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Q & A: supercooling and nucleation

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Most recent answer: 06/28/2011
Q:
Why condensation is delayed in an air-mass , super-saturated with vapor, till there is addition of some condensation nuclei? Can't we induce condensation simply by adding more and more water vapor in an air-parcel, without any condensation nuclei? Similarly what prevents super-cooled water from freezing if there is absence of any freezing nuclei? Can't we induce freezing simply by continued lowering of temperature?
- Saurabh (age 25)
India
A:
Those are very nice questions. In fact, if you keep cooling pure water enough, it will freeze spontaneously, without any added nuclei.  Likewise if you get the water vapor super-saturated enough (say by cooling it), it will start to condense without any added nuclei.

That still leaves your basic question. Let's say that we've got vapor (or liquid) that's supercooled but not enough to condense (or freeze) without some added nucleus, at least in any reasonable period of time. Why does it act that way?

Let's look at the liquid. It should freeze when its free energy can be lowered by forming crystals. That means that the energy it can lose per molecule, U, exceeds TS, where T is the absolute temperature and S is the entropy lost when the molecule lines up into the crystal. So you can see why this tends to happen at low T.

Ok, so here's the point. When you add a molecule to a big crystal, you add energy-lowering contacts on all sides of the molecule. That makes a big U, which can exceed TS.  When you add a molecule to a small crystal, the surface is still very important, and there isn't so much good contact per molecule. U is smaller. If it's just below the freezing point, the free energy actually goes up as molecules are added, until a large enough frozen cluster has formed. Things don't form high free-energy clusters very often, since higher  free-energy means lower probability. (That's why there are fewer air molecules high up, where their energy is higher.) Just below the melting point it takes more or less forever before a big enough cluster is likely to form by accident.

A very similar argument describes the formation of liquid clusters out of a gas.

Follow up if this needs more explanation.

Mike W.

(published on 06/27/2011)

Follow-Up #1: why silver iodide nucleates ice

Q:
Thank you sir. This answer gives me clarity . As you explained here that the crystals should provide energy-lowering contact. So i guess that an ideal "freezing-nuclei" should have crystal structure very similar to that of the ice (am i right ?) so that max lowering of energy occurs and the change in gibbs fee-energy of ice-water system is negative despite a decrease in its entropy. So does Silver Iodide , a commonly used cloud seeding agent, have a crystal structure matching to that of ice? or else What makes Silver Iodide so effective cloud-seeding agent?
- Saurabh (age 25)
India
A:
Your guess about the match in the crystal structure sounds reasonable to me. I found a reputable article ()  which says that is indeed the mechanism. One thing to remember is that it's not necessary for the overall 3-D crystal structure of the nucleating material to match the overall 3-D crystal structure of the ice. What's needed is just that some surface structure of one fits well with some surface structure of the other.

Mike W.

(published on 06/28/2011)

Follow-up on this answer.