I got my viscosity study data which includes shear stress, shear rate, viscosity, and torque and now I want to find out how to calculate shear train, storage modulus, and loss modulus from these data?
If you have the values of torque and shear rate, you can easily calculate the rheometric properties by well-known relations (see https://en.m.wikipedia.org/wiki/Dynamic_modulus). However, the recalculation would have no physical relevance because the viscosity is the property of fluids and is determined by rotational shear testing, while the moduli are the properties of solid bodies and are determined by oscillatory shear testing. Therefore, evaluating both sets of variables makes no sense.
Hi Amir,
i agree with Miomir Miljković in that with a rotational test only viscosity can be measured. Actually shear stress is a measure of the torque, multiplied with a conversion factor, and shear rate is a measure of the rotational speed of your meausuring geometry, multiplied with another conversion factor. Viscosity, finally, is the ratio between shear stress and shear rate.
Storage and loss moduli cannot be generated with a preset shear rate (or shear stress), but you need to perform an oscillatory shear test, with preset strain and(!) frequency. Storage and loss moduli will then calculate from shear stress and strain (their ratio equals the complex shear modulus) and the phase shift angle. This phase shift angle provides information about the viscous and elastic fractions of material behavior, which then translate into loss and storage moduli.
This also means that oscillatory tests can be performed not only on solid materials, but on the entire range of materials from solid, over visco-elastic, to liquid (with suitable measuring geometries). This is, in fact, the particular strength of oscillatory tests compared to rotational tests.
Good news is that oscillatory tests can be performed with the identical setup as for rotational tests, assuming that an (air-bearing) rotational rheometer was used.
If you are interested in the Basics of Rheology i can recommend the following Wiki page:
https://wiki.anton-paar.com/uk-en/basics-of-rheology/
...or eLearnings:
https://wiki.anton-paar.com/uk-en/e-learnings/
Best wishes
Christopher
Dear Miomir and Christopher,
Thank you so much for your helpful answer.
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