Different Types of Rheometers
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Measuring specific viscous and other rheological properties require you to relate two quantities (shear stress and shear rate, as described in the following). The different types of instruments (called rheometers) will then produce and measure those two properties differently, each having its benefits and drawbacks. In this blog, we’ll cover essential questions and principles on the different types of rheometers and the results they can provide. Hopefully, by the end of this article, selecting the right type of rheometer for you will be simple!
How to Measure Viscosity and Other Rheological Properties：
In our blog post “Why Shear Rate Matters in Process Control,” we discussed how viscosity quantifies the amount of force that is needed when deforming a liquid or substance: We all know that it requires greater force to mix cookie dough in a bowl than to stir in a cup of coffee.
How is Viscosity Defined?
On a small scale, imagine viscosity as the friction arising between different “fluid layers” sliding against each other while you shear the fluid.
Think of the layers as a stack of cards – each layer having a slightly different velocity than the neighboring layers. The same happens when shearing a liquid, as illustrated below, where you conceive many “fluid layers” sliding between a fixed lower plate and a moving upper plate. The shear rate is then the difference in velocity between two fluid layers, divided by the layers’ distance. (*) The shear rate is also described below to the right, by the first mathematical relation.
(*) Fluid layers are introduced to aid, helping in the description as no “thick” fluid layers exist in the real world, as real velocity fields vary smoothly.
Now we turn the attention towards the force needed to obtain the fluid’s shearing, illustrated by the force arrow in the above Figure.
The force is distributed evenly over the upper plate. It results from a constantly applied shear stress (denoted σ) given by the force divided by the upper plate area (the second relation).
Viscosity is simply the shear stress divided by the shear rate! – Nothing more, nothing less.
This example of force distribution is the last relation, above, and surprisingly enough, even high-end rheometers today compute viscosities by this simple relation.
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