Viscosity helium, liquid

The effect of temperature on the viscosity of liquid helium is peculiar. The... 

It is known, that viscosity of liquid is in a direct dependence upon the diffusion coefficient. If the molecules of helium 2 were capable to diffuse (and, therefore, to evaporate), the viscosity of helium 2 could not be equal to zero it means that superfluidity could not exist. 

Helium II is often referred to as a superfluid. Immediately below the lambda point, the flow of liquid through narrow slits or channels becomes very rapid. Figure 2.4 shows the viscosity of liquid helium when measured by an oscillating disk. Helium I has a viscosity of about 3 x 10 Pa s, whereas this experiment indicates that Hell has a viscosity of about 10 Pas at... 

Seven isotopes of helium are known Liquid helium (He4) exists in two forms He41 and He411, with a sharp transition point at 2.174K. He41 (above this temperature) is a normal liquid, but He411 (below it) is unlike any other known substance. It expands on cooling its conductivity for heat is enormous and neither its heat conduction nor viscosity obeys normal rules. 

Liquid helium-4 can exist in two different liquid phases liquid helium I, the normal liquid, and liquid helium II, the superfluid, since under certain conditions the latter fluid ac4s as if it had no viscosity. The phase transition between the two hquid phases is identified as the lambda line and where this transition intersects the vapor-pressure curve is designated as the lambda point. Thus, there is no triple point for this fluia as for other fluids. In fact, sohd helium can only exist under a pressure of 2.5 MPa or more. 

Helium is an interesting example of the application of the Third Law. At low temperatures, normal liquid helium converts to a superfluid with zero viscosity. This superfluid persists to 0 Kelvin without solidifying. Figure 4.12 shows how the entropy of He changes with temperature. The conversion from normal to superfluid occurs at what is known as the A transition temperature. Figure 4.12 indicates that at 0 Kelvin, superfluid He with zero viscosity has zero entropy, a condition that is hard to imagine.v... 

The 1996 Nobel Prize in physics went to three researchers who studied liquid helium at a temperature of 0.002 K, discovering superfluid helium. A superfluid behaves completely unlike conventional liquids. Liquids normally are viscous because their molecules interact with one another to reduce fluid motion. Superfluid helium has zero viscosity, because all of its atoms move together like a single superatom. This collective behavior also causes superfluid liquid helium to conduct heat perfectly, so heating a sample at one particular spot results in an immediate and equal increase in temperature throughout the entire volume. A superfluid also flows extremely easily, so it can form a fountain, shown in the photo, in apparent defiance of gravity. 

The density of He I at the boiling point at 1 atm is 125 kg m 3 and the viscosity is 3 x 10 6 Pa s. As we would anticipate, cooling increases the viscosity until He II is formed. Cooling this form reduces the viscosity so that close to 0 K a liquid with zero viscosity is produced. The vibrational motion of the helium atoms is about the same or a little larger than the mean interatomic spacing and the flow properties cannot be considered in classical terms. Only a quantum mechanical description is satisfactory. We can consider this condition to give the limit of De-+ 0 because we have difficulty in defining a relaxation when we have the positional uncertainty for the structural components. 

Homogeneous Liquids. The physical properties important in determining the suitability of a liquid for propellant application are the freezing point, vapor pressure, density, and viscosity. To a lesser extent, other physical properties are important such as the critical temperature and pressure, thermal conductivity, ability to dissolve nitrogen or helium (since gas pressurization is frequently used to expel propellants) and electrical conductivity. Also required are certain thermodynamic properties such as the heat of formation and the heat capacity of the material. The heat of formation is required for performing theoretical calculations on the candidate, and the heat capacity is desired for calculations related to regenerative cooling needs. 

Superfluid. Liquid helium (more precisely the 2He4 isotope) has a "lambda point" transition temperature of 2.17 K, below which it becomes a superfluid ("Helium-II"). This superfluid, or "quantum liquid," stays liquid down to 0 K, has zero viscosity, and has transport properties that are dominated by quantized vortices thus 2He4 never freezes at lbar. Above 25.2 bar the superfluid state ceases, and 2He4 can then freeze at 1K. The other natural helium isotope, 2He3, boils at 3.19 K and becomes a superfluid only below 0.002491 K. 

As helium gas is cooled below -268.95°C, it forms a liquid. At -270.97°C, helium still looks like a liquid, but a liquid with unusual properties. Suddenly, liquid density drops and this "liquid" gains the ability to move through very small holes that helium gas cannot pass through. It flows up the walls of its container defying gravity, and has zero viscosity. Below -270.97°C, helium becomes a superfluid, the only one discovered so far. Helium never changes to a solid. —... 

The high thermal conductivity of liquid helium is related to the abnormally small viscosity. According to Allen, Peierls, and Uddin it has a small. value at the A-point (2 19°K.), is a maximum at about 2 0°K., and decreases again at lower temperatures. 

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