Dossier Wetenschap: Over water, sneeuw, ijs en sneeuwslijk

De basisstructuur (de facet) wordt bepaald door de temperatuur wanneer het gevormd werd én de verzadigingsgraad van water in de omgeving - de 6 puntige structuur is inherent aan de H20 molecule.

http://www.its.caltech.edu/~atomic/snowcrystals/faceting/faceting.htm

De takken worden enerzijds bepaald door de temperatuursfluctuaties wanneer de sneeuwvlok naar beneden dwarrelt, anderzijds hoe de watermolecules door de luchtreizen en condenseren aan de sneeuwvlok:

http://www.its.caltech.edu/~atomic/snowcrystals/dendrites/dendrite.htm

Gezien de temperatuur en luchtvochtigheid heel plaatselijk is en constant fluctueert krijgt elke sneeuwvlok een uniek patroon (gezien er nooit twee sneeuwvlokken op exact dezelfde locatie gevormd worden en hetzelfde pad naar beneden volgen).

Daardoor krijg je de illusie dat er bepaalde externe vormkrachten zijn - dat echter te herleiden is tot de omgeving (temperatuur, luchtvochtigheid, aanwezigheid andere deeltjes) waarin de sneeuwvlok gevormd is en het pad dat de sneeuwvlok volgt. Gewoon omdat we het niet met het oog kunnen zien, maakt het nog geen mysterie. Niettemin een héél boeiend onderwerp!

Voor meer informatie: http://www.its.caltech.edu/~atomic/snowcrystals/

Trouwens een meer wetenschappelijke uitleg dan "Dit is nu eenmaal de manier waarop kristallen zich vormen" vindt u hier: http://www.its.caltech.edu/~atomic/publist/rpp5_4_R03.pdf

Hartelijk dank! Voor de lezers heb ik er enkele dingen uitgehaald...

As the nascent snow crystal grows, faceting will often create a simple hexagonal prism morphology. Diffusion limits the growth as the crystal becomes larger, and eventually this causes branches to form. Because ice growth is so sensitive to the local environment, it frequently happens that an abrupt motion of some kind will cause all six corners of a simple plate-like crystal to sprout arms at the same time.

As the crystal travels through the cloud, it experiences different temperatures and humidities along the way, and thus the growth behaviour changes as a function of time. But of course all six arms experience the same changing conditions as they grow. The result is a rather complex growth pattern for each arm of the crystal, with all six arms developing roughly the same pattern. Under ideal conditions — for which the growth must be unperturbed by collisions with other ice or water particles — a snow crystal can grow into a rather elaborate, six-fold symmetric shape. As this scenario is replayed countless times, the bulk of the liquid water in a cloud is transformed into the solid state.

Many aspects of snow crystal growth are well understood at a quantitative level. For example, we know the crystal structure of ice, the interactions between water molecules, the ice phase diagram, and much of phase transitions in general. Other pieces of the snow crystal puzzle, like diffusion-limited growth and the equilibrium structure of the ice surface, are fairly well understood, at least in a qualitative sense. And then there are some rather basic aspects of this phenomenon, like the snow crystal morphology diagram, that are not yet understood even at a qualitative level.

The normal form of ice has a hexagonal crystal structure, so the basic ice crystal growth form is the hexagonal prism. Cubic ice has nearly the same binding energy as Ice Ih, and there is evidence that the cubic form plays a role in the nucleation of snow crystals, especially twinned and polycrystalline forms. This role is relatively minor, however, so we will mostly ignore cubic ice in this review.

Page 872: One thing that makes snow crystal physics so intriguing is that we do not yet understand the ice surface structure very well, nor its impact on growth, and thus we still cannot explain the snow crystal morphology diagram.

Page 881: One might have thought that understanding the growth of simple snow crystal prisms, the most basic ice crystal form, would be a relatively simple task. Upon closer inspection, however, the problem is surprisingly rich, and many aspects remain quite puzzling. Although considerable effort has been expended in attempts to reveal the physical origins of the morphology diagram, at present we do not have even a satisfactory qualitative picture of the mechanisms that produce such dramatic changes in morphology with temperature. Our understanding of ice prism growth continues to fall short in both theory and experiment. On the theoretical side, we do not yet have a microscopic model that describes crystal growth from the vapour in the presence of surface melting.

In addition to this lack of a basic underlying growth model, there is now evidence that additional growth instabilities, such as the proposed structure-dependent attachment kinetics, are necessary to explain some aspects of prism growth, as described above. These new instabilities are likely quite important in understanding the morphology diagram, but they also complicate the theoretical picture even further.

The simple hexagonal ice prisms discussed above appear when the crystal growth is not so strongly limited by particle diffusion — that is, when the supersaturation is low, the crystal size is small, and/or the background gas pressure is low. As long as at least one of these parameters is small enough, the growth is dominated by attachment kinetics, resulting in faceted crystals.

When some combination of these parameters is increased sufficiently, then particle diffusion begins to dominate the crystal growth dynamics and at some point branching occurs. When the crystal morphology exhibits numerous branches and side-branches, then we say the growth is dendritic, which literally means ‘tree-like’. Dendritic crystal growth is but one example of the more general phenomenon of pattern formation in nonequilibrium growing systems.

Some examples of ice crystal dendrites grown in air are shown in figure 8. While ice dendrites can grow at any temperature, they are most prominent near T = −5°C, where thin ice needles form, and especially near T = −15°C, where the six-fold crystal symmetry produces side-branches that extend at 60° angles from their main branches. In both these cases, the growth of a single ice dendrite is largely two-dimensional, such that most of the large-scale structure is confined to a flat plane.

Page 891: The growth of plain hexagonal ice prisms remains perhaps the most puzzling aspect of snow crystal growth, in spite of its apparent simplicity.

Snow crystal formation provides a fascinating case study of the physics of crystal growth, requiring a deep understanding of the nanoscale dynamics of solidification and pattern formation. By studying snow crystals, one can gain insights into many fundamental aspects of materials science. There is much that remains to be learned.

Hier is dan ook nog een beetje extra info over dit boeiend thema:

- Snowflakes are only hexagonal if formed in very high clouds at 32-35 degrees farenheit.

- A snowflake's shape reflects the internal order of the water molecules. Water molecules in the solid state, such as in ice and snow, form weak bonds - called: hydrogen bonds - with one another. In other words: oxygen atoms like to share their hydrogen atoms with each other. The stabilizing forces behind hydrogen bonding orient the water molecules into a hexagonal pattern.

These ordered arrangements result in the symmetrical, hexagonal shape of the snowflake. During crystallization, the water molecules align themselves to maximize attractive forces and minimize repulsive forces. Consequently, water molecules arrange themselves in predetermined spaces and in a specific arrangement. Water molecules simply arrange themselves to fit the spaces and maintain symmetry.

- But why hexagonal?  Why in the world all snow crystals are bound to have that symmetry?

The answer is to be found deep inside the crystals. We need to look at the atoms and molecules that form our snow crystals to understand. But that is so deep in fact that no microscope can be used to just look and see the answer.

Scientist have devised ways to 'look' at atoms, not just with microscopes but also with many other tricks. There is for instance a way to get pictures of ordered arrays of atoms in crystals by shining X-rays on the crystals and recording the reflected rays. A suitable mathematical treatment of the measured data leads to information on the arrangement of atoms. This technique is known by scientists with the fancy name of  "X-ray crystallography". We will call it "X-ray Vision", and it lets us "see" how Oxygen and Hydrogen atoms are bonded to form water molecules and how those molecules interact with each other in solid ice and snow crystals.

Look at this X-ray vision image of ice !...