What's in the vapor space of my Liquid Air?

So, I’m sitting here screwing around with a can of compressed air - the kind you use to blow the fuzzies off your computer fans, and freeze your friend’s hair with. Just me?

According to the label this stuff is diflouroethane, and it’s obvious the bottom quarter or so of the can is filled with liquid. I understand pretty well the physics of this and the phase diagrams involved - but what about the headspace above? I suspect that it’s a saturated vaporspace of the same stuff; here are my questions:

Is the boundary between the liquid and gas phases discrete? Or is it fuzzed a little due to the pressure? For that matter, what IS the pressure inside one ofthese things? They seem much flimsier than the ~300 psi CO2 tanks I’m familiar with in paintball.

What gives the liquid it’s odd flow characteristics? When shaken it seems to almost ‘foam’ rather than slosh around as I’d expect.

I think there is liquid below a certain surface and gas above, both difluoroethane. I wonder if the feeling you are describing is because you are closer to the critical point than you are used to, for example with water in a bottle.

The critical point is the temperature and pressure above which liquids and gasses are no different, or above which they don’t exist (a poor definition because in many contexts the stuff is called gas above the critical point).

The issue is that in one extreme, where molecules are crowded together but too hot to form a solid, they loosely cling to one another primarily by attractive forces (which dominate in this situation) but can still slide around, and we call it a liquid; whereas in the other extreme they bounce off of one another due to elastic collisions and we call it a gas. However, above the critical point, these two states are no longer distinguished.

I think this is why propane in a tank feels like this; however, propane is also very light weight, low density, so maybe that is why.

I have seen a most beautiful and nifty demonstration device filled with a substance whose critical temperature is around body temperature. It is a little metal disk with windows in the faces, about the size of a bottle cap. When it is cool you can slosh the liquid around, but as you warm it in your hand the division between liquid and gas gets more and more indistinct and eventually turns into vague ripples before vanishing altogether.

I get lots of hits on “supercritical” on YouTube, and clearly many show substances around their critical points.

Look up vapor pressure, and that should explain it pretty well.

I’d guess that the diflouroethane has just about zilch surface tension, and therefore doesn’t act much like water would as far as sloshing goes. Neither does alcohol- have you ever shaken a liquor bottle?

Related to the odd “feel” of the liquid, I think it’s important to point out a couple of things.

First, water is kind of our yardstick for liquids – almost every liquid we deal with is composed largely of water.

However, water has a bunch of properties that are unique or very rare. The surface tension, heat capacity, density, and phase transition points of water (and several other properties) are very different than most liquids. The stuff inside an air duster is actually more of a normal liquid than water is – not dense, low surface tension, low heat capacity, and a smaller difference between phase changes at room temperature.

The feel of a liquid depends a lot on its kinematic viscosity, which is the ratio of viscosity to density. Water is fairly low. Mercury is far lower. If the difluoroethane has a high kinematic viscosity, and feels like the opposite of mercury, that may be a major reason.