Stirring, mixing and homogenizing

Discussion in 'Silver Gelatin Based Emulsion Making & Coating' started by Photo Engineer, Sep 11, 2010.

  1. Photo Engineer

    Photo Engineer Subscriber

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    In response to some questions here are some loose "definitions" for emulsion making.

    Stirring is accomplished most simply by a magnetic stirring bar and a magnetic stirrer but it can be accomplished manually with a spoon or spatula. In gelatin, a magnetic stirrer is good from up to about 700 rpm with a good point being 300 rpm and a volume from 100 - 500 ml.

    Mixing is accomplished most commonly by a propeller on a shaft and is most efficient at about 100 - 5000 rpm and volumes up to 5 L. You must be careful not to beat air into your emulsion.

    Homogenizing is at the upper end of mixing and if the homogenizer has a loose enough gap between the rotor and stator (bigger than the emulsion or say 10 - 20 microns) then it can be used as a "mixer" for emulsion making. However, this gap is critical as a narrow gap can break or shear crystals and fog the emulsion. Generally a homogenizer has a gap of 1 micron or thereabouts and is used to make photographic dispersions of insoluble materials in gelatin. In this latter case a surfactant is needed. There is no limit on size of solution to be homogenized depending on the container used and the size of the rotor/stator. I have used them from test tube size up to liters with the right equipment. Speeds vary up to 25000 rpm or higher. These are dangerous units that can cause severe injury and also they can even overheat the solution being worked with or can shatter glass containers.

    See here for a state of the art rotor/stator homogenizer: http://www.ikausa.com/rotor-stator.htm

    PE
     
    Last edited by a moderator: Sep 11, 2010
  2. dwross

    dwross Subscriber

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    Thank you, Ron, for taking the time and care to post this information.
    d
     
  3. Sirius Glass

    Sirius Glass Subscriber

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    Thanks, now I am all mixed up! :D

    Steve
     
  4. wildbillbugman

    wildbillbugman Member

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    See here for a state of the art rotor/stator homogenizer: http://www.ikausa.com/rotor-stator.htm

    PE[/QUOTE]

    This thing look just like the grinder my father (the chef) used to grind up raw beef liver for leberknoodles. I still have two just like it.
    Bill
     
  5. AgX

    AgX Member

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    I'm mixed up too...

    So far I thought that stirreng is just a way of mxing.
    Now I learn that stirring and mixing are just grades of... Of what? Of blending?
     
  6. Photo Engineer

    Photo Engineer Subscriber

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    Stirring may not mix the ingredients depending on condition. Or, it may just be moving the liquid in a continuous circle with no real change in the up or down direction. Mixing implies an intermingling of ingredients whereas stirring does not. Both use just a rotor. Homogenizing blending imply shearing forces as well that can change any particle size present if set up that way, or it may imply simply dispersing the ingredients forcefully.

    The three are not necessarily identical, but many of the effects involved are the same. Mixing does not imply any shearing or blending force, nor does it involve heat. A blender may generate quite a lot of heat and may boil the liquid(s) involved. So, there is also a difference in speed.

    PE
     
  7. Hexavalent

    Hexavalent Subscriber

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    Is 'vortex mixing' used in commercial emulsion making?
     
  8. Photo Engineer

    Photo Engineer Subscriber

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    Ian;

    No, they are not used due to the generally small sample size that vortex mixing can handle.

    PE
     
  9. Sirius Glass

    Sirius Glass Subscriber

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    PE,

    You are getting me stirred up! :wink:

    Steve
     
  10. dwross

    dwross Subscriber

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    I was hoping your OP was a jump-off to a more detailed discussion. You are very good at salting the forum that way (and I totally approve:smile:), but so far I'm not seeing the kind of follow-up information I was expecting. If 'stirring', 'mixing', and 'homogenization' are meant as vocabulary words, I'd love it if you could add them to your sticky thread, A short dictionary of emulsion making and coating terms. It's my observation/opinion that this forum is getting very bulky and hard to mine for usable information.

