In scientific terms, viscosity is the measure of a fluid’s internal flow resistance. The physical quantity ‘viscosity’ gives information on how thick a fluid is and how easily it flows. This article provides insights into the basics of viscometry, quality control parameters, and application examples. Rotational viscometers are perfectly suited for the determination of the viscosity of samples ranging from liquid (e.g., nose drops, juices) to semi-solid (e.g., waxes, sauces).
Many important parameters for the production control of materials and also for the development of new products are directly related to the product’s viscosity. Rotating-cup rheometers are called Couette systems, while rotating-bob rheometers are called Searle systems, as shown in Figure 3 and Figure 4, respectively.Practically all industries rely on viscosity checks to develop, formulate, and produce a product with consistent characteristics. These rheometers involve an additional design consideration depending on whether the outer cylinder (the cup) or the inner cylinder (the bob) rotates. USP38 Concentric cylinder rotational rheometers are sometimes referred to as cup-and-bob rheometers. Controlled shear rate rheometers measure the shear stress (from the torque on the rotor axis) resulting from a given shear rate (or rotational speed).
Controlled shear stress rheometers measure the shear rates resulting from the application of a given force or torque (the stress).
Both controlled-stress and controlled-rate rotational rheometers are available commercially in configurations with absolute geometries (e.g., very small annular gaps between concentric cylinders) that allow calculation of apparent viscosities for non-Newtonian fluids. Apparatus: In the concentric cylinder rheometer, the apparent viscosity is determined by placing the liquid in the gap between the inner cylinder and the outer cylinder.
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