Emulsion stability?

[spiridij]

We have an oil in water emulsion, with a very low percentage of water (~7%) and a very high percentage of fine particles (~20 micron). It resembles a thick paste similar to cream cheese. We are using a small nozzle system to control the flow (which is very likely plug flow) of the paste to within +/-15%. What I've found is that the paste flows differently over storage time. Each time we open its storage container and pull a small volume out for testing, it flows faster. The paste is not very hydrophilic, as a matter of fact it dries very quickly if left on a bench. So I wonder if there's a stability issue with the emulsion that would cause it to flow faster over time (apparent viscosity). We've done extensive testing with centrifugation to show that the particles do not settle and while the color does appear to get slightly darker over a few months when the storage container is opened and closed multiple times, the color does not change when it's kept in a sealed container for the same amount of time. Each time the storage container is opened it is in a 100% RH environment and kept wrapped with saran. Can anyone give me some ideas (other than oxidation or water adsorption from the humidity chamber) that could be causing instability of flow rate over time? And any testing we can do to see changes (other than rheology which seems not to work very well).

 

[Compositepro]

The material you describe is almost certainly thixotropic. This means that the viscosity will change with time and shear history. There is not likely much you can do about that except to design your process to deal with it. Being able to control flow rate to +/-15% with a simple nozzle is doing pretty well.
Use a positive displacement flowmeter with feedback control.

 

[spiridij]

Compositepro, It certainly is thixotropic, but does that mean that it could be settling just sitting in it's storage container causing a decrease in viscosity?

 

[SuperSalad]

Why can't water adsorption from the humidity chamber not be a culprit? Have you confirmed the moisture content of the original material vs. the periodic samples you pull is the same?

Putting the samples through some physical stress before testing the viscosity would give you a clear indication of whether or not you are dealing with a thixotropic effect.

Andrew H.

http://www.mototribology.com

 

[spiridij]

THat's my theory as well, SuperSalad, but I'm not a chemist and our chemist says that because the viscosity of our paste emulsion is so high it is very unlikely that it would absorb water. Assuming he's correct, would there be anything else that could be at play here? Our flow test occurs at 37C, so the surfactant is likely gelling, but since we store at room temperature, it's likely still a liquid. Could there be some phase transition issues going on?

 

[Compositepro]

Mixtures like your are very complex in how they behave. With enough time and research (experimentation) you may be able to find some improvements. But, if the only problem is flow rate through an orifice, then the cheapest solution with 100% chance of success is what I suggested.
How are you making your emulsion? True emulsions have a particle size of 0.1 to 0.5 microns. 20 microns is huge, and particles will tend to float or settle. At only 7% water you probably will not see much segregation because the particles are already as tightly packed as they can get. If you dilute your emulsion you probably see rapid formation of a water layer. This can be avoided by making the particles smaller by using high shear homogenizers or emulsifiers. You can also add a thickener to the water such as fumed silica, gelatin, or a water soluble polymer. Your mixture will still be thixotropic, so to have consistent flowability at an orifice, you must precondition the material by shearing (mixing) it all uniformly in time and space. Thixotropic materials are generally shear thinning, so they get thinner when mixed but get thicker with time after you stop mixing. With so little water phase though, your material might be shear thickening. In that case you may also run into a problem with air entrainment.

You may find interesting this link to a master's thesis on mayonnaise.

http://lup.lub.lu.se/luur/download?func=downloadFile&recordOId=8916120&fileOId=8916126

 

[spiridij]

Thanks Compositepro. Unfortunately we cannot change the device, it already uses a positive displacement system to push the paste through the nozzles, but we can't add a feedback control in the device, since it's only about 1" x 0.5" x 0.25". I'll check out the link.

 

[IRstuff]

Have you tried agitation prior to each new sampling?

TTFN (ta ta for now)
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[spiridij]

IRstuff, that may be worth another shot. The paste is around 500,000 cP, do you have a suggestion on agitating it?

John