Units in Rheological Measurement

Shear Rate, denoted by the symbol, j , is the velocity gradient established in a fluid as a result of an applied shear stress. It is expressed in units of reciprocal seconds, s . Shear Stress is the stress component applied tangentially. It is equal to the force vector (a vector has both magnitude and direction) divided by the area of application and is expressed in units of force per unit area (Pa). The nomenclature committee of the Society of Rheology recommends that the symbol a be used to denote shear stress. However, the symbol r that was used to denote shear stress for a long time can be still encountered in rheology literature. 

Viscosity, is the internal friction of a fluid or its tendency to resist flow. It is denoted by the symbol t] for Newtonian fluids, whose viscosity does not depend on the shear rate, and for non-Newtonian fluids to indicate shear rate dependence by Depending on the flow system and choice of shear rate and shear stress, there are several equations to calculate. Here, it is defined by the equation  

The preferred units of viscosity are Pa s or mPa s. Some of the older units and their relation to the SI unit are given in Table 1-1. It is clear that the shear rate employed in the calculation must be specified when the magnitude of apparent viscosity is discussed. Apparent viscosity has many useful applications in characterizing a fluid food in particular, in the characterization of shear-thinning fluids, the apparent viscosity at low shear rates, called the zero-shear rate viscosity ( jo), is a useful parameter. 

Because much work was done with viscosity expressed in poise in sensory assessment of viscosity, the same tradition will be continued in Chapter 7 on sensory assessment of viscosity. 

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