RHEOLOGY

By: Aamarpali Puri

Websters define rheology as “the study of the change in form and flow of matter, embracing elasticity, viscosity and plasticity. The word rheology in the most literal sense means the study of flow. However, one may extend the concept of flow of a material to include the idea of any change in its shape, under the action of an agency, which is not instantaneous, and which is not recoverable. The ideal solids deform easily and when the stress is removed, the energy of deformation is fully recovered. The energy of deformation arises from internal friction of the fluid otherwise known as viscosity. In dealing with industrial fluids, perhaps raw materials or manufactured products, introducing viscosity as a single physical constant is not sufficient to make accurate predictions of real flow behavior. This is due to the fact that in many cases a single parameter is not sufficient to describe the fluids flow behavior or rheology. Where a simple coefficient of viscosity is sufficient the flow behavior is referred to as Newtonian. Unfortunately many everyday fluids are non-Newtonian, for example Paints, salad cream, wall paste, printing ink, etc. For studying the rheology of these we need to know shear stress, shear rate, torque and apparent viscosity. A frequent application of the measurement of rheological properties can be found in the area of quality control, where raw materials are consistent. Broadfoot (Broadfoot and Miller, 1990) studied the rheological behavior of massecuites and molasses from Australian sugar factories and found them to pseudoplastic characteristic but their viscosity values varied, as did their composition. Molasses (Kaur, Kaler and Aamarpali, 2002) in the presence of starch exhibited pseudoplastic behavior and the shear stress shear rate relationship obeyed the power law model. Capillary rheometer ((Nguyen, Geiger and Fritz, 20002) was used for rheological characterization of thermo tropic liquid crystalline polymer. Vane geometry is used as an efficient rheometric measurement system for determining the Newtonian, non-Newtonian and viscoelastic properties of complex fluids. Rodrigue (Rodrigue, 2002) made use of its generalized models to take into account the shear-thinning character of the fluids via the simple power-law viscosity model for inelastic non-Newtonian fluids. One of the instrument is ABEMAT refractometer for analysis of brix of the Fluid. See Figure.
The direct assessment of process ability is the principal objective of the flow behavior studies. The flow behavior is an indirect measure of the product consistency and quality. The rheological studies provide information on how to best control the flow properties of the product so that the desired end product can be prepared and contribute also to a better understanding the underlying mechanism of momentum and heat transfer process.

References:
1. Broadfoot, R and Miller, K.F. (1990). “Rheological studies of massecuites and molasses”. Int. Sug. J., 92 {1098} 107-115.
2. Kaur, S.; Kaler, R.S.S and Aamarpali. (2002). “Effect of starch on the rheology of molasses”. J. Food. Eng. (U.K), 55, 319-322.
3. Nguyen, T.N.; Geiger, K and Fritz, H.G. (2002). “Rheological properties and shear induced texture development of thermo tropic liquid crystalline polymer melts”. Appl. Rheol., 12, 142-150.
4. Rodrigue, D. (2002). “A simple correlation for gas bubbles rising in Power-Law fluids”. Can. J. Chem. Eng., 80, 289-292.

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