i have viscosity diagram vs shear rate
i want to use cross model
but i dont know how to calculate the C and m and n zero and n infinity
the equation is in the attachment
please some body explain in detalis
thanks
Dear Elmira,
The viscosity at infinite shear rate cannot be determined from the data shown. The viscosity curve does not show a plateau at high shear rates. Just assume its value to be zero. The viscosity at zero shear rate (etha 0) is simply the value shown in your data at zero shear.
Now make a double logarithmic plot of the viscosity versus shear rate. The slope of the curve at high shear rates now equals -m. Extrapolation of the tangent towards shear rate 0 is equal to ln(etha 0) - m ln(C).
Dear Elmira and Hans;
It seems to me the parameters can be easily determined by curve fitting with a logistic function:
y = A2 + (A1-A2) / (1 + (C x) ^ m)
thus; A2 = n0, A1 = n8
For the top curve as an example, I obtained n0 = 5.73; n8 = 2.01; C = 0.0065, m = 2.01
Is there any reason why this approach is not valid? Regards, Jinan Cao
thanks all.....please dr. Jinan please check the following attachment
Most of the rheology instrument softwares are powerful and can be used to fit the cross model. If you have used a ARES instrument then TRIOS software can be used for fitting the data. Just get the software and become familiar with it that's all.
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