I'm planning to build a small cloud chamber.

The design is the usual: alcohol evaporates at the top by gently heating it with a thin wire and small current. The vapour sinks to the bottom of the chamber, where the temperature is significantly lower than at the top, and causes the vapour to super-cool. Ion tracks caused by fast moving particles can be observed as mist tracks.

My question is about the bottom plate temperature.

The usual design uses a (near) room temperature solvent and very cold bottom plate to achieve a big temperature gradient.

Could a cloud chamber be constructed that would use just a slightly cold bottom plate, and a more aggressively heated liquid? I'm thinking about ethanol or isopropyl alcohol evaporated at 60°C, and a bottom plate temperature at maybe only 0°C

Could this work? Is it just the temperature gradient, or the temperature itself is important?

  • $\begingroup$ I expect that for supercooling it is the temperature itself, not the gradient.en.wikipedia.org/wiki/Supercooling $\endgroup$ – anna v May 22 '14 at 10:40
  • $\begingroup$ That's not what's going on in a chamber. The alcohol is always below it's boiling point and always above it's freezing point in both "traditional" dry-ice or peltier cooled chambers and the "warm chamber" described in the question. The chamber works by cooling the air with alcohol vapour until it is too much alcohol vapour in it. My question is that is the low temperature is a requirement for the tracks to form and be seen, or just the gradient is enough. If it's not enough then why. $\endgroup$ – netom May 22 '14 at 10:49
  • $\begingroup$ isn't it a phase transition that is not happening that the passage of the ionizing particle seeds? The phase diagrams are with temperature not gradients $\endgroup$ – anna v May 22 '14 at 10:55
  • $\begingroup$ Yes, but you can get alcohol vapour into the air below the boiling point, and also can get alcohol mist. The phase transition happens not because the temperature goes from above boiling point down below that, but because the air gets supersaturated. I actually did an experiment with this, and you can get alcohol mist by evaporating alcohol at about 20-30°C and then cooling the air / vapour mixture to something like 0°C. (No tracks though, it was just a quick experiment) $\endgroup$ – netom May 22 '14 at 15:04

This is a generic phase diagram :

phase diagram

I looked up the construction of a demonstration for classes cloud chamber using alcohol. For the air/alcohol gas line 3, constant pressure, change in temperature is where you want to work for your cloud chamber. Cooling it without condensation. The gradient of temperature in your chamber will define the thickness over the iced bottom where supersaturation can be maintained. So your choice of alcohol should depend on how slowly the phase diagram changes with temperature as to have a larger distance from the cold plate where the vapors will be supersaturated and tracks can form.

I do not think it is the gradient in the chamber that is decisive, but the one from the triple point to the vapor phase in line 3.The hot on top is to generate the vapour phase in the chamber. It will all depend on the phase diagram of your specific choice for vapor.

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  • $\begingroup$ Thank you very much, I have to contemplate on this for a moment :) $\endgroup$ – netom May 24 '14 at 7:07
  • $\begingroup$ Ok, I still have some questions: 1: "The gradient of temperature in your chamber will define the thickness .. where supersaturation can be maintained". Why is it so? 2: Do you suggest that I need an alcohol with a slowly changing phase diagram in order to create this large gradient (to have more "temperature space" to work with)? $\endgroup$ – netom Jun 5 '14 at 5:57
  • $\begingroup$ Yes, more or less. How far the cold from the floor can penetrate into the chamber depends on the composition of the gas means actually how slowly the heat from the chamber is transferred to the cold floor. If the heat flow is slow enough then a larger part of the bottom will stay cool enough to be supersaturated. $\endgroup$ – anna v Jun 5 '14 at 8:45
  • $\begingroup$ So cool air-alcohol mixture is easier to supersaturate? This is the point of the low temperature? If it is, then why? $\endgroup$ – netom Jun 6 '14 at 12:02
  • $\begingroup$ Yes, you get alcohol fumes on the top, which do not condense on the cool bottom until there is a seed to condense around. Like the tracks of jets in the sky $\endgroup$ – anna v Jun 6 '14 at 13:29

Yes, the original cloud chamber user water to form the supersaturated vapour, and ethylene glycol has also been used. Alcohol just happens to be very convenient as it's easy to get a lot of vapour and it doesn't freeze at the the chamber operating temperatures.

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  • $\begingroup$ And of course bubble-chambers run in super-heated mode (achieved by dropping the pressure rather than changing the temperature, but still) with a variety of working fluids. $\endgroup$ – dmckee --- ex-moderator kitten May 22 '14 at 14:49

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