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Superfluidity of helium refers to the phenomenon where helium, when cooled below a critical temperature, enters a phase with unique properties, flowing without viscosity or resistance. This occurs in liquid helium-4 (at 2.17 K) and helium-3 (at much lower temperatures, around 0.0025 K).

Key Properties of Superfluid Helium:

1. Zero Viscosity: It flows without any resistance, allowing it to pass through tiny pores or cracks that normal liquids cannot.

2. Climbing Walls (Film Flow): Superfluid helium forms a thin film and "creeps" up and over container walls due to its lack of viscosity and quantum effects.

3. Vortex Quantization: When rotating, superfluid helium forms quantized vortices instead of behaving like a normal spinning liquid.

4. Second Sound: It can carry heat via waves (like sound waves), where temperature oscillations propagate through the liquid.

5. Macroscopic Quantum Phenomenon: Superfluidity arises from quantum mechanics, with helium atoms behaving as a single quantum entity (Bose-Einstein condensation for helium-4, Cooper pairing for helium-3).

Applications:

Used in cryogenics and cooling superconductors.

Fundamental research in quantum mechanics and low-temperature physics.

Superfluidity showcases the interplay between quantum mechanics and macroscopic systems, making helium a unique substance for studying quantum phenomena.