gelatin tests

Dichromated Gelatin.
Joe Farina
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gelatin tests

Post by Joe Farina »

Did two tests regarding noise (scattering) in gelatin. The first one was to compare possible noise due to lower-temperature heating of the mixed emulsion (40C for 30 minutes) as opposed to 50C for 2 hours. I used silicone cube molds (for making candy, available on Amazon). The gelatin was pure, without dichromate. Using a 405nm laser pointer, the beam was shone through the cubes. I couldn't tell any visual difference between the two, with regards to scattering. The holographic performance may be different, but for this simple visual test, they seemed similar to each other.

The second test was with regards to filtration. The first sample had no filtration, and the second one was filtered through a warm 1.5 micron syringe filter. The filtered cube of gelatin was better, with less scatter. It was noticeable. I would like to do further tests with smaller pore sizes, since the warm gelatin easily went through the 1.5 micron filter. Some other available pore sizes are 1.2, 1.0, and 0.7 micron.

The two gelatins were Great Lakes (B, bovine hide) and Knox (unknown). The Knox was much clearer than the Great Lakes (less yellow, that is), suggesting that it may be in part (or all) type A pig hide. I have yet to try Gelita photographic gelatin (B-bone) and Superclear (A-hide).
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Joe Farina
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Joined: Wed Jan 07, 2015 2:10 pm

Re: gelatin tests

Post by Joe Farina »

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Din
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Re: gelatin tests

Post by Din »

If you want to quantify your results, you'll need a beamsplitter, a mirror and two meters (see below). The reading on the two meters will be RI (where I is the intensity of the laser and R is the reflectivity of the beamsplitter) and RS(1 - RI), where S is the scattering intensity. The power of the beam, I should be easy to get, the reflectivity of the BS and the mirror (R) can be measured or calculated easily. You should therefore get a table of scattering intensity against the various types of gelatin, which may be useful for DCG holographers. Software to calculate and tabulate is pretty straightforward (even chatGPT may be able to do it :D )
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Din
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Re: gelatin tests

Post by Din »

Another thing I noticed was that you were illuminating your samples with a narrow, raw laser beam. This means the volume illuminated inside the gelatin sample may be a small ratio of the beam's volume*. In other words, if the scattering centres are widely dispersed, a narrow laser beam illuminating a larger gelatin volume may not pick up many scattering centres and the scattering may seem weak. It may be necessary to widen the beam, or use a smaller sample. Remember, when you record a hologram, a large area of the plate is uniformly illuminated, so even sparsely distributed scattering centres may scatter.

*If the beam radius is r, and the gelatin sample is a cuboid of side a, and length l, then the ratio ( beam volume)/(cuboid volume) = 2πr/(a²)
Joe Farina
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Joined: Wed Jan 07, 2015 2:10 pm

Re: gelatin tests

Post by Joe Farina »

Thank you, Din, I'm grateful for your help. The method you illustrated would be a precise way of determining scattering, I will keep it in mind. Since the silicone molds produce cubes of the same size, I was also thinking of measuring the simple transmittance of different cubes of gelatin. I'll have to weigh the advantages of your method. Thanks for noting the importance of the beam size vs. the sample size. It will be easy to add a couple lenses to produce a wider beam.

Regarding scattering, I'm interested in your earlier comment about micro-bubbles. With the cubes, there was an alarming amount of macro-bubbles (I always use magnetic stirring). These were easily visible. When I coat plates with the Mayer bar, I remove them with a toothpick. (These usually appear at the edge, at the beginning of the draw.) But I never considered the possibility of difficult-to-see micro-bubbles.

As a recent batch of gelatin was being heated/stirred, I noticed what I used to call "turbidity." These were swirls within the gelatin, which seemed to have a slightly different index of refraction. These swirls did not dissipate, and were still present after 2 hours of heating/stirring at 50C on the hot plate/stirrer. I am beginning to think these may have been micro-bubbles, rather than some kind of inhomogeneity within the gelatin.

I want to try vacuum de-airing of an emulsion batch. I already have this system for de-airing epoxy.
Martin
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Re: gelatin tests

Post by Martin »

Joe,
you might remember Stojanoff's work... See e.g. https://www.researchgate.net/publicatio ... _art_no_62

He stressed on the importance of the drying conditions (prior to the holographic exposure).
Joe Farina
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Joined: Wed Jan 07, 2015 2:10 pm

Re: gelatin tests

Post by Joe Farina »

Thanks Martin. I've printed a copy of that paper, and will be taking a closer look.

Once, I did a test using a hot-dried DCG plate. The gelatin never got the chance to gel (which was intentional). It produced a weak hologram, so I didn't pursue it further. I have an old paper which talks about hot-dried gelatin layers (amorphous) vs. gelled, cold-dried (crystalline) layers. In the paper, Stojanoff confirms the amorphous vs. crystalline difference:
The internal structure of the gelatin layer determines the holographic properties of the film. The molecular composition and the transition from one structure into another determine the sensitivity and the range of refractive index modulation. The latter is a function of the cross-linking capacity of DCG-layers. Two distinct molecular structures are present in the gelatin film—the triple helix structure and the amorphous pattern of the proteins. Our experience with DCG-films shows that this order or disorder type transition is responsible for the variation of the holographic properties.
This is actually a source of concern for me. For color DCG, my coating method has been to warm the plate to 35C, coat, rapidly cool to gel, and store at room temperature. (For regular DCG, it's the same, but I refrigerate the plates.) I may have to do further tests to see if variations produce obvious results.
Din
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Joined: Thu Mar 12, 2015 4:47 pm

Re: gelatin tests

Post by Din »

One property not mentioned is that the helical structure of gelatin makes it slightly birefringent. This sometimes manifests itself in what appear to be odd holographic effects.
Joe Farina
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Joined: Wed Jan 07, 2015 2:10 pm

Re: gelatin tests

Post by Joe Farina »

Just a note about gelatin gelling and drying. After a few days in the fridge, the gelatin (no dichromate) dried, and became like a hard plastic, almost like acrylic. Didn't realize how hard it gets. Also, it shrinks enormously, compared to the gel state. The photo below shows the same gelled cubes I posted earlier.
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Din
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Re: gelatin tests

Post by Din »

Joe,
Just saw your comment about turbitity in the gel while stirring. This may be due to the stirrer whirling at too high a speed. The stirrer should create a circular laminar flow. however, if the stirrer is too rapid, you may start to get turbulence. This could be the cause of the bubbles. Also, the hardening and distorting of your gel cubes could be due to the shrinkage of water at low temperatures. It's not normally noticed because the percentage shrinkage is small and the absolute shrinkage for a large volume is too small to be noticed. But, for the small quantities of water in the gel cube the absolute shrinkage could be high. One reason why you don't put DCG plates in a fridge!

In our case, Joy rigged up a "drying chamber" with a hood from a kitchen cooker above an enclosure. On the face of the enclosure was a plastic sheet. On the base, about 2 feet below the hood, she placed a large sheet of paper on which were placed pennies (NIST approved spacers!) and then placed the plates above the pennies. This ensured that the plates were kept flat while drying - to avoid gel creep across the plates - and were subject to a uniform flow of air. The plastic sheet ensured that no dust settled on the plates while drying.
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