Influence of Rotational Mode on Rheometry of Low-Viscosity Fluids


When working with low-viscosity fluids at high shear rates using a concentric cylinder system with a rotating bob and a fixed cup (Searle mode), inertial forces may cause secondary flow effects, which are known as Taylor vortices. These vortices are the result of centrifugal forces that push the low-viscosity sample from the inner cylinder towards the outer one. These pairwise counter-rotating Taylor vortices show patterns perpendicular to the direction of the main flow with a vortex height which is roughly equal to the gap between the inner and outer cylinders.

Above a certain critical value in angular velocity, or shear rate, the superposition of the laminar flow by the Taylor vortices increases the energy dissipation, causing an erroneous increase of the measured torque and the calculated viscosity of the sample[2][3].

Contrary to the Searle mode, there is no formation of Taylor vortices in the case of the Couette mode with a fixed inner cylinder (bob) and a rotating outer cylinder (cup)[4] . Nevertheless, along with Taylor vortices, further types of secondary flow appear at higher shear rates (e.g. vortices below the bottom of the inner cylinder), which may lead to inaccurate results in both Searle and Couette modes.

The aim of this application report was to compare the formations of secondary flow depending on the rotational mode of the geometry used (Searle, Couette or counter-rotation mode).

Furthermore, the impact of the rotational mode on the measuring performance at low torques and shear rates was investigated.

To eliminate the influences of geometry factors and surface properties of the measuring geometry, the measurements were carried out with the same geometry while the TwinDrive option of an MCR was used to enable investigations in Searle, Couette or counter-rotation mode.

[2] Macosko, C. W. (1994). Rheology Principles, Measurements and Applications. Wiley-VCH, New York

[3] Malkin, A.Y., Isayev, A.I. (2006). Rheology: Concepts, Methods and Applications. ChemTec Publishing, Toronto

[4] Anonymous, (2008), DIN 53019-3:2008-09: Viscosimetry - Measurement of viscosities and flow curves by means of rotational viscometers – Part 3: Errors of measurements and corrections, DIN German Institute for Standardization, Berlin

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