Supercriticality in the Palm of Your Hand

After watching a video on supercritical carbon dioxide from Applied Science I decided to make my own carbon dioxide pressure chamber. I used a three-piece design which is mechanically the simplest possible. With half an hour of handicraft, I was able to produce a working device for seeing phase transitions that most people never see in their lives.

When I first heard of supercritical fluids, I thought that they were some kind of unattainable sci-fi magic. But then I did some research and discovered that supercriticality may not be magic at all. It's just the densities and other properties of gases and liquids merging at higher pressures and temperatures. At least that's how I rationalised it. But when I built my pressure chamber and played around with it, I discovered that supercriticality really IS magic. Seeing liquid expand until it fills the whole volume available, witnessing a seemingly empty space magically fill with fog only to give way to two phases an instant later. Gas bubbling out from nothing. Words don't suffice.

But be aware of the dangers when you undertake this build. Read the "Safety" section and do your calculations. You are responsible for the damage that an exploding pressure chamber may cause.

I have to add that the pilot chamber I made didn't last very long. The metal is of course impervious to carbon dioxide but the plexiglass window gave out. It is your choice whether to improve on the design by using real glass or find some other way to make the plastic last longer.

Supplies

Materials needed:

A 1/2 brass plug

A 1/2 to 15 mm compression coupler - only the nut is necessary

A piece of at least 5 mm (1/5 inch) thick plexiglass

A piece of dry ice

Tools needed:

Sandpaper for polishing the sealing surface of the plug

A hacksaw and a rasp for processing the window material

Wrenches to put it together

Step 1: Safety

This project entails high pressures from 40 up to 90 bar. Make sure your construction can withstand the forces created by this pressure. By my calculations the force acting upon the viewing port of my design was equivalent to about 40 kg. If inadequate materials of wrong thickness are used, the pressure may cause the window to fracture and shards may fly into your eyes.

Since the critical temperature of carbon dioxide is quite mundane (around 30 degrees centigrade, 300 K), operation of the chamber probably won't cause a thermal hazard. The only dangerous temperature is that of the dry ice used to fill the chamber (about -78 degrees centigrade, 195 K).

In some countries this device may be classified as a "dry ice bomb" and thus be illegal. Be aware of the appropriate legislation.

Step 2: Design

The part of the plug that will form the seal with the acrylic glass will have to be polished to make a good seal. Remove larger dents with a coarse sandpaper and finish it with a finer one. Be careful not to make the surface slanted. It needs to be even in order to contact well.

When buying the plug, make sure that the threaded part is long enough so that the sealing surface reaches the plastic.

For the window, cut out a square of plexiglass and file it into a circle that fits into the nut. If you have the same type of nut as me, you'll have to make a chamfer as well. This will make the piece fit nice and snug. The plastic is easy to work so this step shouldn't be too hard. If you like challenges, try improvising a lathe from, say, a power drill (use your imagination). By turning you can make a perfect circle. I didn't bother with anything this complex but it came out just as well.

Initially I used an o-ring seal but since supercritical carbon dioxide seeps into materials quite well, it turned out that the seal caused leakage. Without the seal, things were better. Plexiglass will give a little under the pressure from the brass plug and a nice seal will form on its own. Actually, the seal between the plastic and metal will probably only get better when you tighten and untighten it, the system kind of "wears in".

Put a piece of dry ice into the plug and screw the thing together. Fast.

And that's it.

Step 3: Operation

There are two controllable parameters in this system: the amount of dry ice and the temperature. Pressure inside the chamber will be dependent on both of the aforementioned parameters. With increasing temperature the pressure will rise, corresponding to a up-and-right movement on the phase diagram. The changes in temperature and pressure will cause phase changes. Study the phase diagram of carbon dioxide and draw arrows of the transitions that you see when you play with the chamber.

The first phase transition will be dry ice melting. Which is kind of an oxymoron since the ice won't be so dry anymore. Or is the liquid that it produces dry? In any case, this phase transition is interesting, since most of us will only ever witness the sublimation of dry ice, which is of course also a very interesting phenomenon in itself. The melting of solid carbon dioxide refers to a transition above the triple point, whereas sublimation happens below the triple point.

Now the dance around the critical point is more complex and I'll better not dive into this because it is not quite clear for me yet. While playing with this device, I saw things I hadn't expected to see. Maybe it's been described somewhere else.

The phase diagram is from Wikipedia.

Step 4: Further Improvements on the Design

Since carbon dioxide in either the liquid or supercritical state is a solvent, it will interact with the plastic that the window is made of. Glass is a better window material but with glass you need a seal. This can be made of teflon or maybe copper. I'm not actually sure, I haven't experimented with different seals yet.

Glass can produce shards upon rupture and these shards should be kept from flying around. The solution to this is a sandwich design. Maybe a protective film like the one used on smartphone screens will work. I'd go with acrylic as a top layer.

If substances of higher critical temperature are experimented with, no plastic can be used. Also, the different thermal expansion coefficients of different materials have to be taken into account.

Producing a glass of suitable shape is probably described in another instructable.

Thanks for reading and be safe.