Lessons Learned, and Pitfalls to Avoid in Modern Rheometry
Arno Talmon (TU Delft - Mechanical Engineering, Deltares)
Ebi Meshkati (Royal Boskalis Papendrecht, Deltares)
Floris van Rees (Deltares, Universiteit Utrecht)
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Abstract
Rheometry is the measurement of rheological properties. Rheometry is essential to understanding and quantifying the flow behaviour of non-Newtonian fluids. Results may differ from one research group to another, or from machine to machine. We will address discrepancies that may arise from the conversion of the machine’s rotational velocity to physical shear rate, ill-defined protocols, the occurrence of wall slip, (bottom) gap size variations, and differences in sample preparation. We confine ourselves to materials consisting of fine colloidal constituents, which form a homogenous carrier fluid with eventual coarse material augmenting the rheology, but are inert by themselves and prone to jamming your rheometer or may settle during the rheometric measurement. Wall slip may occur in rheometry, especially in smooth-walled geometries. Wall slip transpires when the composition of the mixture near the wall differs from that in the interior. A method to identify wall slip in rheometry involves comparing flow curves obtained from various geometrical dimensions, such as pipe/capillary diameter, sheared gap size, or radical testing with vane. Coarse constituents may promote wall slip, but surprisingly, we also encountered it in natural fluid mud without coarse particles. Measurements show that the narrow bottom gap, the mechanical default of the concerned rheometer using vanes, impacts results. The application of dedicated protocols to measure specific properties is another crucial aspect that is emphasized in this paper.