View - Martin Kröger - ETH Zürich

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1304 FANG, KRÖGER, AND ÖTTINGER FIG. 1. Dimensionless shear viscosity as function of dimensionless time after startup of steady shearing at many dimensionless shear rates predicted by our model. first-normal-stress difference coefficient 1 in Fig. 4. The overshoot in 1 occurs at a higher shear rate than the shear viscosity does. Here, our model underpredicts the magnitude of the overshoot at the highest shear rate. Not shown are the predictions by FCS and MLDS models: they are very comparable with our model for the two lower shear rates. For the highest rate, again the FCS model overpredicts the overshoot and the MLDS model gives a better but underpredictive fit. In Fig. 5 we plot the magnitude of the strain p at which the maximum in the overshoot of the stress occurs as functions of shear rate predicted by the models and the experiment. It appears that the values of p stay nearly constant at low shear rates and increase with the increasing shear rate at high rates. Here, our model predictions are in good agreement with the experimental data, while the MLDS and FCS models overpredict. Not shown are the model predictions and FIG. 2. Transient growth of normalized viscosity as function of time under startup of steady shear flow at several shear rates predicted by our model lines and experiment symbols.
THERMODYNAMICALLY ADMISSIBLE REPTATION MODEL 1305 FIG. 3. Transient growth of normalized viscosity as functions of time under startup of steady shear flow at ˙ 10 s 1 predicted by the models and experiment. the experimental data of p for the first normal stress difference, which have a qualitative shape similar to that seen in Fig. 5 but differ by approximately a factor of 2. B. Cessation of steady shearing In Fig. 6 we plot the relaxation of the shear stress, normalized by its initial value, after cessation of steady shear flow for two shear rates. All the model predictions not shown are the predictions by MLDS which almost overlap with ours compare rather well with the experimental data. The stress relaxation rate increases with the increasing previous shear rate. Later in the relaxation process, at times much larger than s , the relaxation rate is governed only by the reptation process and constraint release due to double FIG. 4. Transient growth of normalized first-normal-stress difference coefficient as functions of time under startup of steady shear flow at several shear rates predicted by our model lines and experiment symbols.
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View - Martin Kröger - ETH Zürich

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