Abstract
Abstract This paper analyzes various techniques to use viscometers equipped with vane spindles to characterize rheological properties of yield stress fluids. Specifically, application of Brookfield viscometers to this end is discussed. A wide selection of toothpastes and lotions were tested. It is shown that a simple method based on apparent shear rate and stress, commonly referred to as a representative viscosity method, works well for moderately non-Newtonian samples but may significantly underestimate viscosity for samples with a more pronounced yield stress behavior. To get more accurate data an integral equation relating torque to angular velocity needs to be solved which can be easily done numerically to get a good agreement between the data collected on an inexpensive viscometer and the data from high-end rheometers.
Highlights
Brookfield viscometers are ubiquitous in food and consumer product industries as well as many others
3.1 Transient step-shear tests such tests can be performed and may be important for certain applications, in the context of this work relatively short 20-second-per-point test suffices. This allows to wipe-out the history of the sample loading into the narrow-gap DIN geometry and represents a typical condition for a quality control test like those performed with Brookfield viscometers
We will show that the results of such tests performed on a rheometer with the standard DIN geometry agree quite well with those performed on a Brookfield viscometer using immersion vane geometry
Summary
Brookfield viscometers (currently manufactured by AMTEK) are ubiquitous in food and consumer product industries as well as many others. While there are attachments which allow using these viscometers to perform more rigorously defined rheological measurements (narrow-gap concentric cylinders or cone-plate) this immersion technique remains by far the most widely used in Brookfield viscometers work as any strain-controlled rheometers – since they apply angular velocity (reported in rotation-per-minute units, RPM) and measure torque (reported in percentage of the maximum torque specific for the given type of the instrument). They convert measured torque into viscosity based on the calibration for a Newtonian standard. Anderson and Meeten [5] presented an extensive analysis of how the data obtained with T-bar tools can be converted into meaningful rheological parameters in terms of Herschel-Bulkley equation rather than reporting them as some “viscosities” indexes
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