Hi Jose,

Mixing highly viscous fluid uniformly is always a challenging problem. To test performance of the liquid mixer (degree of homogeneity), probably colorimetric test is the best option. There are few options--

a) DIMST (double indicator system for mixing time):

    Two standard acid-base indicators, methyl red (red to yellow, pK ~ 5) and thymol blue (yellow to blue, pK ~ 9). These indicators can be added to the both

unmixed liquid A & B.

If these two liquids are well mixed (~95% homogeneous) after adequate mixing time is provided, the solution should look yellow. If the test is carried out in a transparent  mixing vessel and photographed, in the images, distinct red and blue regions as well as the later emergence of yellow regions can be seen.

This is a general method where any suitable color combination can be used e.g. blue and white should yield indigo.

After the images are acquired, a suitable image processing must be carried out using tools like MATLAB or any other software e.g. imageJ, to calculate the standard deviation/covariance of the pixel values in the mixing zone. An uniform homogeneity would be assured when these statistical parameters fall within the desired accuracy limit e.g. 0.05 - 0.01 (95%-99% homogeneity).

b) PLIF (planar laser indued fluorescence)

Planar laser induced fluorescence methods provide excellent spatial resolution,

for a selected plane (thin laser light sheet) within the mixing vessel using either simple dilution or acid-base reactions.Most of the acid-base fluorescence studies use fluorescein, whose fluorescence turns on for pH>4.

Hope it helps,

SM

You could also consider sampling different parts of the mixture and measure their viscosity. The degree of variability in viscosity should be linked to the heterogeneity of the mixture.

Subhasish and Jan, Thank you very much for your comments. 

My comments to your input: 1) Our final product has to be medical grade approved, so the introduction of new components will require verification (costly and time consuming) that they do not create a problem (biocompatibility)*. As a method to learn how to mix, I believe it sounds a very good idea. Question: Any documentation on recommended percentages? I am about to look for any information, but if you (Subhasish) have any document to share or recommend I would appreciate. 2) Regarding Viscosity, it was proposed internally and for now we will compare the viscosity of a "good-performing" mix, vs. a "bad-performing" (believe to be not homogeneous) mix. It is possible to take samples from different location of the container, but my concern is that viscosity is a macro characteristic, and may average the response. The lack of homogeneity is perceived while passing the mix through a very small (~1-10 µm) orifices. Again, thank you for your inputs. 

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*It would also be a battle for marketing.   

Dear Jose, 

To measure the degree of mixing of these liquids non-invasively during mixing in the  CST, inline NIR spectroscopy to monitor compositional variations would probably be very suitable. Please contact me at [email protected] if you would like more information.

Regards,

Rut

you could try using the Tyndall effect used for colloids. If you have two glass beakers, you place the solution in one and the colloid mixture in another. Then, shine a light through both beakers. ... That means one example of a colloid mixture is fog. Try it at different positions of your fluid and check for " scattering". I used this many years back to check homogeneity in frack fluids (cross-linked polymers of polysaccharides e.g guar, with borate). I know that you are not looking for a colloid but the test maybe useful!