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Author Topic: Info needed: sub-zero resin/casting  (Read 1115 times)
quatch
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« on: October 05, 2010, 08:24:49 pm »

Hi guys, I need a spot of help for some actual science. I'm a grad student studying snow, and I'd love to be able to cast a volume of snow for structure analysis. Since I've never done anything like this before, I don't know where to start, and I figure you folk are much better informed about this than I am.

Does anyone know of a resin (or similar thing) that could be poured and set below freezing?

The ideal process would be: at the field site, section a small volume of snow (say a 5-10cm diameter, ~50cm long cylinder with a piece of pvc pipe), pour in some sort of resin, allow to cure. In lab, saw in half, melt out/remove snow, do science on the shape of the hollows.

Snow is actually quite hollow and well connected, but will obviously melt if the resin is at all warm.

Any advice at all is welcome.
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Herr Döktor
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« Reply #1 on: October 05, 2010, 09:28:00 pm »

Hmmm, difficult one: most resins, urethanes especially, are exothermic- ie. they 'evolve' heat as part of the curing process; polyester resins, conversely, tend to need some warmth to 'kick' them into curing; even silicone rubber will need some heat to aid the catalytic reaction...

So, no, not sure: oh, hang on, possibly something like alginate might work- maybe someone else knows more about it...
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Narsil
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« Reply #2 on: October 05, 2010, 10:15:41 pm »

There are two big challenges here. Firstly the obvious one of temperature, certainly all of the resins conventionally used for casting, epoxy, polyester, polyurethane etc won't go off at all at these temperatures.

The other thing is finding something with good enough flow properties to penetrate such a fine structure.

Alginate is sensitive to temperature but I'm not sure if freezing temperatures would stop it from curing completely or just slow the process down a lot, it might be worth a try, although the moulds do start to degrade very quickly ie 24-48 hours and its usualy to make a plaster cast almost immediately for best results.

It might also be worth trying gelatine as this definitely does set at low temperatures.  

One highly speculative option is to use a hot casting method , if the sample is cold enough then something with a low melting point may freeze fast enough to preserve the structure you want. When you cast hot materials the material in contact with the mould freezes first and forms a skin, after that it doesn't matter if the mould melts  as you've already captured the detail, as long as the outer material doesn't re-melt. As I say its an outside chance but it might be at least trying something like wax or woods metal. There are certainly instances of people casting iron into ice moulds so there is some hope.

Another outside chance might be using something like very fine clay slip (a thin suspension of clay in water) or maybe even a starch solution, possibly mixed with salt or alcohol so you can pour it at sub zero temperatures then re-freeze the whole thing and freeze dry it ie thaw it in a vacuum so that the water content sublimates off, hopefully leaving the solids behind. The end result would be pretty fragile but you might be able to get a resin or alginate cast off it.

« Last Edit: October 05, 2010, 10:17:50 pm by Narsil » Logged







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« Reply #3 on: October 05, 2010, 10:47:44 pm »

Another problem to consider is the weight of the material used in the casting; snow is quite brittle. If you poor something on it, it might compact to such an extend that it will not easily allow for fluids to pass through...

Hmm.. perhaps it would be easier to reduce your snow sample size, then use optical imaging to create a 3d render of it on a computer? would that work?
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Dr cornelius quack
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« Reply #4 on: October 05, 2010, 10:58:15 pm »

What?.... You want to take a cast of some snow?

Sounds redundant to me.

To what degree of detail do you want to preserve the structure?

Individual ice crystals?

Mass structure of many thousands of flakes?

Large scale shapes of snow drifts?

The way in which consecutive falls of snow interact?

If you are looking for a means of preserving the geometry of how individual ice crystals interlock, then I don't think that a physical 'moulding' of a real specimen is the way to go. I'd suggest that some form of computer modeling using the known properties of ice crystals would give better results.

I'm pretty sure that no casting medium is going to have the properties you need to capture such a fine structure without changing it in some way. The degree of fluidity required to conform to shapes at a molecular level is mind boggling.
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« Reply #5 on: October 06, 2010, 02:40:18 am »

Seen something on tv in order to see the bug they coat it with golddust to reflect the ray from electronic microscope. I know nothing about the process, mabye even I remember wrong but you can search a bit. mabye works coating an individualy flake and mass produce it Tongue... sorry 4Am and I need sleep

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quatch
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« Reply #6 on: October 06, 2010, 04:26:19 am »

3D snow work has been done with MRI and electron microscopy, which are pretty crazy. The kind of crazy I do not have access to, sadly.

I'm interested in finding a method of preparing 2D slices, which would be digitized and compared with several other methods (NIR photography tries to do the same thing, but it is a nightmare to calibrate).

Basically the properties of interest here are grain size, packing arrangement, and orientation. Trying to measure these things on your knees in -20 -> -40C with wind, polar bears, and only frozen PB&J to eat gets a little trying Smiley

WRT gelatin and alginate, could they be made with antifreeze to be liquid just below zero? I can ensure the whole thing freezes after the pour with some liquid nitrogen if needs be. I could use an excuse to get my hands on some.

WRT metal or other very hot pours, I suspect that because the snow grains are so small that I couldn't get the liquid to penetrate fast enough, and the top portions would remelt.

WRT starch or clay, I like that idea. I can easily get some clay (I do pottery) to try alongside any other ideas before the field season starts.

@Dr cornelius quack, you're on the right track with successive snowfalls. The vertical layering of different grain sizes and packing makes it a very complex system to model. As for computer modelling of the actual pack growth, maybe in 20 years. The physical processes by which snow forms and changes aren't well understood.
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« Reply #7 on: October 06, 2010, 12:31:49 pm »

Not sure what compound to use, but the basic theory would be:
1 drop sample in liquid nitrogen to harden.(though this would in all probability still cause small crystal growth from the moisture trapped in the air pockets)
2 place sample in chilled container of whatever medium you can find that will set below freezing.
3. place sample under vacuum to ensure all voids fill in with low viscosity fluid.
4. place in temp controlled area to ensure sample does not melt until after the fluid has set.

Don't forget to take into account the lower boiling point due to vacuum.

22" should be fairly easy to pull on commercial equipment. Maybe 10-12" off an air conditioning vacuum pump. 

And using the thinnest samples you can provide without damaging the structure.   
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Dr cornelius quack
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« Reply #8 on: October 06, 2010, 08:04:33 pm »

Maybe some sort of Tomography would be a better approach.
http://en.wikipedia.org/wiki/X-ray_microtomography
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Mr. Boltneck
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« Reply #9 on: October 08, 2010, 06:24:26 pm »

Perhaps a cyanoacrylate vapor, as done to bring up latent fingerprints? I'm only guessing, but given that it polymerizes in the presence of water, this seems like a possibility. On the other hand, vaporizing it without melting your sample might be tricky. Perhaps a two-chamber system, one filled with snow and evacuated of most of its air, the other containing a supply of cyanoacrylate and a heater. After some time to build a supply of vapor, open a valve to flood the sample with rapidly-cooling vapor. Sort of a combination of vapor-deposition and vacuum casting.
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