Superfluids
Right, since some of you have asked about superfluids I'll do a short article on them. I'll try to keep the technical stuff to a minimum so as many people as possible can understand it, but if anyone wants a bit more depth then feel free to ask.
What is a Superfluid?
A superfluid is like a fluid but super! It's basically a fluid that has a viscosity of zero. This leads to some interesting effects, such as being able to leak out of a container, high thermal conductivity and being able to go along tiny tubes (superleaks) that normal fluids can't fit down.
How do you get a Superfluid?
To make a superfluid, you take some helium 4 (helium 3 works too but at lower temperatures) and you make it cold. This is usually done by reducing the pressure on a cryostat (big thermos flask) containing liquid helium 4. As we all know, reducing the pressure on a liquid lowers its boiling point and a liquid cannot exist above its boiling point since it turns to gas at that point. Therefore by lowering the boiling point you lower the temperature.
When the temperature hits 2.17K the bubbling caused by boiling ceases and you have a superfluid on your hands. What has happened is that all the atoms that make up the helium have dropped into their ground state. This causes the entire fluid to behave as if it were a single atom (a Bose Einstein Condensate). This is why a superfluid is sometimes known as a macroscipic (seeable) quantum phenomenon. That's all the technical stuff for this article so you can relax again!
Actually, at 2.17K there will be a mixture of normal fluid and superfluid coexisting in the cryostat. These are generally described as interpenetrating but non-interacting. That means that they both occupy the same container but are unaware of each others existence. As you get colder, more and more of the normal fluid becomes super, until you get pure superfluid at about 1K.
Some fun things to do with Superfluids
Zero viscosity is a fun thing to play with. If you put superfluid in a dish it will appear to leak out of the bottom of the dish. Clearly any normal fluid will not do this. What happens is that it sticks to the sides of the container and rises up them (just like a normal fluid would rise up a capillary tube). It gets to the top and falls down the outside, emptying the container pretty quickly!
The high thermal conductivity can be used to fun effect too. This is the reason the bubbling stops: bubbles are caused by local pockets of fluid rising above the boiling point of the fluid and turning into gas, this gas then rises through the fluid causing bubbles. In a superfluid local heating is impossible because heat is spread evenly throughout the whole fluid so quickly that bubbles don't have time to form.
The concentration of normal to super fluid is also strictly adhered to and is spread uniformly throughout the fluid. This also happens very quickly. You can create an interesting fountain by placing a heater just below a capillary tube that is pointing upwards. The heater converts superfluid to normal fluid, messing up the concentration locally. Superfluid then rushes over to restore the concentration and gets shot up the capillary tube. You can get heights of about a metre!
One useful application of this is as a pumping mechanism for helium in space, where it's naturally superfluid anyway, since it's so bloody cold up there!
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Originally Posted by Bootsiuv
). If you managed to swallow the superfluid (which would rapidly turn to gas as it came into contact with the heat of your body), the cold would cause your oesophagus to contract right up and burn it and you'd suffocate. If the liquid got into your stomach it would turn to gas. Helium gas is 700 times the volume of helium liquid, and your oesophagus would be sealed shut, so you'd explode (or at least have massive internal haemorraging (sp?)).
, but they don't let undergrads play with expensive equipment like that 
