I've seen (and done) supercritical CO2 before, but I'd never seen supercritical propane attempted. Propane's critical point is 97 degrees C (206 F) and 42 atmospheres pressure (616 psi). It's a little hotter and a little lower pressure than CO2's critical point, but the temperature is tantalizingly close to the boiling point of water, meaning it's a piece of cake to attain - just drop a sample in a vat of boiling water! The 42 atmospheres pressure is a little trickier - luckily I had sitting around a 1/2" 2000 psi rated ball valve (from ebay) and - even better - a 1750 psi rated sight glass! It's no fun if you can't see it!
I transferred some propane into this contraption by connecting it to a small propane torch tank with an adapter that had 1/8" NPT threads. I opened the propane tank to the contraption and then disconnected and depressurized it to purge a good bit of the air out. I reconnected the tank and then dunked the sight glass end in a cup of ice water, which reduced the pressure on that end and caused propane to flow from the cylinder into the valve/sight glass combo. I let it flow in until the chamber between the ball and the sight glass was about half full of liquid propane. Closing the ball valve trapped the liquid propane on the sight glass side of the valve, and I disconnected the propane tank.
I filled a one liter flask with water and wired up a harness to suspend the ball valve in the water. Heating the water subsequently heated the propane in the valve. Once it hit about 190 F, the propane in the chamber began to boil. As the temperature rose, the liquid fraction appeared to decrease, and finally, about 207 F, it disappeared completely - supercritical! At that point, the vapor and liquid phases decided to settle their differences like Rocky and Apollo, and they should have the same surface tension (none), density, should be miscible with other fluids, et cetera. They become the best of friends, alike in every way. But then Ivan Drago killed... ok this analogy is getting out of hand.
Here it goes! I apologize for the nerdy commentary - actually no, I take that back because this is awesome.
So what's really going on in here? As the pressure and temperature get closer and closer to the critical point, the differences in all properties between the gas and liquid phases start to get smaller and smaller. The density of the liquid phase decreases, while the density of the gas increases. The viscosities likewise approach each other. Then, suddenly, the difference between the two vanishes abruptly. Through the transition, it drags a couple of properties of each. For instance, the surface tension, which is basically what makes a liquid a liquid, disappears, making the fluid gas-like in that sense. The fluid has no tendency to stick together anymore, and is completely miscible (mix-able) in any proportion with any other gas or supercritical fluid. Some liquid-like properties are dragged through too - a supercritical fluid can have strong solvating power, dissolving other substances. These weird properties lead to many of the ways supercritical fluids are used, like in dry cleaning, for their strong solvating power - making them good at getting rid of oily stains on clothes, mixed with their lack of surface tension - making them able to get into all nooks and crannies to dissolve tough-to-get-at stains. Best of both worlds - ain't it great when you can have it all? Sometimes Mother Nature throws us a bone - and looks damn fine doing it in that squeaky clean getup.
Edit: Here's a slightly better video of the transition