    If you intended more discussion, I'll poke into some potential details. 1) What "insoluble materials" does one typically need to disperse into an emulsion with an homogenization rotor? and 2) Are you saying that a properly selected and implemented rotor can be used to change the grain size distribution of an emulsion that had been originally precipitated with uniform-sized grains? (And if so, to what end? More precise grain size control? Or, a way to economize on vat set-ups and emulsion runs?)
     
  11. Photo Engineer

    Photo Engineer Subscriber

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    Well, Denise, this thread was an adjunct to the dispersion making thread that I posted the same day, at least in part. The other thread demonstrates the use of homogenization or "blending".

    No method should ever be used with emulsion crystal making that breaks up or changes the grain size by force, as this will fog the emulsion. This is destructive and uses a lot of energy which is expended on the emulsion in the form of shear and heat both. The method used must be mixing action that does not shear the emulsion and that does not allow gelatin or silver halide to accumulate on the bottom of the container. It must not whip air into the emulsion.

    Basically then, to summarize, this thread was to point out that:

    1. Up to about 500 ml, stirring at about 300 - 700 rpm will work well for most emulsion making.

    2. Over 500 ml, you need a mixer running at that same speed or higher, but not high enough to cause air to be whipped into the emulsion.

    3. Over 500 ml, you may need a mixer and a stirrer both to prevent gelatin from "burning" or just collecting at the bottom of your container, especially if you use a hot plate.

    4. Over about 3L and with most modern emulsions at any scale, you need some sort of mixer running at up to 5000 rpm.

    5. To make dispersions, you need a homogenizer.

    6. Modern emulsions need a special sort of mixer that can act like a homogenizer but without cracking or shearing grains.

    PE
     
  12. dwross

    dwross Subscriber

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    What "sort"?

    (Apologies if you've answered this somewhere else. It is hard to keep track unless you read every word posted -- and who has time for that, especially with all the extraneous stuff that creeps in :blink:)
     
  13. Photo Engineer

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    I'm working on that post. It will be here in this thread Denise.

    PE
     
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  15. Photo Engineer

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    A special sort of mixer!

    The ideal mixer will not heat the emulsion, cause shear forces nor will it beat air into the make. It will be a perfect mixer for any and all emulsions.

    One way to do this is in a totally enclosed chamber with no air gaps and into which you add your starting mix and the making ingredients. A high speed impeller or prop type mixer would virtually fill this chamber and the conditions would be met.

    But wait! How do we add ingredients? The answer is to have an on-line UF unit to allow for constant volume throughout the make. However, this still limits you to a given fixed size, that of the chamber. This is usually a cylinder, with closed top and bottom and a full sized mixing impeller inside the chamber.

    This does work, but is rather limited in scope.

    A broader scope is allowed by taking the cylindrical side off the chamber, drilling holes in the top and bottom of the "lids" on the above, and then submerging it in the large kettle. Then you can make any size emulsion you wish.

    This device is called a shrouded turbine. It sucks liquid in from the top and bottom (both kettle ingredients and newly added ingredients) and the reactor is within the top and bottom lid and just before expulsion due to the rotational force of the turbine or impeller.

    Coming out the edges of the shroud (the top and bottom of the original cylinder), the reactants are spread evenly by the high mixing energy and form rising and falling walls of mixture at the edges of the kettle. Baffles break up any eddy currents and provide full mixing.

    Since there are no shear forces, there is no excessive heat generated, crystals are not broken, but good mixing is achieved. This type of unit is a unique mixer, particularly adapted to emulsion making. Further refinement can adjust this device to be fully scalable and to also have constant throughput regardless of make size or input rate.

    Unfortunately, I have found that emulsions designed to take advantage of this type of mixer are difficult to make with other types of mixer and are difficult to scale with other types of mixer.

    So, there you have a special sort of mixer explained. Earlier, we were in the midst of a huge thunderstorm with heavy rain and hail. I deferred posting this until we had an "all clear".

