Rheology time effects

The self-similar spectrum is not valid at short times, X < X0, where the details of chemical structure become important (glass transition, entanglements, etc.). The cross-over to the glass transition at short times is typical for all polymeric materials, for both liquids and solids. The critical gel is no exception in that respect. X0 could be used as a characteristic time in the CW spectrum since it somehow characterizes the molecular building block of the critical gel however, it has no direct relation to the LST. At times shorter than X0, the LST has no immediate effect on the rheology. Indirect effects might be seen as a shift in the glass transition, for instance, but these will not be studied here. 

As mentioned in Part 7.1, for polymer blends the relation between the steady-state shear viscosity and concentration can be quite complex. In the following discussion, the constant stress (not the constant rate) viscosity, corrected for the yield and time effects, will be considered. To illustrate flexibility of Equation 7.125 to describe (and thus to facilitate interpretation of the rheological results) r vs. < > dependence examples of computations are shown in Figures 7.24-7.31. 

Many examples have been given (Baird, 1982 Hemqvist, 1983 Princen, 1983 Hermansson, 1994 Morrison, 1994 Breitschuh and Windhab, 1997) that illustrate the complexity of the rheological behavior of lipids-based foods. The rheology of lipids, as in other food systems, is the science of deformation and flow of real materials, in terms of stress, strain, and time effects, not merely those of ideal... 

Non-Newtonian Flows Newtonian behavior has been reported for dilute emulsions or emulsions subjected to slow deformation. As the concentration and deformation rate increases, the flow becomes more complex One, frequently neglected, complication is the time effect - at lower concentration the emulsions may show thixotropy, while in highly concentrated emulsions, 0.9, rheopexy dominates. Once the time effects are subtracted, the emulsion usually shows a pseudoplastic character, often with the yield stress. Evidently, the rheological response is a reflection of the inner structure of the emulsion hence a change of its morphology is inherent to flow. 

A check of actual rheological data for the system in English units (Fig. 10-6) shows that the line must be shifted to fit the preceding. Further, in the actual calendering case, there will be both an end effect and a time effect. 

The basic principles of rheology and the various experimental methods that can be applied to investigate these complex systems of food colloids have been discussed in detail in Chapter 7. Only a brief summary is given here. Two main types of measurements are required (1) Steady-state measurements of the shear stress versus shear rate relationship, to distinguish between the various responses Newtonian, plastic, pseudo-plastic and dilatant. Particular attention should be given to time effects during flow (thixotropy and negative thixotropy). (2) Viscoelastic behaviour, stress relaxation, constant stress (creep) and oscillatory measurements. 

LOY is characterized by low spinning tension, mostiy rheological effects, Httie orientation, amorphous stmcture, low tensde strength, and high elongation. The spun filament must be drawn, usually three to six times its initial length, and heat-treated before it develops useful properties. Nearly all PET staple is spun this way. 

The fact that the appearance of a wall slip at sufficiently high shear rates is a property inwardly inherent in filled polymers or an external manifestation of these properties may be discussed, but obviously, the role of this effect during the flow of compositions with a disperse filler is great. The wall slip, beginning in the region of high shear rates, was marked many times as the effect that must be taken into account in the analysis of rheological properties of filled polymer melts [24, 25], and the appearance of a slip is initiated in the entry (transitional) zone of the channel [26]. It is quite possible that in reality not a true wall slip takes place, but the formation of a low-viscosity wall layer depleted of a filler. This is most characteristic for the systems with low-viscosity binders. From the point of view of hydrodynamics, an exact mechanism of motion of a material near the wall is immaterial, since in any case it appears as a wall slip. 

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