Chemistry Reference
In-Depth Information
10
3
Rheometer measurement
Power law fit
10
2
10
1
10
0
10
−1
10
−1
10
0
10
1
10
2
Shear rate (per second)
Fig. 3.5
The flow curve of a Carbopol suspension with the points measured with the
rheometer and the corresponding power law fit.
3.7.3
Gradient method
For the gradient method, the viscosity is calculated directly using Equa-
tions 3.9 to 3.11. The resulting minimum shear rate from the flat part
of the profile towards the pipe centre is limited by the resolution of
the velocity measurement and possible small distortions. Due to the
convolution (Section 3.7.1), the maximum measurable shear rate is
shifted away from the wall, therefore lower than the actual wall shear
rate ˙
γ
w
.
3.8
EXAMPLES
The application of the technique is demonstrated with two different
model fluids which have the advantage to be reproducible and mea-
surable with a conventional rheometer. The latter allows to verify the
in-line (UVP) measurement, which is a recommendable first step.
3.8.1
Carbopol solution
Aqueous Carbopol solution (0.5% wt) is a yield shear-thinning fluid.
The flow curve is shown in Fig. 3.5. In the double logarithmic plot, the
