Unveiling the properties of helium and liquid helium: The production of liquid helium
Liquid helium Helium gas
To obtain liquid helium, one must first compress and cool helium gas to the temperature of liquid hydrogen, and then allow it to expand, causing the temperature to further decrease, so that the helium gas can turn into a liquid.
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Liquid helium is a transparent and easily flowing liquid, resembling a soda bottle with the cork removed, constantly splashing small bubbles. Liquid helium is a unique liquid that boils at -269°C. At such low temperatures, hydrogen also turns into a solid. It is crucial not to allow liquid helium to come into contact with air, as the air will immediately freeze on the surface of the liquid helium, forming a hard layer. Unlike ordinary liquids, whose viscosity increases as the temperature drops, the viscosity of HeI is almost independent of temperature when it drops to around 2.6K, with a value of approximately 3×10-6 Pascal-second, much lower than that of ordinary liquids. Below 2.6K, the viscosity of HeI decreases rapidly as the temperature drops. The viscosity of HeII drops to a very small value (less than 10-12 Pascal-second) at temperatures below the λ point. This property of almost no viscosity is called superfluidity. When experiments are conducted using capillaries of different thicknesses, it is found that the thinner the flow tube, the more pronounced the superfluidity. In flow tubes with a diameter less than 10-5 centimeters, the flow rate is almost independent of the pressure difference and the length of the flow tube, and depends solely on temperature, with no loss of kinetic energy during flow. Any wall of the apparatus that comes into contact with HeII is covered with a liquid film, which contains only the superfluid component with no viscosity, known as a helium film. The presence of a helium film allows liquid helium to move along the wall of the apparatus to the lowest possible position. When an empty beaker is partially immersed in HeII, the liquid helium outside the beaker will climb up the outer wall of the beaker and enter the beaker until the liquid levels inside and outside the beaker are equal. Conversely, when a beaker containing liquid helium is lifted out of the liquid helium surface, the liquid helium inside the beaker will continuously transfer outside the beaker and drip down along the wall. The rate of this transfer of liquid helium is independent of the height difference of the liquid surface, the length of the path, and the height of the barrier.
The theoretical study of the properties of He II was first conducted by F. London. 4He atoms are bosons with integer spin, and London regarded He II as a Bose gas composed of bosons, obeying Bose-Einstein statistics, which allows different particles to be in the same quantum state. London proved the existence of a critical temperature Tc. When the temperature is below Tc, some particles will simultaneously be in the zero-point vibrational energy state (i.e., the ground state), known as condensation. The lower the temperature, the greater the number of particles condensing to the zero-point vibrational energy state. At absolute zero, all particles condense to the zero-point vibrational energy state, a phenomenon known as Bose-Einstein condensation. L. Tisa believed that the superfluidity of He II originated from Bose-Einstein condensation. Since He II atoms that have condensed to the ground state have the lowest zero-point vibrational energy, they have a very large mean free path and can pass through extremely thin capillaries almost unimpeded. Tisa first proposed a two-fluid model, which was later revised and supplemented by L. D. Landau. The two-fluid model posits that He II consists of two separate, mutually permeable fluids: one is the condensed part in the ground state, with entropy equal to zero and no viscosity, which is a superfluid; the other is the normal fluid in an excited state (not condensed), with entropy not equal to zero and viscosity. The sum of the densities of the two fluids equals the total density of He II. When the temperature drops to the λ point, the normal fluid begins to partially transform into a superfluid. The lower the temperature, the greater the density of the superfluid, and the smaller the density of the normal fluid. At absolute zero, all atoms are in a condensed state, and the entire fluid is a superfluid. This two-fluid model can explain many mechanical and thermal properties of liquid helium.
