This month's Challenge Question: Specs & Techs from IHS Engineering360:
Entering a laboratory, you notice a motionless clear
cylinder resting on a table that is filled with a liquid circulating around
inside. After hours of observation, you are puzzled to find that the liquid's
motion has not changed. Knowing that no outside force or energy has entered the
cylinder since you began observing, how is this possible?
And the answer is:
The liquid in the cylinder is superfluid helium-4.
Helium becomes a liquid when cooled to a temperature below
its boiling point of 4.22 K (-268.93° C).
Taken down a couple more degrees past its "lambda point" of 2.17 K
(-270.98° C), liquid helium starts to exhibit unusual properties. It is at this
point that a fraction of the helium has transitioned into a "superfluid." In
this curious state of matter, the helium behaves like a fluid with zero
viscosity. With no friction to slow its motion, the superfluid helium flows past
the surface of the cylinder, continuing to circulate endlessly (or until it
warms up and transitions back into its more regular liquid or gaseous states).
Unlike most liquids, helium doesn't turn into a solid when
cooled down. Its atoms are light and weakly attracted to each other, so even
when cooled to temperatures at which regular heat vibrations are absent, helium
doesn't settle into a solid. At low temperatures its atoms wiggle with
zero-point motion, a slight momentum bestowed by the quantum uncertainty
principle. Instead of settling in a solid state, liquid helium undergoes a
transformation known as Bose-Einstein condensation. Its atoms start acting in
harmony, behaving like one big particle, no longer colliding together. It is
these quantum effects that grant superfluids their remarkable properties.
For more on superfluid helium, including a detailed
description of how superfluid helium is actually a mixture, with normal and
superfluid components, see The physics of superfluid helium[pdf].
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