Frictionless things

Superfluids are interesting things. They move without friction. This is a... strange concept, especially in the world we live in - everything, at some point, moves with friction; something falling from the sky experiences air resistance, anything moving on a surface experiences friction, all movement we engage in, more or less, involves the friction between us and the thing we've pushing against. But we also have the capability to imagine its' non-existance; we have experiences of weak friction, in the form of fluids spreading across surfaces, slipping and falling, sliding things along a frozen path. So we can at least understand somethings about a frictionless substance.

For instance, if you had big drop of frictionless liquid hit a flat surface, it would spread. But when would it stop? If it was water, it would form a puddle (imagine water on a flat, marble surface, so it doesn't drain away). Is that due to friction? At some point, the friction between the water and the surface becomes greater than whatever force is pulling it apart (the water seeking to go to it's lowest point - water flows downhill, in the direction of gravity)? If that's the case, in a frictionless liquid, would it ever form a puddle? Or would it continue to spread until it covered the entire surface in a very thin film. It's the later - it continues to spread, at least until it reaches such a thin film that the internal forces start to play a part.

But what if you put this same drop of frictionless liquid in a cup? It climbs up walls. This seems daft, but it's climbing the walls of the glass because there's nothing to stop it doing so. Sure, gravity acts on it, but imagine a skateboarder in a frictionless u-pipe. He goes up and down forever, to exactly the same height. Add another skateboarder. They collide, but they are also frictionless skateboarders. So the only place the energy can go is into each other - one gains more, the other gains less. So one skateboarder is going higher than the other. Add lots and lots of skateboarders, and eventually, you'll have skateboarders flying up and over the edge of the U-pipe, just because of an uneven spread of energy. They'll never really fly, if you assume that in each collision not much energy is changed (imagine a skateboarder at the top of our chaotic pile of skateboarders - another one hits him, but if they are bother quite high, then only a small amount of energy will switch between them).

It's a bit of a limited analogy, but it sort of fits. But if you put a frictionless liquid in a semi-spherical container, hold it above the ground, then the frictionless liquid will flow up the sides of the container, and then down the outside, to the very bottom of the container, and then drip off. Sort of like water on a window - if you have a drop on water and a ball on a horizontal glass surface, and rotate the surface so it's vertical, then the droplet and the ball slide down. But if you continue to rotate it, the ball would fall to the ground, but the droplet wouldn't fall off, at least straight away - it would slide down the surface of the glass, even though the most direct way of going down would be going straight down, as the ball does.

So these sorts of things are imaginable, if rather counter intuative. But they're also a lot of fun, and the properties of them can be explained through one of my favourite branches of physics, statistical mechanics.

So it's fun to think about superfluids, and, by extension, superconductors. And once "fun" is added, in whatever means, the whole process of learning becomes a pleasure.

Need to remember the fun.