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Superfluids, such as helium-4 below the lambda point, exhibit many unusual properties.
Second sound is also observed in superfluid helium-3 below its lambda point 2.5 mK.
The specific heat capacity tends towards infinity as the temperature approaches the lambda point.
Below the lambda point, however, helium does not boil, and it expands as the temperature is lowered further.
Helium-4 is a special case that presents a triple point involving two different fluid phases (see lambda point).
This model states that there will be two components in liquid helium below its lambda point (the temperature where superfluid forms).
The heat removed via evaporation lowers the temperature of the cooling coil closer to the lambda point.
Liquid helium below its lambda point begins to exhibit very unusual characteristics, in a state called helium II.
The helium vessel is often pumped to a pressure below helium's superfluid lambda point to take advantage of the superfluid's thermal properties.
Second sound is observed in liquid helium at temperatures below the lambda point, 2.1768 K, where He becomes a superfluid known as helium II.
The speed of second sound is close to zero near the lambda point, increasing to approximately 20 m/s around 1.8 K, about ten times slower than normal sound waves.
Liquid Helium II,Superfluid:demonstrations of Lambda point transition/viscosity paradox /two fluid model/fountain effect/creeping film/ second sound.
The Lambda point is the temperature below which normal fluid helium (helium I) transitions to superfluid helium II (approximately 2.17 K at 1 atmosphere).
Liquid helium-4 was discovered by Pyotr Kapitza, John F. Allen, and Don Misener to exhibit this property when it is cooled below a characteristic transition temperature called the lambda point.
In this model, liquid helium below the lambda point is viewed as containing a proportion of helium atoms in a ground state, which are superfluid and flow with exactly zero viscosity, and a proportion of helium atoms in an excited state, which behave more like an ordinary fluid.
Below its boiling point of 4.22 kelvins and above the lambda point of 2.1768 kelvins, the isotope helium-4 exists in a normal colorless liquid state, called helium I. Like other cryogenic liquids, helium I boils when it is heated and contracts when its temperature is lowered.