Rotational Rheometer
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Rotational rheometer COUETTE-Type- und SEARLE-Type rheometer (Author: Prof. Dr. H.-J. Radusch)
Measurement principles
Rotational rheometers are used in rheology to determine fluid properties. They are characterised by two rotationally symmetrical components (e.g. circular plates) arranged on a common axis, between which the fluid to be tested is located [1].
The angular velocity ω of the rotating part determines the shear rate dγ/dt, and the applied torque Md determines the shear stress τ. The measuring principle of rotational rheometers is standardised in ISO 3219 [2]. There are two ways to use the geometry underlying rotational rheometers to determine flow characteristics:
- CS rheometers (CS = controlled stress), in which a defined shear stress is specified and the velocity gradient, which is proportional to the viscosity, is determined, and
- CR rheometers (CR = Controlled Rate), in which a defined shear rate is specified and the resulting shear stress is determined.
Another distinguishing feature of rheometers is the way in which one of the two active elements is driven. A distinction is made between COUETTE and SEARLE measuring systems.
COUETTE measurement principle
In the COUETTE measuring principle, the outer cylinder or lower plate is driven by an electric motor M1 (Fig. 1). The liquid to be characterised is made to flow in the measuring gap, whereby the resistance to shear transfers a viscosity-proportional torque Md1 to the inner cylinder or upper rotating body.
| Fig. 1: | CR rotational rheometer according COUETTE [3] |
The inner cylinder is coupled to a second motor M2, which can apply a torque Md2 opposite to that of motor M1. The torque ΔMd, which is proportional to the viscosity and is transmitted by the outer cylinder or the lower plate via the fluid, is determined by adjusting the torque of motor M2 until the inner cylinder remains in its rest position despite the flow of the test substance in the measuring gap. The compensatory measured electrical power of motor M2 is thus the measured variable for the torque. The speed difference results from the specified speed of the outer cylinder or lower plate.
SEARLE measurement principle
SEARLE measuring systems are characterised by the fact that the outer cylinder or lower plate is stationary. The inner cylinder, rotating body or rotor is driven by a controlled electric motor M, for which defined torque values can be specified (Fig. 2). Any increase in electrical energy is converted linearly into corresponding torque values on the rotating body axis.
| Fig. 2: | CR and CS rotational rheometer according SEARLE [3] |
Due to the resistance that the fluid exerts against the torque or the shear stress generated, the rotating body can only rotate at a certain rotational speed, i.e. a certain speed gradient, which corresponds to the viscosity of the fluid being tested. The resulting rotational speed n is measured with an optical sensor, which also allows small rotation angles φ to be detected.
In SEARLE-type rheometers, both the applied torque and the resulting rotor rotational speed n act on the same rotor axis [3]. The CR or CS principle can be coupled with the SEARLE as well as with the COUETTE measuring system (see Figs. 1 and 2).
The most important different geometric and technical designs of rotational rheometers are:
- cone-plate rheometers,
- plate-plate rheometers and
- coaxial cylinder rheometers.
Acknowledgements
The editors of the lexicon "Polymer Testing & Diagnostics" would like to thank Prof. Dr.-Ing. habil. Hans-Joachim Radusch, Martin Luther University Halle-Wittenberg and Polymer Service GmbH Merseburg for this guest contribution.
See also
References
| [1] | Radusch, H.-J.: Determining Process-related Properties. In: Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser, Munich (2022) 3rd Edition, pp. 48–50 (ISBN 978-1-56990-806-8; E-Book: ISBN 978-1-56990-807-5; see AMK-Library under A 22) |
| [2] | ISO 3219 (2021-05): Rheology – Part 1: Vocabulary and Symbols for Rotational and Oscillatory Rheometry and Part 2: General Principles of Rotational and Oscillatory Rheometry |
| [3] | Schramm, G.: Einführung in die Rheologie und Rheometrie. Gebrüder Haake GmbH, Karlsruhe, 2nd Edition (2004) ([https://www.polymerservice-merseburg.de/fileadmin/inhalte/psm/veroeffentlichungen/Rheologie_Inhaltsverzeichnis.pdf Content as pdf) |