Joe Flow 06 - Rotational Measurements
With the right know-how you can quickly and easily characterize the viscous properties of your samples using rotational tests.
What is a rotational measurement and what are the most frequently used types of rotational measurements?
With rotational measurements the measuring bob turns in one direction while the lower plate or measuring cylinder does not move. This causes laminar flow in the shear gap of the sample.
There are different types of rotational tests. In these tests there are two different types of test settings which can be used:
- In a controlled shear rate test (CSR or CR test) the speed or shear rate is set and controlled and the torque or shear stress is measured.
- In a controlled shear stress test (CSS or CS test) the torque or shear stress is set and controlled and the speed or shear rate is measured.
The different test types:
Single-point measurements: In this test type the viscosity is measured at constant settings. This measurement is only suitable for Newtonian substances, whose viscosity remains constant independent of the set load. However, it is often used for quality control of non-Newtonian substances.
Time test: This test is used to evaluate the time dependence of a sample under constant settings, e.g. during curing or gelification.
Flow and viscosity curves: To produce flow and viscosity curves a shear rate range is set and the viscosity is measured as a function of the shear rate. With a flow curve the shear stress τ is usually plotted on the y-axis and the shear rate γ ⋅ on the x-axis. With a viscosity curve, the viscosity η is usually plotted on the y-axis and the shear rate γ ⋅ on the x-axis. Most samples show shear-thinning behavior. This means the viscosity decreases with increasing shear rate. Figure 1 gives an overview of the viscosity functions for substances without a yield point.
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