I once conducted a set of experiments in which I had to form ice by direct vapor deposition (not condensation of liquid water followed by freezing, aka riming) onto a Peltier-cooled glass-coated copper rod.

The experiments were conducted outside at night within a small wind tunnel, with ambient air temperatures ranging from -2 oC to -25 oC and the glass substrate was cooled from 1 oC to 10 oC below the ambient air temperature. Occasionally supercooled water droplets would condense, followed by freezing (forming rime ice) rather than direct deposition of ice crystals from the vapor phase. These events would ruin my time-consuming experiments.

The only relevant parameter under my control was the temperature of the glass-coated copper rod. The only relevant parameters I could measure during the experiment were the air temperature and humidity. I also introduced trace-levels (single to hundreds of ppb) of several C1 to C4 alcohols in order to later measure their incorporation into ice during ice deposition from the vapor phase. However, the low levels of these compounds did not affect whether riming or direct vapor-ice deposition would occur, as based on a series of 'blank' experiments without the trace gases.

If I were having trouble with liquid water condensation, in what way would I want to alter the glass substrate temperature so that only the direct deposition of solid ice could occur?

  • $\begingroup$ This is hardly a chemstry question. Otherwise, the heat of condensation can easily heat your glass rod locally by a few degrees. Heat conductivity of glass is lousy, and how would you cool a glass rod effectively and uniformly, especially to a different temperautre than the surrounding air? The way you describe it, this experiment cannot work reliably and reproducibly. $\endgroup$ – Karl Jan 11 '17 at 5:31
  • $\begingroup$ As I stated @Karl, I control the temperature of the glass rod. Obviously it took engineering and proper calibration but it was very doable. And very publishable. And if phase transitions are not relevant to chemistry, then I guess I don't know what field I'm in. $\endgroup$ – airhuff Jan 11 '17 at 5:42
  • $\begingroup$ Meteorology is physics. And sorry, from your description it sounds like a misguided experiment. Perhaps you can describe it better? Is your "glass rod" actually a pipe with flowing coolant? What temperatures did you measure? What "bulb"? How did you know the actual temperatue of the glass rod, if you didn't measure it? $\endgroup$ – Karl Jan 11 '17 at 5:53
  • $\begingroup$ You've asked at least five questions about my experimental conditions since stating in one post then reaffirming in another that you do not believe my question is related to chemistry. If I'm off topic, at least it's by accident. If others also believe it to be so or if there is a general lack of interest then I will without hesitation voluntarily remove this Answer-Your-Own-Question post. $\endgroup$ – airhuff Jan 11 '17 at 6:36
  • $\begingroup$ I find your question interesting, but I'm not on SE:physics. ;-) I think you can migrate it with comments and everything. $\endgroup$ – Karl Jan 11 '17 at 16:20

Surprisingly, at a glance, I had to run my experiments with the glass substrate at higher temperatures in order to ensure ice would form and not liquid water. When you think in terms of the difference between the saturation vapor pressure of liquid water and that of ice at the same temperature, this solution is obvious. The temperature has to be above the dew point but remain below the frost point, which is always higher than the dew point at temperatures below 0 oC. The reason for this is that at any given temperature, liquid water has a higher vapor pressure than ice.

As a real-world example of the ice-growth mechanisms observed in my trace-gas uptake experiments, what was a problem for my experimental conditions actually has important implications for ice-particle growth in clouds, in which context it’s called the “Wegener-Bergeron-Findeisen Process”. Here, when there is a mixture of ice and supercooled water droplets, water vapor condenses onto the ice particles so that the humidity with respect to liquid water decreases enough for the liquid water to further evaporate. Thus there is a net transfer of water mass from liquid droplets to solid particles and the solid particles may then grow large enough to precipitate (as in fall to the ground in this context).


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