How do water crystals grow in freezing temperatures?

[GoldenEagles]

https://www.youtube.com/watch?v=_S1ZiNCz6CA

In this video, the chief researcher is shown collecting a water sample from a very beautiful location in Japan. He takes it back to the lab, and divides the sample into 50 petri dishes. The amount of water in each petri dish is 1 ml. He then freezes the lot for 3 hours at (minus) -25 deg. C (-13 deg. F). He then takes out one of the fifty petri dishes at a time, and places it under a microscope. The microscope lab is kept at -5 deg C (23 deg. F). It is my understanding that the temperature differential is calculated to induce water crystal formation, like you see in snowflakes, which form as they fall from colder temperature regions, into (comparatively) warmer temperature regions. This video shows water crystal growth as it happens (under the microscope).

I would like to know by what mechanism do these water crystals grow? For starters, where does the material come from, that makes up the growing crystal? It is derived from the ice block below? Or are water molecules in the air attaching themselves, in a very orderly manner, to create this pattern? In other words, does the crystal actually grow out of the hexagonal point? Or does the crystal gather its water molecule substance from the surrounding air? Or perhaps by another mechanism? Would someone be so kind as to explain this to me? Thank you.

 

[Mapes]

Hi GoldenEagles, welcome to PF. The crystals grow when water molecules from the air attach to the existing crystal. Since the cold room is already at -5°C and likely has very low humidity, I wonder if the water supply is coming from the researcher's own breath.

 

[GoldenEagles]

Hi GoldenEagles, welcome to PF. The crystals grow when water molecules from the air attach to the existing crystal. Since the cold room is already at -5°C and likely has very low humidity, I wonder if the water supply is coming from the researcher's own breath.

Thanks Mapes, for the welcome, and for your observation. Yes, this sounds like the most reasonable explanation (at first glance) though I wanted to put the question out there to see if someone was aware of some other mechanism operating that was not obvious. Without a doubt, the breath of the researcher must add to the humidity, and thus to the store of water molecules in the air from which the growing crystal draws.

Obviously (I think) since the lower block of ice is frozen, it would seem impossible for the crystal to "grow" like a plant leaf for example, which draws (most of) its resources from within, resources that flow into the area of the leaf that is growing, relying on cell division to expand. Since the structure of the ice crystal is static in that respect, like a piece of concrete (?) that leaves only the water molecules in the air to draw from, as far as I can see.

I wonder what is it, then, that causes these water molecules to attach themselves in such a singularly beautiful and orderly manner. Why don't we see just a growing lump there, like grains of sand pile up on a beach, instead of this christmas tree like structure which builds molecule upon molecule upon only the six intersecting points of the underlying crystal hexagon?

 

[Mapes]

Anisotropic* growth is an universal aspect of crystal growth that's shared by ice, metals, polymers, and so on. As the solid accumulates by a phase change from a liquid or vapor, the arriving atom or molecule--water, in this case--will typically attach to the solid in an orderly way. A certain connected position orientation will be most energetically favorable, like fitting a jigsaw puzzle piece tightly in place.** The growth rate is also direction dependent, so dendrites will rapidly extend in certain directions (that is, additional molecules will attach in these directions; the solid part doesn't move). The underlying atomic-scale symmetry of this process becomes evident in the macroscale symmetry of the final crystal.

*Not the same in all directions; direction dependent.
**If solidification is rushed (if the liquid is supercooled, for example), the atoms or molecules may not have time to fit nicely into place, and the new solid consequently may be amorphous rather than crystalline. This is easier to accomplish in some materials than others; it's hard to achieve in metals, but all window glass is amorphous; the crystalline form is quartz.

 

[GoldenEagles]

Anisotropic* growth is an universal aspect of crystal growth that's shared by ice, metals, polymers, and so on.

While this property that you refer to as "anisotropic" may be common to ice, metals, polymers, and so on, as you say (I will take your word for that) i.e. that their mode of growth is direction dependent, isn't it also true, that the kind of crystal growth that we see here in these water crystals is singularly unique in the nature kingdom? I have never seen anyone get excited about metal flakes, polymer flakes, and so on, like we see with snow flakes. If I am mistaken, please correct me.

 

[Mapes]

While this property that you refer to as "anisotropic" may be common to ice, metals, polymers, and so on, as you say (I will take your word for that) i.e. that their mode of growth is direction dependent, isn't it also true, that the kind of crystal growth that we see here in these water crystals is singularly unique in the nature kingdom?

I must tell you, this couldn't be farther from the truth. Consider diamond, sapphire, emerald, ruby, aquamarine, topaz, garnet, salt, pyrite, quartz, and a million other examples (but not necessarily all gems and minerals; opal, for example, is amorphous). In fact, anisotropic growth is such a hallmark of crystal growth that even glass (which is amorphous) can be marketed as a "crystal" to the unknowing, as long as it is cut in a dipyramidal shape.

 

[GoldenEagles]

I was not denying the application of this word "anisotropic" to these other crystal categories. I will stipulate that they grow in a direction dependent manner. The point I was trying to make, is that I think water is unique in its capacity to produce single crystals which grow in six perfectly symmetrical directions all at the very same time (as we see in this video). I don't believe we see that anywhere else in nature. Can we agree on that?

 

[Lok]

The point I was trying to make, is that I think water is unique in its capacity to produce single crystals which grow in six perfectly symmetrical directions all at the very same time (as we see in this video). I don't believe we see that anywhere else in nature. Can we agree on that?

I'm sure that these properties are not unique, by properties i mean it's hexagonal structure. H2S might be a candidate although the S is a bit too big and it has to be cooled to -90 C'.

 

[Mapes]

I was not denying the application of this word "anisotropic" to these other crystal categories. I will stipulate that they grow in a direction dependent manner. The point I was trying to make, is that I think water is unique in its capacity to produce single crystals which grow in six perfectly symmetrical directions all at the very same time (as we see in this video). I don't believe we see that anywhere else in nature. Can we agree on that?

Well, http://graphitecrystals.com/images/graphite/SnowflakeDendrite_Harrisville.jpg" (cinnabar is where we get mercury from) form equivalent hexagonal dendrites. I'd say the notable thing about water snowflakes is that they routinely form in our atmosphere rather than on the surface or underground. But rarely are effects in Nature unique, especially under such a common theme as crystal structure and growth.

EDIT: Water snowflakes also form by a vapor-to-solid phase transition (as opposed to liquid-to-solid). I suspect that this is unusual for natural crystal formation; perhaps the geologists could clue us in.