So, let's demonstrate two simple scientific principles through this tube. That's right, two principles in one!
let's start with temperature. Temperature is an AVERAGE of how fast the particles are bumping into each other. Because it's an average, it means that air is composed of fast AND slow particles. It's hot and cold at the same time! (see right of third picture) How will this be proved? Well, I think that if we just separated the hot and cold, that'd do just fine for proving!
How's it going to work? By utilizing inertia! Which is defined as: "the property of an object to remain at constant velocity unless acted upon by an outside force." Basically, once you get a boulder rolling, it's not going to want to stop. And trust me, it'll resist you changing it's velocity!
There are two things that make up inertia- mass, and velocity. More of either means more inertia.
Now imagine, there's a wall that curves 90 degrees. A boulder and ping pong ball are rolling towards it. (see fourth picture for the explanation) now, as you saw, the pingpong ball was just pushed away with ease. Now, say the pingpong ball was the same size as the boulder (but still very much lighter) and on the right instead of the left. The boulder would still force its way to the wall. If you had a steady stream of boulders and giant pingpong balls, and this was a circle instead of just a curve, it wouldn't be long before there were only boulders rolling along the wall, and all the pingpong balls were all pushed to the inside.
Now, one step further, now because inertia = mass x velocity, say you had lots of molecules of the same weight going around that circle. But some were moving really fast, and others slow, because the faster ones are moving much faster (and have more inertia), they'll push the slower ones out of the way. (yes, just like the boulder and pingpong ball) and before you know it, there's the hot (fast) molecules around the edge, and slow (cold) in the center! (see left side of third picture) and that's exactly what the inside of the vortex tube looks like!
to put it very simply all the vortex tube does is get those molecules moving in one direction (rather then the chaotic right of the third picture) so that the separation will begin!
There ya go, you've learned two scientific principles, and the basic idea of how this machine works. If you want to know the history and a step by step process, go to step one! If not, and just want to build, just move to the step after that.