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Copyright © 2004 jsd

1  How to Make Fog

If you toss hot water into cold air, a spectacular cloud or fog is formed. See reference 1 including especially the video (reference 2).

Let us try to figure out what’s going on, by evaluating various hypothetical contributions. I suspect item (1) is the main contribution, while the others are relatively minor:

  1. In order to make a really impressive fog, there is a huuuuge premium on small particles. Suppose you have a constant amount of water, but you are given the choice of a single 100-micron particle, a thousand 10-micron particles, or a million 1-micron particles. The smaller, more-numerous particles will be vastly more effective at scattering light. They will also stay in suspension better.

    The scenario I have in mind is that a modest percentage (less than 1/3rd, as calculated in section 2) of the water gets turned to vapor, high-density saturated vapor at fairly high temperature. This vapor then cools by contact with the surrounding cold air, becomes supersaturated, and condenses into lots of reeeeally teeny particles, far teenier (and far more numerous) than you could produce just by the violence of throwing the water.

    The other more-than-2/3rds of the water doesn’t disappear. It is still there ... it is just relatively inconspicuous compared to the spectacularly dense cloud.

    We can see why having very hot water is important: the vapor pressure is a steep exponential function of temperature, and we want to push out as much vapor as we can.

  2. There is a very interesting factor we might call the "sizzle" effect or the "rocket" effect. Suppose a drop of hot liquid breaks into two droplets by accident; the question is whether the droplets will coalesce to re-form a larger drop. If the water is hot and the ambient air is very dry, each droplet will be outgassing like crazy. The outward flow of vapor will tend to push the droplets apart. (A single isolated droplet will feel no net force, since the outgassing shoots off in all directions equally. But when two such droplets get near each other, each is repelled by the other.)

    This effect is also exponentially favored by high temperatures.

  3. Hot water has a lower surface tension. Of course this is not the whole story, but it is not negligible either. After all, surface tension is what holds drops together; without surface tension dispersal would be super-easy. OTOH the surface tension does not go to zero even at 100C.
  4. Hot water has lower viscosity. This factor is not overwhelmingly significant; the viscosity of boiling water is not overwhelmingly less than for cold water.
  5. For completeness let’s consider the hypothesis that hot water is useful because you start out with large drops but they become small by evaporation. That’s a nice guess, but the numbers don’t work out. Only about 1/3rd of the water can be lost to evaporation in this way. Sure, some evaporation occurs, making the droplet get smaller, but only by about 13 percent (since the cube root of 2/3rds is 0.87).
  6. It may be worth noting that after the ice cloud has been created, the cloud will be very persistent, much more persistent than the water-droplet fog you’d get under warmer conditions. A big contributing factor is the fact that ice doesn’t stick to ice very well when it’s cold, unlike water droplets that instantly bond due to surface tension effects. This has got nothing to do with the nominal topic of this thread ("how cold does it have to be to make water disappear") but it does address the implicit question of why it is fun to disperse water into the air.

IMHO we now have a pretty decent picture of what is (and isn’t) happening. Remember, the objective is to make an impressive fog. It is exponentially important to have hot water in order to drive a lot of H2O into the vapor phase. It is important to do a decent job of tossing the water, in order to create sufficient surface area for evaporation to occur. It is necessary to have reasonably cold air, to cause recondensation to occur. Extreme cold is not necessary, but doesn’t hurt, and an ice-fog will be more persistent than the other kind of fog.

Note that if you do a really good job of dispersing the water by mechanical means (e.g. an ultrasonic atomizer, such as found in humidifier appliances and Hollywood fog machines) there is no advantage to starting with hot water; indeed it is disadvantageous.

By way of contrast, note that if your objective was to make the water disappear, you wouldn’t do it by tossing water into cold air. Rather, you would do it by spraying a fine mist of water in to warm dry air. You can easily demonstrate on a warm afternoon in the desert, using the mist from a garden nozzle.

2  What Fraction Evaporates?

Suppose we start out with hot water and let it cool by evaporation. How much of the water evaporates? We can analyze this by calculating the enthalpy involved. You could look it up in tables, or you could estimate it as follows:

  1. Cooling water from 100C to 0C: heat capacity is about unity, so we get 100 cal/g.
  2. Cooling ice from 0C to (say) -40: heat capacity is about half, so we pick up another20 cal/g.
  3. Latent heat of freezing, IIRC about 80 cal/g for a grand total of 200 cal/g available enthalpy.
  4. Compare that with the latent heat of evaporation, roughly 600 cal/g, and discover that you can’t evaporate more than 1/3rd of the water by this means.

The only way to evaporate the other 2/3rds of the water is to take enthalpy out of the air. That’s right, when the ice cloud finally disappears, it makes the already-cold air colder. Also note that since the SVP is so low (exponentially low, as tabulated in reference 3) at the air-temperatures of interest, making it disappear (i.e. vaporize) involves enormous volumes. It is an easy yet instructive exercise to calculate the volume of vapor at -40C that corresponds to (say) 200ml of liquid water.

3  References

1.
“Fun With Cold” http://adelie.harvard.edu/ed/Activities/FunWithCold.html

2.
“Video of Making Fog” http://adelie.harvard.edu/ed/Images/WaterToss.mov

3.
Vapor pressure of water, tabulated in “Phase Diagrams” http://www.science.uwaterloo.ca/~cchieh/cact/c123/phasesdgm.html
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Copyright © 2004 jsd