Transient Shear Stresses

Transient shear stresses following either a flow reversal or a step-up in shear rate show pronounced oscillations, and the period of these oscillations scales with the shear rate [161]. 

FIG. 16.35 Transient shear stress vs. time upon starting steady shear flow at three different shear rates for a 20% (w/w) solution of PpPTA in concentrated sulphuric acid. Reproduced with permission from Doppert HL and Picken SJ (1987) Mol Cryst Liq Cryst 153,109. Copyright Taylor and Francis Ltd., http //www.informaworld.com. 

Mason, P. L., Bistany, K. L., Puoti, M. G., and Kokini, J. L. 1982. A new empirical model to simulate transient shear stress growth in semi-solid foods. J. Food Process Eng. 6 219-233. 

Several studies were conducted on the stress overshoot and/or decay at a constant shear rate. Kokini and Dickie (1981) obtained stress growth and decay data on mayonnaise and other foods at 0.1, 1.0, lO.Oand 100 s . As expected from studies on polymers, shear stresses for mayonnaise and other food materials displayed increasing degrees of overshoot with increasing shear rates. The Bird-Leider empirical equation was used to model the transient shear stresses. 

Figure 1.10 Transient shear stress a and first normal stress difference A i after start-up of steady shearing for a low-density polyethylene melt, Melt I, at a shear rate y = 1 sec . (From Laun 1978, reprinted with permission from Steinkopff Publishers.)...

Figure 43. Evolution of transient shear stress for 7.5 wt% silica suspensions in 2.0% aqueous HPMC solution (176).

Another indication of fewer entanglements is the magnitude of the overshoot in the transient shear stress. For high shear rates, the stress exhibits a maximum, followed by a steady-state value determined by the degree of entanglement under the particular conditions of flow. If the shearing is stopped, the subsequently observed stress overshoot will be smaller. However, its magnitude... 

FIGURE 6.20 Maximum in transient viscosity after initiation of shear flow varsus the duration of a quiescent interval between shearing. The growth of the overshoot peak is caused by reentanglement of the solution. The inset shows representative transient shear stress data (Roland and Robertson, 2006). 

The formulation of Eqs. (70)-(72) was compared with experiment for an SMR-5 nature rubber with 0.2 volume fraction N-326 carbon black. The steady-state viscosity is compared with experiment in Fig. 8(a), and the transient shear stress buildup at the start of flow is considered in Fig. 8(b). The shear flow-rest-shear flow data are contained in Fig. 8(c). The agreement is, in general, good. 

Fig. 6. Transient shear-stress measurements presented in dimensionless variables for a...

Fig. 13 Transient shear stress recorded after difierent step shear rates (a) 7= 1.2 s, (b) 2s and (c) 5 s for a semidilute sample of CPQ/NaSal (12 wt. %) in 0.5M Nad brine at a temperature of T = 20.3 °C. All the applied shear rates belong to the plateau region. Reprinted from Berret [138]...

Figure 9.9 shows a classical data set by Meissner on the low-density polyethylene whose transient shear stress was shown in Figure 2.6. The tensile stress divided by the stretch rate is plotted versus time, together with three times the transient development of the zero-shear viscosity. The data deviate from a single curve at values of the strain (stretch rate multiplied by time) of about 2. There is a plateau at low stretch rates at a Trouton ratio of 3, but the plateau is followed by a sharp increase, and in general the tensile stress greatly exceeds three times the shear viscosity. (The shear viscosity for this polymer decreases with shear rate, so the deviation from three times the viscosity is greater than it would appear when the comparison is based only on the zero-shear viscosity.) The fact that the data lie above the band of three times the shear values at short times is probably an experimental artifact. A steady-state stress is not reached in these experiments, except perhaps at the highest and lowest stretch rates. An apparent steady state has been reported in other measurements.

Figure 44. Transient shear stress behavior at the shear rate of 0.1 s for 10 wt.% polypyrrole (PPy) coated polyethylene (PE) particle doped with 1.5 g FcClj-bHiO dispersed in mineral oil. The inset is the same material doped with 0.75 g FeCl3 6H20. , K=1.5 kV/mm F>2.0 kV/mm. Reproduced with permission from Y. D. Kim, D. H. Park, Synthetic Metals 142 (2004) 147.

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