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Rheology Tools: Taming non-Newtonian Data with the WRM Correction

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Weissenberg-Rabinowitsch-Mooney Correction (WRM)


Most fluids are non-Newtonian. What that means is that as the sample is exposed to shear rates, the viscosity can change (increase or decrease). Some samples display Newtonian behavior until a certain shear rate point where it changes the viscosity so it's imperative to be aware of all possibilities and expand your testing capabilities as wide as possible so you are prepared for all unknown. 

When measuring non-Newtonian samples on rheometers and viscometers, such solutions often violate the assumptions programmed into the instrumentation. Diving deeper, the assumption that the change in shear stress over shear rate is linear would then be violated. However, there is a way to work with non-Newtonian samples in such cases by using the Weissenberg-Rabinowitsch-Mooney Correction (WRM). 

In this webinar, we will go over the mathematics of the WRM correction through a step by step tutorial on how to deal with non-Newtonian samples in such cases and specifically for use with RheoSense viscometers powered by VROC® technology (listed in USP as chapter 914, Rectangular slit method). 

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1. What is Viscosity?

Viscosity of often referred to as the thickness of a fluid. You can think of water (low viscosity) and honey (high viscosity). However, this definition can be confusing when we are looking at fluids with different densities.

At a molecular level, viscosity is a result the interaction between the different molecules in a fluid.This can be also understood as friction between the molecules in the fluid. Just like in the case of friction between moving solids, viscosity will determine the energy required to make a fluid flow.  

https://www.rheosense.com/what-is-viscosity

 

2. How many shear rates do you recommend to analyze whether or not your sample is Newtonian or non-Newtonian?

We recommend a minimum of three different shear rates when analyzing your samples to determine whether they are Newtonian or non-Newtonian. Separately, the shear rates should have a wide range. The reason for the wide range is because many are complex. Sometimes, your sample will not show shear thinning or shear thickening behavior at low shear rates but will at high shear rates. 

Overall, there is just a lot to your sample that you may not know about until you actually test it. As a result, we recommend testing as much of a range as possible to ensure full knowledge of how your sample behaves. 

When it comes to your injections, the best way to design your experiments is by focusing on the injection rate that your sample will be exposed to when going from the syringe into a patient. Testing with those numbers in mind will help give you confidence and know what is the unknown.

Questions?

Any questions regarding RheoSense viscometers or general rheology inquiries?
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About the Viscometers

Interested in the viscometer that was mentioned in this webinar? 

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