Micrographs of Precipitation Nozzles
One of the perks of being at GEH is the conservation lab. Not only do they have a bunch of awesome microscopes, but they have people that know how to use them!
Mark Osterman brought in a couple of old Polaroid precipitation nozzles (needles for introducing silver-halide into the emulsion pot) and I wondered how big the openings were. I asked Ralph Wiegandt if he could measure them somehow and he showed me how to do it on the microscope. These are at 50x.
There are 3 images; two metal nozzles that were originally used by Polaroid and a glass nozzle, hand pulled by Mark that we use in the emulsion lab here at GEH. The metal ones measure 0.6mm and 1.2mm; the glass one is also (miraculously) 0.6mm.
In hypodermic needle gauges, this translates to 16g and 21g.
To me, this brings up an interesting quandary. The same quantity of silver-solution can be introduced into the gel/halide "pot" in a set amount of time, yet the nozzles can be bigger or smaller. The effect being, to increase the pressure at which solution enters the pot.
On one hand you can have an incredibly fine stream of high pressure going through a very small nozzle, and on the other hand you can imagine a large nozzle that just "drips" out.
So how important is the nozzle size and the pressure?, or is rate of addition the more important factor??
Nice to see you again. Good to see you're having so much fun at GEH!
re: "So how important is the nozzle size and the pressure?, or is rate of addition the more important factor??"
It's hard to imagine all the alchemy and uber engineering that was engaged in to make Polaroid products, but this film, http://thelightfarm.com/cgi-bin/showvideo.py, starting at 7:56 minutes gives an idea of how important extreme precision was to Kodak, not all that long ago. (Spoiler alert: NOT )
A "jet" compared to a "drop" delivering both the same flux (volume per time) would have a larger surface.
Haha, wow Denise, you're not kidding. There's no way they were really making film like that by that point... right?? I didn't quite realize what all went into getting that film translated; what a great service you guys did in doing so!
A "jet" would also reach further into the solution and produce much more turbulence on its journey. I posed this question to Ron and he said that a high pressure jet will remain "cohesive" (I believe this is the word he used...?) longer and thus delay the reaction slightly. But on the other hand there is more surface area, and you'd have to think that the turbulence that it encounters would also aid in quick mixing.
Who knows though; it's all arm-chair emulsion making at this point. It'd be nice to make the same emulsion with a "drip" and a "jet", with the same total rate of addition, and compare the two.
I use 24 gauge needles, directed as closely as possible to the stirrer. So far, I've never had any "crunchies" or pepper grain, except when I once used a larger bore needle. 24 ga. and finer does require quite a bit of pressure for quick delivery!
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Very interesting Ian; that's quite a fine nozzle indeed. How large of a bore are you referring to when you mention the crunchies??
You bring up a good point about delivery; a high pressure nozzle would make a long addition easier and more consistent, compared to low pressure where the accuracy of delivery is all controlled by your muscles. With high pressure you kinda just 'give it hell' and it ejects as fast as it can. We've been using Mark O's bulb thingy which works on that principle; you squeeze it 100% and the rate is more or less controlled by the constant pressure you've created in the system.
And then there's this option (warning: eye candy)...
Chris, the larger bore item was one of those hardware store syringe thingy designed for glue, probably 1mm. It was very tricky to achieve a steady delivery. Basically useless.
I 've made myself a hand crank syringe pump and use 18 gauge needles and nylon (I think) tubing. The tubing came from empty hair spray bottles. It's not really not that hard to control delivery rate. Below is a picture.
The tubing arrangement is likely to change a little because the clothespin tends to move around on the side of the pot and the tubes don't really point were I want them to. Also, been thinking about changing to a smaller sized needle.
I think the part about the cohesiveness Ron mentioned to you is that the high pressured streams push away the other liquids and allow the salt and silver reactants to combine at a more closely controlled ratio. So, the crystals are nucleated within the stream and then carried away into the rest of the emulsion. You want to have the reactants introduced sub-surface.
All this has happened before, and all this will happen again.
Wow, you know I knew mass production of film was a major industrial undertaking and it was in its time the largest consumer of silver but that never really hit home till I saw the office building sized film base machine or the guy shoveling in silver ingots by the gross. Very cool movie.
Originally Posted by dwross
I see what you mean Ian; it does seem like one would need a little back pressure to get a steady delivery.
Jason, wow, that's great! Just this week the idea of syringe pumps hit me, but it's great to know that others have already been thinking about this stuff.
I've been mulling over a design in my head... imagine that the syringe (or pump) is seated on the counter, next to the kettle, with a length of plastic tubing leading to a hypodermic needle which is held in position over the pot by a lab stand. The stand could clamp a cork, through which you push the needle.
Regarding an 18-gauge needle; this is the size that seemed like a good fit to me, but I haven't used it in practice. I'd be very curious to know why you might be looking for something smaller; was it not ideal?
I was able to find some 18g needles in 4" length, which seems like a good length to work with. (here)