Measurement of Flow and Viscoelastic Properties

Techniques for measuring rheological properties of polymeric materials have been well described previously by others (e.g., Whorlow, 1980 Macosko, 1994). The text by Van Wazer et al. (1963) is still a valuable reference that explains in detail many facets of earlier attempts to measure rheological properties of polymeric materials as well as basic equations of viscometric flows. The unique nature of fluid foods prompted this author to review both the rheological properties of fluid foods and their measurement about 30 years ago (Rao, 1977a, 1977b). Subsequent efforts on rheology of foods include those of Rao (1992, 2005) and Steffe (1996). 

The concepts of laminar and fully developed flow are frequently used in studies on flow and heat transfer. Both are best explained using flow in a straight tube as an 

Viscosity measurements are done under laminar flow conditions. Under conditions of turbulent flow of Newtonian fluids, the measured viscosity will be higher. In general, because non-Newtonian fluid foods are highly viscous, usually laminar flows are encountered. 

In a rotational viscometer, the shear rate is derived from the rotational speed of a cylinder or a cone. Often, considerable effort is spent on obtaining low-shear viscosity data, such as zero-shear viscosity, that are difficult to obtain in view of the low shear rates needed for example, 0.01 s The maximum rotational speed that is used is left to the discretion of the operator. If the flow properties of the food are needed to design for processing operations, it is necessary to use shear rates that span the range expected to be used in the process and these may be quite high. 

In a concentric cylinder geometry, the shear stress can be determined from the total torque (A/)  

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