Steady flow rheological properties

Some of the most difficult material properties of fluid and semisolid foods to determine experimentally are viscometric functions and steady shear rheological properties. The flow properties of a liquid and semisolid food system should be measured in the following instances ... 

The steady and dynamic drag-induced simple shear-flow rheometers, which are limited to very small shear rates for the steady flow and to very small strains for the dynamic flow, enable us to evaluate rheological properties that can be related to the macromolecular structure of polymer melts. The reason is that very small sinusoidal strains and very low shear rates do not take macromolecular polymer melt conformations far away from their equilibrium condition. Thus, whatever is measured is the result of the response of not just a portion of the macromolecule, but the contribution of the entire macromolecule. 

Fig. 3.1 Examples of simple, viscometric, shear-flow rheometer geometries, la, 2a and 3 are steady while lb and 2b are dynamic rheological property... 

To characterize Newtonian and non-Newtonian food properties, several approaches can be used, and the whole stress-strain curve can be obtained. One of the most important textural and rheological properties of foods is viscosity (or consistency). The evaluation of viscosity can be demonstrated by reference to the evaluation of creaminess, spreadability, and pourability characteristics. All of these depend largely on shear rate and are affected by viscosity and different flow conditions. If it is related to steady flow, then at any point the velocity of successive fluid particles is the same at successive periods of time for the whole food system. Thus, the velocity is constant with respect to time, but it may vary at different points with... 

We can see that Eqs. (2 101) (2-104) are sufficient to calculate the continuum-level stress a given the strain-rate and vorticity tensors E and SI. As such, this is a complete constitutive model for the dilute solution/suspension. The rheological properties predicted for steady and time-dependent linear flows of the type (2-99), with T = I t), have been studied quite thoroughly (see, e g., Larson34). Of course, we should note that the contribution of the particles/macromolecules to the stress is actually quite small. Because the solution/suspension is assumed to be dilute, the volume fraction is very small, (p 1. Nevertheless, the qualitative nature of the particle contribution to the stress is found to be quite similar to that measured (at larger concentrations) for many polymeric liquids and other complex fluids. For example, the apparent viscosity in a simple shear flow is found to shear thin (i.e., to decrease with increase of shear rate). These qualitative similarities are indicative of the generic nature of viscoelasticity in a variety of complex fluids. So far as we are aware, however, the full model has not been used for flow predictions in a fluid mechanics context. This is because the model is too complex, even for this simplest of viscoelastic fluids. The primary problem is that calculation of the stress requires solution of the full two-dimensional (2D) convection-diffusion equation, (2-102), at each point in the flow domain where we want to know the stress. 

Buevich, Yu. A. and Shchelchkova, I. N., Rheological properties of homogeneous finely divided suspensions. Steady-state flows, J. Eng. Phys. Thermophys., Vol. 33, No. 5, 1977. 

For Newtonian lipid-based food systems, it is sufficient to measure the ratio of shearing stress to the rate of shear, from which the viscosity can be calculated. Such a simple shear flow forms the basis for many rheological measurement techniques. The rheological properties resulting from steady shear flow for variety of food systems have been studied by many laboratories (Charm, 1960 Holdsworth, 1971 Middleman, 1975 Elson, 1977 Harris, 1977 Birkett, 1983 Princen, 1983 Shoemaker and Figoni, 1984 Hermansson, 1994 Kokini et al., 1994, 1995 Morrison, 1994 Pinthus and Saguy, 1994 and Meissner, 1997). 

Nishitani, Y. Sekiguchi, I. Hausnerova, B. Zdrazilova, N. Kitano, T. (2007). Rheological Properties of Aminosilane Surface Treated Short Glass Fibre Reinforced Polypropylenes. Part 1 Steady Shear and OsHllatoiy Flow Properties in Molten State. Polymers Polymer Composites, Vol.l5, pp. 111-119 ISSN 0967-3911... 

In marked contrast to measurements of shear rheological properties, such as apparent viscosity in steady shear, or of complex viscosity in small amplitude oscillatory shear, extensional viscosity measurements are far from straightforward. This is particularly so in the case of mobile elastic liquids whose rheology can mitigate against the generation of well-defined extensional flow fields. 

RHEOLOGICAL PROPERTIES OF LIQUID CRYSTALLINE POLYMERS Steady Shear Flow... 

Rheological properties under steady state and oscillatory shear flow of isotropic and nematic solutions of PpPTA, PBT and PBO were studied by Baird [70] and Berry et al. [46]. Baird observed shear thinning for a series of PpPTA solutions in sulfuric acid (4-15%). These results also suggest that at higher shear rates very little difference exists between the anisotropic and isotropic phases. Steady-state viscosities as a function of the temperature observed for solutions of PBO in methane sulfonic acid showed a sharp increase near T,, a behavior which has also been reported for PpPTA and PpBA solutions [46],... 

---