    PE
     
  16. wildbillbugman

    wildbillbugman Member

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    Hello to All,
    I am curious about the minimum amount of stirring required for what is usually the last addition to any emulsion. the hardener. What would be the minimum mixing RPM and time for,say 250 ml of emulsion. Can it be "over mixed" I usually give it 5 minutes of rapid mixing on a magnetic stirrer.
    Bill
     
  17. Photo Engineer

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    Bill;

    Overmixing just before coating is represented by the generation of bubbles in the coating or the whipping of air into the emulsion. Correct mixing does not generate bubbles by cavitation or by just whipping the emulsion but does mix all ingredients thoroughly. I generally figure that the hardener is mixed in by a good hand stirring with a spatula or glass rod at 40C for about one minute. That should do it.

    PE
     
  18. Photo Engineer

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    Well, about 2 weeks or so ago, I took delivery of a shrouded turbine which I have posted a picture of elsewhere. If you want another post, just scream a bit.

    Today, my high speed mixer arrived. It is capable of up to 6000 RPMs. I've already cranked it up with the shrouded turbine in a 1 L beaker and really mixed things up right and proper. At 300 RPM, it mixes a drop of dye into 1 L in less time than I can measure, giving a uniform solution. It causes little if any vortex, and a small metal baffle removes the tiny vortex that doe form giving a smooth, nearly ripple free surface to the liquid.

    At present, the turbine I am using must be rebalanced properly since this was the first use, and I am also getting one 1/2 the size as this one is designed for 1 L and larger and I need one for 1 L and smaller as well. So, I had a fun day in the lab. What about all you guys?

    Pictures of the setup to follow as I can get time.

    PE
     
  19. Hexavalent

    Hexavalent Subscriber

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    Ron,
    That sounds like a real MIXER - I've come to the conclusion that a standard magnetic "stir bar" is good for making things go in circles, but not for the quick and intimate mixing of emulsion.

    I trust most of the test mixture stayed in the beaker:smile:
     
  20. Photo Engineer

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    The test did indeed stay in the beaker.

    And, the OP and my following posts describe that stirring will work for emulsions smaller than about 500 ml or even maybe 1L and that stirring is only reasonable at up to about 300 RPMs in gelatin.

    PE
     
  21. Ray Rogers

    Ray Rogers Member

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    Rebalanced?
     
  22. Photo Engineer

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    Ray;

    Any rapidly rotating object must be in perfect balanced. Just as you balance the tires on your car or other similar objects, this high speed turbine must be balanced to prevent wobble or vibration at working speeds.

    If you do not, the the wobble or vibration induced by the imbalance will be detrimental to your health by throwing the moving parts all over the place. :wink:

    PE
     
  23. Ray Rogers

    Ray Rogers Member

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    Is the balance "adjustible"... or does it have to be reground or something?
     
  24. Photo Engineer

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    Ray;

    This is done in much the same fashion as with other similar rotating machinery such as fans and tires, but unlike tires there is no wear on these which would change balance. They are more like fan blades.

    Stay tuned. It will become clear with time. I am not at that point yet by any means. I am just at the point of testing and proving the adjustment is needed, I am not at the stage of even determining if it is harmful or what the speed limitation is (if any) let alone doing it, let alone having the designer do the adjustment.

    PE
     
  25. holmburgers

    holmburgers Member

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    Looking forward to the photos PE.
     
  26. Photo Engineer

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    Results and photos

    To cross reference the thread on the shrouded turbine (not the Shroud of Turin :D ), I now have some results to compare with the reference there to my friend Ingo's web site, referenced in that thread.

    I have now compared a shrouded turbine (PEPA) mixer with a propeller mixer and have the following pictures taken at 2000 RPM. The left photo is a propeller and the right is a Planar Emulsion Precipitation Apparatus (PEPA), both at the same speed. You can see the difference visually. To make them visually the same with respect to turbulence it is necessary to reduce prop speed to about 900 - 950 RPM or about 50% of the speed of the PEPA.

    The prop mixer directs an approximately cylindrical vortex downward to the bottom and then upwards to create a "V" in the beaker. The PEPA creates a horizontal "T" vortex with 2 random cylinders that mix more effectively with less entrained air.

    I now plan on making some complex emulsions that will benefit from the better mixing supplied by the PEPA.

    PE
     

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