19.11.2020 by Milena Riedl

Working Principle of Capillary Rheometers

Traditionally, capillary rheometers have been used to measure the shear viscosity and elasticity of viscous materials at high shear rates. In this article and video, Natalie Rudolph (PhD) explains the working principle, parameters and their relations of the capillary rheometer.

In the previous blog article about capillary rheometry, we answered the important question why there is a need for a capillary rheometer, what parameters can be measured and for which purposes the parameters can be used.

Natalie Rudolph (PhD) explains the working principle, parameters and their relations of the capillary rheometer in the video below.

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What does a characteristic viscosity flow curve look like?

For each applied piston speed, a shear rate (dependent on the die diameter) is applied on the sample. An equilibrium pressure is recorded for each piston speed to calculate shear viscosity. The shear viscosity is calculated from the velocity and pressure prevailing at different shear rates.

Most samples (especially polymers) follow a characteristic viscosity flow curve. In the zero shear region, the viscosity is shear rate independent. This happens at low shear rates as the deformation is not large enough to disentangle the long polymer chains. The Shear ThinningThe most common type of non-Newtonian behavior is shear thinning or pseudoplastic flow, where the fluid vis­cosity decreases with increasing shear.shear thinning region describes the region where the viscosity continues to decrease with increasing shear rates. Once the polymer chains are elongated and stretched to the maximum extent, increased shearing cannot further reduce the viscosity. This is the so-called infinite viscosity region.

Figure 1: Characteristic viscosity flow curve

In order to obtain accurate viscosity curves, an understanding of the impact factors on the measurement results is required. These factors can be based on the setup, but also on the material properties like the shear thinning behavior itself.

Bagley and Rabinowitsch Corrections

In the video, Dr. Rudolph explains both the Bagley and Rabinowitsch correction in detail and demonstrates their importance. The Bagley correction is required due to the typical flow conditions pressing material from a bigger reservoir into the smaller die. The latter Rabinowitsch correction is most important for Non-NewtonianA non-Newtonian fluid is one that exhibits a viscosity that varies as a function of the applied shear rate or shear stress.non-Newtonian materials like polymers.

Next week, we will share details on the RH 2000 and show in a quick test run how easy to operate the instrument is.