Einstein statistics, Liquid helium, Quantized vortices, Second sound, Superconductivity