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 viscosity is calculated using the formula:
The Different Test Types
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.
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 ŷ. With a flow curve the shear stress τ is usually plotted on the y-axis and the shear rate ŷ⋅ 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
Figure 1. Viscosity functions: (1) ideally viscous or Newtonian flow behavior, (2) shear-thinning flow behavior, (3) shear-thickening flow behavior.
This test is predominantly used to determine thixotropy, which is the structural decomposition and regeneration of a substance. This is the topic of the next "Tips and Tricks from Joe Flow".
Under constant settings the viscosity is determined as a function of the temperature.
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This information has been sourced, reviewed and adapted from materials provided by Anton Paar GmbH.
For more information on this source, please visit Anton Paar GmbH.