One feature of the teaching at Dalhousie University’s Physics Department is a laudable emphasis on demonstrations.
Visiting Professor Tom Duck there, I was delighted to be shown a demonstration I had heard of, but never seen: the phenomenon of critical opalescence in carbon dioxide.
In my previous articles I described the phenomenon in two immiscible liquids which is an exact analogy for the physics of critical opalescence in a pure substance. But it’s not what physics students read about in text books.
Michael: What are you going on about?
The phenomenon occurs when one heats a liquid in a container with a small amount of free space.
- As the liquid heats up, it expands causing its density to fall.
- The liquid also evaporates causing the vapour (gas) pressure to increase.
- The critical point is where the density of the liquid matches that of the vapour.
Above this ‘critical’ temperature and pressure, the substance forms a single fluid with no distinct liquid state. In the movie you can see that the meniscus at the top of the liquid just gradually disappears – there is now no ‘surface tension’.
At the critical point, the density of the fluid is typically one third of the liquid density at atmospheric pressure. Because there is no difference between liquid and gas, the latent heat associated with evaporation (when molecules move from the liquid to the gas) and condensation (when molecules move from the gas to the liquid) falls to zero.
Critical Opalescence can be seen when cooling just below the critical temperature.
The random motion of the molecules in the fluid causes some regions to transiently have densities that are slightly greater than the average (more typical of the liquid) – and others to have densities more typical of the gas.
Because the latent heat and the surface tension are very close to zero, these microscopic fluctuations can grow dramatically. Spontaneous fluctuations can cause regions as large as a thousandth of a millimetre – containing thousands of billions of molecules -to fluctuate into and out of the liquid state – forming droplets.
Although the difference in density (and hence refractive index) between the liquid droplets and the gas is tiny – it is just enough to scatter light – like a fog – a phenomenon which someone poetically named ‘opalescence’ rather than fog.
Critical Opalescence is mentioned in Physics course, but it is rarely seen. The high pressures involved (more than 73 atmospheres in this case) present a hazard that few people are prepared to tackle. I suspect that students at Dalhousie may not appreciate how lucky they are!
P.S. It turns out that carbon dioxide above its critical point is an excellent non-toxic solvent for caffeine and so when you sip your de-caff latte tomorrow – you can now imagine the physics that describes the fluid which took away the caffeine.