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I am currently conducting a method validation for the analysis of a suspension. This would be the first time we will be conducting viscosity analysis via rotary viscometer and the below statements are what we know about the analysis:

  1. P (Percentage) should be between 10 to 90% for an accurate reading of viscosity.
  2. Suspension and other Non-Newtonian liquids have different readings as per the speed and temperature of the analyte.
  3. The variables we can change in the analysis are the Spindle type, Spindle RPM, Sample temperature

What I am confused with is I get different viscosity reading for different P values even if we keep it within the 10 - 90% allowable. How can I choose the proper spindle and speed for my analysis and what is the basis for it.enter image description here

Attached image is the data we have gathered so far. The data set was acquired for a single sample. In red are the data set within the allowable P values.

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  • $\begingroup$ What is $P$? Is it some kind of setting on the instrument? Or is it a range of allowed values that measured viscosities must lie in? Is it a sample property? $\endgroup$
    – Curt F.
    Nov 10, 2022 at 18:08
  • $\begingroup$ Is it the percentage of solids in your suspension? If so, is it a volume % or a mass %? $\endgroup$
    – Curt F.
    Nov 10, 2022 at 18:08
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    $\begingroup$ @CurtF. P is displayed on the rotary viscometer and it stands for percentage. If I search for percentage on the internet, the ff is shown: When the viscosity of a fluid is not known. If the reading is below 10% or above 100%, the user should choose a different speed to obtain a reading in the recommended range. If changing speeds doesn't provide readings between 10% and 100%, the user should try another spindle. $\endgroup$
    – Kent de los Reyes
    Nov 10, 2022 at 20:36

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You alluded to non-Newtonian fluids in your question. Some examples of these are dilatant fluids and pseudoplastic fluids. For these types of fluids, apparent viscosity is a function of the shear stress. This is because the slope of the shear rate vs. the shear stress is not constant. But at least for these non-Newtonian fluids, viscosity remains essentially a "state function"; all you need to know is (a) the fluid in question and (b) the sheer stress in order to obtain an apparent viscosity.

However, things can be worse. Thixotropic and rheopectic fluids have complex, time-dependent shear behavior. This means that the vicosity not only depends on (a) the fluid in question and (b) the shear stress at a particular time, but also (c) the entire history of shear stress applied over the lifetime of the fluid sample in order to be able to characterize its viscosity. For such fluids, viscosity is no longer really a state function.

The problem you are having may be that your suspension may be thixotropic or rheopectic. For example, if the particles of solid suspended in your liquid phase may have complex shapes whose net average orientation changes over time, or those particles may aggregate together slowly over time, and these aggregates may be (slowly) disrupted by shear stress. This means that repeated measurements may not give you the same viscosity.

Of course, there are many other possible causes of this issue, but without a detailed description of your sample types, it will be hard to examine most of these.

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  • $\begingroup$ So getting the density of my sample would only be possible if I keep some of my variables as a constant and set only one variable to be read in the analysis? $\endgroup$
    – Kent de los Reyes
    Nov 11, 2022 at 15:17

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