Paracetamol crystal surprise and a question

[Beatsy]

Inspired by Banania's post of panadol under crossed polarisers ( http://www.photomacrography.net/forum/v ... hp?t=33615 ) the next day I spent a couple of hours trying something similar. I used generic paracetamol tablets in methanol and filtered the solution. I didn't try cellophane 'retarders' but did install a deSarnemont compensator (quarter wave plate?) and partially uncrossed the polarisers - something I'd not thought of doing before. As expected, with so many variables changed, I didn't get the same results and still much prefer Banania's images. But here's a couple of my results for context...




While making the evaporated alcohol samples I decided to try a melt too. I placed a coverslip containing one of the crystallised samples face down on a slide and heated it to 169C. The sample melted, spread out under the slip and was then allowed to cool slowly. Unexpectedly, it stayed clear and didn't crystallise at all, so I put it to one side. The next day I noticed a very thin crust around the edge of the coverslip which grew about 0.5mm more by the day after that. There was little interesting in this crust but crystals were obviously growing (albeit very, very slowly) so I put the slide away and didn't look at it again until today (about 10 days later). Wow! It has nearly fully crystallised now and grown some lovely concentric rings in places - like this (long edge covers about 1mm).



My question: I am struck by the amazingly accurate circularity of the concentric rings, and the fact that there are clearly defined rings of different structure at all. Since the alcohol had evaporated there shouldn't be any saturation gradients (should there?) so how do the circles stay so regular and precise and what triggers the structural changes to form the rings? Can anyone point me to articles that discuss the mechanisms behind this? Thanks.

[curt0909]

I don't have an answer but I've noticed the same. I can never get circle crystals with an alcohol or water solution, only with the 'pure' melt.

[banania]

You see perfect circles (or concentric circular isochromes) when you are looking straight down the optical axis of an uniaxial crystal. Gemologist use this method to find out whether a mineral is uniaxial/biaxial isotropic/nonisotropic. You are also seeing in the circle a black center spot, called the melatope, and a cross centered in the melatope and the cross is called the isogyre.

My experience is that you get these circles quite often, but I don't know what determines whether you get circles or something else. There are quite a lot of varibales at play. The difference between water and acetone/methanol is the speed of evaporation. With acetone crystallization takes place very fast while it takes hours for the water to evaporate from the slide. Melting introduces heat and many minerals have different crystal structures depending on the temperature during crystallization. Available space is also affecting the growth of crystals and you can manipulate this aspect by using very thin layers of the saturated solution and also by tilting the slide during crystallization so that you force the solution to spread as thinly as possible. Or you could make a small pile of paracetamol on the slide and keep on dropping acetone on the pile repeatedly. This would result in a thicker crystals.

When you are forcing the crystallization to take place under a cover slip or between slides, you are forcing the crystallization to take place "2-dimensionally". This is probably also affecting the crystallization very much, and maybe also result in crystal growth perpendicular to your natural axis of view. Then you would be looking "automatically" down the optical axis of the crystals... maybe...

So I would say that the main factors at play here are heat, speed of crystallization and spatial freedom to grow. The interference colors are also affected very much by the thinness/thickness of the crystals. I don't have any litterature to offer, gemology or petrology textbooks could be a place to search.

I have images of many different crystal forms. Might create another post to display this incredible variation, even with a single substance like paracetamol.

If you are using semicrossed polarizers the colors vanish, as compared to crossed polarizers. To reintroduce new and different colors, one needs to try out various wave plate retarders of various thicknesses. I have found out that the clear cellophan wrap used by florestry shops to wrap around the flowers is quite qood for this purpose. One can cut pieces from this wrap and make thicker retarders by attaching these on top of each other. I have a number of these ready made and try out different thicknesses and different amounts of crossing the polarizers. Addicting stuff as the shapes and colors just keep on transforming.

Hope you don't mind me posting an image here. This is just to give an idea of using cellophan wraps as wave plate retarders

Photographing light: DIY wave plate retarder by Henri Koskinen, on Flickr

[Sumguy01]

very nice.
I like #1 best.
Thanks for sharing.

[zzffnn]

The melt photo is amazing!

Funny how the same question came to my mind when I see that melt photo, before I read on and saw Beats' question. Then I kind of made a guess on the answer (before reading Banania's answer) - limited space during slow crystalization (such slow crystalization may create a concentration gradient). But banania's answer is much better than mine.

Maybe try a melt on a flat well slide, without cover slip? Also a deeper covered space, by sandwiching a few cover slips under the cover? Those may get you some more interesting photos, though variables may be too many to allow controlled repetition or provide an answer.