At the Mercy of Diffusion

Both the mechanism and appearance of hoarfrost are sublime. Hoarfrost results when water vapor encounters a very cold surface and undergoes reverse sublimation. Such ice crystals normally grow on the ground, but it can also form on twigs or on a poorly insulated pane of glass.The latter is usually called  “window frost” because of its flatter appearance. But it’s no less interesting.

103hoarfrostEnergy considerations maximize hydrogen bonding and determine the hexagonal nature of snow crystals, yet it will not predict the pattern of window frost. In this picture above, there are parts that resemble radial spider webs; other sections look like the chaotic streets of downtown Boston. And there are artsy stickmen and elongated patterns resembling those of airport runways.

In order to grow, ice crystals need a “seed”, and each imperfection along the surface of the glass provides the nucleation site. The distribution of dust or soap residue is rarely uniform. Scratches are random. And who’s to say that every point on the surface is exactly at the same temperature? All of these differences are make each hoarfrost event unique.

If we go back in time, we can ask: why was the water vapor that crystallized evenly distributed in the air when it evaporated from concentrated sources? As water molecules evaporate from a living body or from a body of water, they are initially concentrated among themselves. As a result, they are more likely to collide with molecules of their own type. But the collisions send them in random directions. Slowly, air molecules which are themselves bouncing all over the place find themselves mixed in that thinning concentration of water molecules. As long as temperature is high enough, there’s not much of a force discriminating among water vapor and other air molecules. Everything eventually becomes evenly mixed, similar to but faster than the way a dye can spread evenly in water without any human stirring.

Of course, a cold surface slows all molecules that collide with it, but now the fact that attractions between water molecules can be much stronger than those between air molecules becomes significant and leads to the loss of that freedom of random motion that they previously “enjoyed”.

It’s similar to the blogosphere. So much of what is typed, even though it may be advertised through a hashtag or by word of mouth, comes across billions of other thoughts and messages, and the ideas of a specific blog spread thinly among the vast volume of what’s in real or virtual libraries. The content must find the right surface under the right conditions to crystallize in some other mind. And even then, like the frost itself, there’s usually no permanence. Most often, it does not even get to that stage. Our words are like molecules at the mercy of diffusion.

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