Dear researchers,
I tried to investigate the rheological properties of okra mucilage under shear and oscillation condition. When I make a comparison between shear viscosity and complex viscosity, the result is shown as attached (The complex viscosity increases suddenly when frequency is high enough). Can someone give me a reason for this phenomenon, please?
Thank you very much.
Sincerely,
Dear Minh Nguyen, this is the normal behavior. The correlation between the two types of viscosities is possible only at low shear in the linear viscoelestic domain, at high shear level, the arising of thixotropy make the distinction between viscosities types. My Regards
https://doi.org/10.1081/DIS-200054555
https://link.springer.com/article/10.1007/s00397-012-0621-2
Dear Abdelkader BOUAZIZ,
Thank you very much for valuable information.
Sincerely yours,
As far as I know, no one could theoretically explain why the simple shear flow viscosity should match the complex viscosity in the first place. The simple flow is measured at steady conditions, while the oscillation combines a high-shear phase, a low-shear phase recovery, a high-shear phase in the opposite direction, and another recovery. Yes, we have the well-known empirical Cox-Merz rule, but it is known to fail for more complex materials such as nanocomposites, some blends, block copolymers, etc. Abdelkader BOUAZIZ is right that it is likely caused by the non-linearity under large amplitude oscillatory shear (LAOS). I recommend this work to learn more about the different LAOS behaviors:
Article A Review of Nonlinear Oscillatory Shear Tests: Analysis and ...
Besides, what rheometer are you using? Stress-controlled (or CMT) rheometers hugely suffer from inertia at high frequencies, and the applied correction easily exceeds the measured signal. That, in turn, may lead to a systematic error in your data at high frequencies.
Dear Dr. Petr Lepcio,
Thank you very much for your valuable comments.
I will check the nonlinear behavior under LAOS. However, what do you think about the shear thickening behavior of okra mucilage in this case (the reason maybe the temporary hydro-clusters form of polymer under hydrodynamic interactions)?
In addition, I used a HAAKE RS600 rheometer, steady-shear mode in the range of 0.01–1000 (rad/s) for shear viscosity measurements, Oscillatory measurements were performed at a strain of 0.1%, frequencies (f) of 0.01-10 Hz.
Thank you very much.
Best Regards, .
HAAKE RS600 looks like a stress-controlled rheometer, but I didn't go through the full specification. Look into the manual and check what it says about the inertia correction at high frequencies. Our rheometer (TA Instruments) can show the "raw phase" to help you identify the measurement reliability.
Yes, the reason for the shear thickening is the creation of additional structures. Shear enhances the destruction and creation rates of weak bonds and the respective structures, but the shear dependency is different for both. Thus, you can get either shear thinning, shear thickening, or an overshoot regarding the associative tendency inside your material (see the above-referenced paper).
Dear Dr. Petr Lepcio,
HAAKE RS6000 is a stress-controlled rheometer. I will check it again.
Thank you very much for your support.
Best Regards.
You are welcome. If you found my answers useful, please consider recommending them so they might help the other researchers as well. Thank you.
Even in the Cox-Merz rule I would expect a relation between the steady state viscosity and the real part of the complex viscosity. The complex viscosity (the absolute values) includes both the real and the imaginary parts (the latter belongs to melt elasticity)
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