Jorn Avezaat

Looks like you're getting ready for the festive season.

You need a rheometer not a viscometer, for studies such as this. Google 'shear thickening' and 'shear thinning'. I think the latter is occurring.

You can confirm the performance of your instrument with an appropriate standard. Many of these standards are Newtonian fluids (another Google for you).

Eggnog and similar materials have rheology modifiers (and other materials) added to them to control the flow processes under low and high shear - they are not Newtonian liquids.

Alan F Rawle This is by no means my area of expertise. The data is used to create a forcasting model for the viscosity of eggnog based on weekly tests performed on the same sample. The data received is from tests performed at 60, 30 or 15rpm.

I receive this data straight from the factory. Most tests have been performed at 60rpm and show a positive linear relation as the weeks go by. However sometimes the samples are tested at 30 or 15rpm instead. This data shows a much lower viscosity compared to the tests performed at 60rpm.

e.g. 60rpm = viscosity level of 70, 30rpm = viscosity level of 120, 15rpm = viscosity level of 250.

Week 1 test at 60rpm would be 70, one week later at 30 would be 120, another week later at 15 it would 250 and another week later at 60rpm it would be 80.

Do you think I can discard the data from tests performed at 30 and 15rpm because the way they measure at the factory is inaccurate.

If I only use the data of each week for tests performed at 60rpm, I do see a positive linear relationship.

Thanks a lot! (ps. I don't have control over the way of testing)

Jorn Avezaat

You job as a scientist is to formulate how all the results can be correct. The instrument measures what it is given under the corresponding measurement conditions. Never disregard data unless you know that the measurement tool is uncalibrated...

Jorn Avezaat, as you may have guessed, you are seeing the shear thinning behavior of the egg nog. This is very common that liquids reduce in viscosity with increasing shear rates, eg paints, yoghurt, gels, etc. The reduction of viscosity isn't an artifact, but rather a real material property, as the structure of the beverage (from whatever causes it to be viscous) is partially breaking down under higher shear rates (rpms). This is a temporary phenomenon, and the material recovers with time after shearing ceases.

The use of a single value for the viscosity measurement is therefore somewhat lacking for egg nog, however if a constant speed (eg.60 rpm) is used, the results should be consistent. We ideally would run a sweep of speeds and fit a model such as the "power law model" to the results to describe the shear thinning nature more fully.

It sounds like you also have slight thickening with age of the material as well... this is common as any solid powder additives in the sample can take time to fully hydrate, if for instance there were gel thickeners, flavors, etc.

If you were to say that the material actually shows a higher viscosity at higher rpms, this would be due to an artifact known as Taylor vortex flow (effectively turbulent flow). The data where the viscosity starts to increase would clearly be turbulent and incorrect. I hope this helps, but please feel free to reach out if you like for more info.