I have plots of shear stress vs shear rate and viscosity vs shear rate. I want to check which rheological model do my systems (hydrogels) follow and calculate the parameters form the best fitting model. Can anyone give me idea on how to perform this analysis? Thanks a lot
Instructions on how to adjust the rheological model constants based on data in Matlab can be found in my last article, link below: https://www.researchgate.net/publication/371790588_Computational_fluid_dynamics_validated_by_micro_particle_image_velocimetry_to_estimate_the_risk_of_hemolysis_in_arteries_with_atherosclerotic_lesions
Hi Ritu Thapa ,
The best way to choose an appropriate rheological model is to consider why any particular model is most appropriate for your sample in its application. Obviously, you don't want to fit a model only as a data reduction process, but to analyze the data for suitability for some cause. Having a good theoretical thought process is helpful before you start.
Having said this, you may select 2-3 models that would seem applicable. Normally, you'd then use a non-linear regression algorithm to select the best fitting model from the Chi squared value being nearest to 1.000. Most rheometers have built-in model fitting in their software, so you don't necessarily need to use Origin, Matlab etc, but if you're using an esoteric model, that may be the case.
Hope this is helpful!
Hi Krystian and Philip, Thank you so much for your answers. It was really helpful for me. I fitted my Shear stress vs shear rate and viscosity vs shear rate data. However, in all the models (Herschel-bulkley, power law, bingham) that I tested for stress vs shear rate, R2 values were less than 0.9.. and in all the models that I fitted for viscosity vs shear rate (power law, carreau, carreau-yasuda and sisko) the R2 value is 0.99..Is it ideal to use both plots for analysis to find the best fit models? or is it ok to go with only viscosity vs shear rate? will it be biased if I go with viscosity vs shear rate only? And why are the values that I get while fitting power law in both plots not giving same R2 when both the lots are obtained with a same set of experiments? Thank you so much..
Hi @Rita Thapa, Are you trying to extrapolate or interpolate to find viscosity at a particular stress, or shear rate? If so, the accuracy of model fitting is of prime importance, and it doesnt really matter which rheological model it is, so long as it looks sensible outside the collected data.
The reason that the model may fit differently when plotted vs viscosity cf stress and raye maybe the log or linear natire of the data and emphasis of the nonlinear regression algorithm?
Hi Philip, I used viscosity vs shear rate to find zero and infinite shear viscosity and cason model to determine yield stress.. however, i also wanted to determine the flow behaviour index (n) and consistency coefficient (k) using model fitting from shear stress vs shear rate data but my R2 values are less than .9 in all cases
Ms Ritu Thapa, First observed the shear stress v/s shear rate or Viscosity v/s shear rate data on linear scale. observe the curve pattern (very important aspect for model fiiting and explain the parameters), weather any shear stress is observed at zero shear rate? that means there is a exisitence of yield stress - fit the models which is having a yield stress parameter, other wise power law model may obeys. Most of the standard rheometers having inbuilt softwares for calculating the parameter or transfer data to any statistical software having non-linear estimation programme and calculate the parameter having high R2 value and low error values. The primary thing is observe the graph of shear stress v/s shear rate or Viscosity v/s shear rate data on linear scale.
S. S. Manjunatha Thank you so much for your response. I really appreciate it.
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